rust-clippy/clippy_lints/src/methods/mod.rs

// Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.


use crate::rustc::hir;
use crate::rustc::hir::def::Def;
use crate::rustc::lint::{in_external_macro, LateContext, LateLintPass, Lint, LintArray, LintContext, LintPass};
use crate::rustc::ty::{self, Ty};
use crate::rustc::{declare_tool_lint, lint_array};
use crate::rustc_errors::Applicability;
use crate::syntax::ast;
use crate::syntax::source_map::{BytePos, Span};
use crate::utils::paths;
use crate::utils::sugg;
use crate::utils::{
    get_arg_name, get_trait_def_id, implements_trait, in_macro, is_copy, is_expn_of, is_self, is_self_ty,
    iter_input_pats, last_path_segment, match_def_path, match_path, match_qpath, match_trait_method, match_type,
    match_var, method_chain_args, remove_blocks, return_ty, same_tys, single_segment_path, snippet, span_lint,
    span_lint_and_sugg, span_lint_and_then, span_note_and_lint, walk_ptrs_ty, walk_ptrs_ty_depth, SpanlessEq,
};
use if_chain::if_chain;
use matches::matches;
use std::borrow::Cow;
use std::fmt;
use std::iter;

mod unnecessary_filter_map;

#[derive(Clone)]
pub struct Pass;

/// **What it does:** Checks for `.unwrap()` calls on `Option`s.
///
/// **Why is this bad?** Usually it is better to handle the `None` case, or to
/// at least call `.expect(_)` with a more helpful message. Still, for a lot of
/// quick-and-dirty code, `unwrap` is a good choice, which is why this lint is
/// `Allow` by default.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// x.unwrap()
/// ```
declare_clippy_lint! {
    pub OPTION_UNWRAP_USED,
    restriction,
    "using `Option.unwrap()`, which should at least get a better message using `expect()`"
}

/// **What it does:** Checks for `.unwrap()` calls on `Result`s.
///
/// **Why is this bad?** `result.unwrap()` will let the thread panic on `Err`
/// values. Normally, you want to implement more sophisticated error handling,
/// and propagate errors upwards with `try!`.
///
/// Even if you want to panic on errors, not all `Error`s implement good
/// messages on display.  Therefore it may be beneficial to look at the places
/// where they may get displayed. Activate this lint to do just that.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// x.unwrap()
/// ```
declare_clippy_lint! {
    pub RESULT_UNWRAP_USED,
    restriction,
    "using `Result.unwrap()`, which might be better handled"
}

/// **What it does:** Checks for methods that should live in a trait
/// implementation of a `std` trait (see [llogiq's blog
/// post](http://llogiq.github.io/2015/07/30/traits.html) for further
/// information) instead of an inherent implementation.
///
/// **Why is this bad?** Implementing the traits improve ergonomics for users of
/// the code, often with very little cost. Also people seeing a `mul(...)`
/// method
/// may expect `*` to work equally, so you should have good reason to disappoint
/// them.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// struct X;
/// impl X {
///    fn add(&self, other: &X) -> X { .. }
/// }
/// ```
declare_clippy_lint! {
    pub SHOULD_IMPLEMENT_TRAIT,
    style,
    "defining a method that should be implementing a std trait"
}

/// **What it does:** Checks for methods with certain name prefixes and which
/// doesn't match how self is taken. The actual rules are:
///
/// |Prefix |`self` taken          |
/// |-------|----------------------|
/// |`as_`  |`&self` or `&mut self`|
/// |`from_`| none                 |
/// |`into_`|`self`                |
/// |`is_`  |`&self` or none       |
/// |`to_`  |`&self`               |
///
/// **Why is this bad?** Consistency breeds readability. If you follow the
/// conventions, your users won't be surprised that they, e.g., need to supply a
/// mutable reference to a `as_..` function.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// impl X {
///     fn as_str(self) -> &str { .. }
/// }
/// ```
declare_clippy_lint! {
    pub WRONG_SELF_CONVENTION,
    style,
    "defining a method named with an established prefix (like \"into_\") that takes \
     `self` with the wrong convention"
}

/// **What it does:** This is the same as
/// [`wrong_self_convention`](#wrong_self_convention), but for public items.
///
/// **Why is this bad?** See [`wrong_self_convention`](#wrong_self_convention).
///
/// **Known problems:** Actually *renaming* the function may break clients if
/// the function is part of the public interface. In that case, be mindful of
/// the stability guarantees you've given your users.
///
/// **Example:**
/// ```rust
/// impl X {
///     pub fn as_str(self) -> &str { .. }
/// }
/// ```
declare_clippy_lint! {
    pub WRONG_PUB_SELF_CONVENTION,
    restriction,
    "defining a public method named with an established prefix (like \"into_\") that takes \
     `self` with the wrong convention"
}

/// **What it does:** Checks for usage of `ok().expect(..)`.
///
/// **Why is this bad?** Because you usually call `expect()` on the `Result`
/// directly to get a better error message.
///
/// **Known problems:** The error type needs to implement `Debug`
///
/// **Example:**
/// ```rust
/// x.ok().expect("why did I do this again?")
/// ```
declare_clippy_lint! {
    pub OK_EXPECT,
    style,
    "using `ok().expect()`, which gives worse error messages than \
     calling `expect` directly on the Result"
}

/// **What it does:** Checks for usage of `_.map(_).unwrap_or(_)`.
///
/// **Why is this bad?** Readability, this can be written more concisely as
/// `_.map_or(_, _)`.
///
/// **Known problems:** The order of the arguments is not in execution order
///
/// **Example:**
/// ```rust
/// x.map(|a| a + 1).unwrap_or(0)
/// ```
declare_clippy_lint! {
    pub OPTION_MAP_UNWRAP_OR,
    pedantic,
    "using `Option.map(f).unwrap_or(a)`, which is more succinctly expressed as \
     `map_or(a, f)`"
}

/// **What it does:** Checks for usage of `_.map(_).unwrap_or_else(_)`.
///
/// **Why is this bad?** Readability, this can be written more concisely as
/// `_.map_or_else(_, _)`.
///
/// **Known problems:** The order of the arguments is not in execution order.
///
/// **Example:**
/// ```rust
/// x.map(|a| a + 1).unwrap_or_else(some_function)
/// ```
declare_clippy_lint! {
    pub OPTION_MAP_UNWRAP_OR_ELSE,
    pedantic,
    "using `Option.map(f).unwrap_or_else(g)`, which is more succinctly expressed as \
     `map_or_else(g, f)`"
}

/// **What it does:** Checks for usage of `result.map(_).unwrap_or_else(_)`.
///
/// **Why is this bad?** Readability, this can be written more concisely as
/// `result.ok().map_or_else(_, _)`.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// x.map(|a| a + 1).unwrap_or_else(some_function)
/// ```
declare_clippy_lint! {
    pub RESULT_MAP_UNWRAP_OR_ELSE,
    pedantic,
    "using `Result.map(f).unwrap_or_else(g)`, which is more succinctly expressed as \
     `.ok().map_or_else(g, f)`"
}

/// **What it does:** Checks for usage of `_.map_or(None, _)`.
///
/// **Why is this bad?** Readability, this can be written more concisely as
/// `_.and_then(_)`.
///
/// **Known problems:** The order of the arguments is not in execution order.
///
/// **Example:**
/// ```rust
/// opt.map_or(None, |a| a + 1)
/// ```
declare_clippy_lint! {
    pub OPTION_MAP_OR_NONE,
    style,
    "using `Option.map_or(None, f)`, which is more succinctly expressed as \
     `and_then(f)`"
}

/// **What it does:** Checks for usage of `_.filter(_).next()`.
///
/// **Why is this bad?** Readability, this can be written more concisely as
/// `_.find(_)`.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// iter.filter(|x| x == 0).next()
/// ```
declare_clippy_lint! {
    pub FILTER_NEXT,
    complexity,
    "using `filter(p).next()`, which is more succinctly expressed as `.find(p)`"
}

/// **What it does:** Checks for usage of `_.map(_).flatten(_)`,
///
/// **Why is this bad?** Readability, this can be written more concisely as a
/// single method call.
///
/// **Known problems:**
///
/// **Example:**
/// ```rust
/// iter.map(|x| x.iter()).flatten()
/// ```
declare_clippy_lint! {
    pub MAP_FLATTEN,
    pedantic,
    "using combinations of `flatten` and `map` which can usually be written as a \
     single method call"
}

/// **What it does:** Checks for usage of `_.filter(_).map(_)`,
/// `_.filter(_).flat_map(_)`, `_.filter_map(_).flat_map(_)` and similar.
///
/// **Why is this bad?** Readability, this can be written more concisely as a
/// single method call.
///
/// **Known problems:** Often requires a condition + Option/Iterator creation
/// inside the closure.
///
/// **Example:**
/// ```rust
/// iter.filter(|x| x == 0).map(|x| x * 2)
/// ```
declare_clippy_lint! {
    pub FILTER_MAP,
    pedantic,
    "using combinations of `filter`, `map`, `filter_map` and `flat_map` which can \
     usually be written as a single method call"
}

/// **What it does:** Checks for an iterator search (such as `find()`,
/// `position()`, or `rposition()`) followed by a call to `is_some()`.
///
/// **Why is this bad?** Readability, this can be written more concisely as
/// `_.any(_)`.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// iter.find(|x| x == 0).is_some()
/// ```
declare_clippy_lint! {
    pub SEARCH_IS_SOME,
    complexity,
    "using an iterator search followed by `is_some()`, which is more succinctly \
     expressed as a call to `any()`"
}

/// **What it does:** Checks for usage of `.chars().next()` on a `str` to check
/// if it starts with a given char.
///
/// **Why is this bad?** Readability, this can be written more concisely as
/// `_.starts_with(_)`.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// name.chars().next() == Some('_')
/// ```
declare_clippy_lint! {
    pub CHARS_NEXT_CMP,
    complexity,
    "using `.chars().next()` to check if a string starts with a char"
}

/// **What it does:** Checks for calls to `.or(foo(..))`, `.unwrap_or(foo(..))`,
/// etc., and suggests to use `or_else`, `unwrap_or_else`, etc., or
/// `unwrap_or_default` instead.
///
/// **Why is this bad?** The function will always be called and potentially
/// allocate an object acting as the default.
///
/// **Known problems:** If the function has side-effects, not calling it will
/// change the semantic of the program, but you shouldn't rely on that anyway.
///
/// **Example:**
/// ```rust
/// foo.unwrap_or(String::new())
/// ```
/// this can instead be written:
/// ```rust
/// foo.unwrap_or_else(String::new)
/// ```
/// or
/// ```rust
/// foo.unwrap_or_default()
/// ```
declare_clippy_lint! {
    pub OR_FUN_CALL,
    perf,
    "using any `*or` method with a function call, which suggests `*or_else`"
}

/// **What it does:** Checks for calls to `.expect(&format!(...))`, `.expect(foo(..))`,
/// etc., and suggests to use `unwrap_or_else` instead
///
/// **Why is this bad?** The function will always be called.
///
/// **Known problems:** If the function has side-effects, not calling it will
/// change the semantic of the program, but you shouldn't rely on that anyway.
///
/// **Example:**
/// ```rust
/// foo.expect(&format!("Err {}: {}", err_code, err_msg))
/// ```
/// or
/// ```rust
/// foo.expect(format!("Err {}: {}", err_code, err_msg).as_str())
/// ```
/// this can instead be written:
/// ```rust
/// foo.unwrap_or_else(|_| panic!("Err {}: {}", err_code, err_msg))
/// ```
declare_clippy_lint! {
    pub EXPECT_FUN_CALL,
    perf,
    "using any `expect` method with a function call"
}

/// **What it does:** Checks for usage of `.clone()` on a `Copy` type.
///
/// **Why is this bad?** The only reason `Copy` types implement `Clone` is for
/// generics, not for using the `clone` method on a concrete type.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// 42u64.clone()
/// ```
declare_clippy_lint! {
    pub CLONE_ON_COPY,
    complexity,
    "using `clone` on a `Copy` type"
}

/// **What it does:** Checks for usage of `.clone()` on a ref-counted pointer,
/// (`Rc`, `Arc`, `rc::Weak`, or `sync::Weak`), and suggests calling Clone via unified
/// function syntax instead (e.g. `Rc::clone(foo)`).
///
/// **Why is this bad?** Calling '.clone()' on an Rc, Arc, or Weak
/// can obscure the fact that only the pointer is being cloned, not the underlying
/// data.
///
/// **Example:**
/// ```rust
/// x.clone()
/// ```
declare_clippy_lint! {
    pub CLONE_ON_REF_PTR,
    restriction,
    "using 'clone' on a ref-counted pointer"
}

/// **What it does:** Checks for usage of `.clone()` on an `&&T`.
///
/// **Why is this bad?** Cloning an `&&T` copies the inner `&T`, instead of
/// cloning the underlying `T`.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// fn main() {
///    let x = vec![1];
///    let y = &&x;
///    let z = y.clone();
///    println!("{:p} {:p}",*y, z); // prints out the same pointer
/// }
/// ```
declare_clippy_lint! {
    pub CLONE_DOUBLE_REF,
    correctness,
    "using `clone` on `&&T`"
}

/// **What it does:** Checks for `new` not returning `Self`.
///
/// **Why is this bad?** As a convention, `new` methods are used to make a new
/// instance of a type.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// impl Foo {
///     fn new(..) -> NotAFoo {
///     }
/// }
/// ```
declare_clippy_lint! {
    pub NEW_RET_NO_SELF,
    style,
    "not returning `Self` in a `new` method"
}

/// **What it does:** Checks for string methods that receive a single-character
/// `str` as an argument, e.g. `_.split("x")`.
///
/// **Why is this bad?** Performing these methods using a `char` is faster than
/// using a `str`.
///
/// **Known problems:** Does not catch multi-byte unicode characters.
///
/// **Example:**
/// `_.split("x")` could be `_.split('x')`
declare_clippy_lint! {
    pub SINGLE_CHAR_PATTERN,
    perf,
    "using a single-character str where a char could be used, e.g. \
     `_.split(\"x\")`"
}

/// **What it does:** Checks for getting the inner pointer of a temporary
/// `CString`.
///
/// **Why is this bad?** The inner pointer of a `CString` is only valid as long
/// as the `CString` is alive.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust,ignore
/// let c_str = CString::new("foo").unwrap().as_ptr();
/// unsafe {
///     call_some_ffi_func(c_str);
/// }
/// ```
/// Here `c_str` point to a freed address. The correct use would be:
/// ```rust,ignore
/// let c_str = CString::new("foo").unwrap();
/// unsafe {
///     call_some_ffi_func(c_str.as_ptr());
/// }
/// ```
declare_clippy_lint! {
    pub TEMPORARY_CSTRING_AS_PTR,
    correctness,
    "getting the inner pointer of a temporary `CString`"
}

/// **What it does:** Checks for use of `.iter().nth()` (and the related
/// `.iter_mut().nth()`) on standard library types with O(1) element access.
///
/// **Why is this bad?** `.get()` and `.get_mut()` are more efficient and more
/// readable.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// let some_vec = vec![0, 1, 2, 3];
/// let bad_vec = some_vec.iter().nth(3);
/// let bad_slice = &some_vec[..].iter().nth(3);
/// ```
/// The correct use would be:
/// ```rust
/// let some_vec = vec![0, 1, 2, 3];
/// let bad_vec = some_vec.get(3);
/// let bad_slice = &some_vec[..].get(3);
/// ```
declare_clippy_lint! {
    pub ITER_NTH,
    perf,
    "using `.iter().nth()` on a standard library type with O(1) element access"
}

/// **What it does:** Checks for use of `.skip(x).next()` on iterators.
///
/// **Why is this bad?** `.nth(x)` is cleaner
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// let some_vec = vec![0, 1, 2, 3];
/// let bad_vec = some_vec.iter().skip(3).next();
/// let bad_slice = &some_vec[..].iter().skip(3).next();
/// ```
/// The correct use would be:
/// ```rust
/// let some_vec = vec![0, 1, 2, 3];
/// let bad_vec = some_vec.iter().nth(3);
/// let bad_slice = &some_vec[..].iter().nth(3);
/// ```
declare_clippy_lint! {
    pub ITER_SKIP_NEXT,
    style,
    "using `.skip(x).next()` on an iterator"
}

/// **What it does:** Checks for use of `.get().unwrap()` (or
/// `.get_mut().unwrap`) on a standard library type which implements `Index`
///
/// **Why is this bad?** Using the Index trait (`[]`) is more clear and more
/// concise.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// let some_vec = vec![0, 1, 2, 3];
/// let last = some_vec.get(3).unwrap();
/// *some_vec.get_mut(0).unwrap() = 1;
/// ```
/// The correct use would be:
/// ```rust
/// let some_vec = vec![0, 1, 2, 3];
/// let last = some_vec[3];
/// some_vec[0] = 1;
/// ```
declare_clippy_lint! {
    pub GET_UNWRAP,
    style,
    "using `.get().unwrap()` or `.get_mut().unwrap()` when using `[]` would work instead"
}

/// **What it does:** Checks for the use of `.extend(s.chars())` where s is a
/// `&str` or `String`.
///
/// **Why is this bad?** `.push_str(s)` is clearer
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// let abc = "abc";
/// let def = String::from("def");
/// let mut s = String::new();
/// s.extend(abc.chars());
/// s.extend(def.chars());
/// ```
/// The correct use would be:
/// ```rust
/// let abc = "abc";
/// let def = String::from("def");
/// let mut s = String::new();
/// s.push_str(abc);
/// s.push_str(&def));
/// ```
declare_clippy_lint! {
    pub STRING_EXTEND_CHARS,
    style,
    "using `x.extend(s.chars())` where s is a `&str` or `String`"
}

/// **What it does:** Checks for the use of `.cloned().collect()` on slice to
/// create a `Vec`.
///
/// **Why is this bad?** `.to_vec()` is clearer
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// let s = [1,2,3,4,5];
/// let s2 : Vec<isize> = s[..].iter().cloned().collect();
/// ```
/// The better use would be:
/// ```rust
/// let s = [1,2,3,4,5];
/// let s2 : Vec<isize> = s.to_vec();
/// ```
declare_clippy_lint! {
    pub ITER_CLONED_COLLECT,
    style,
    "using `.cloned().collect()` on slice to create a `Vec`"
}

/// **What it does:** Checks for usage of `.chars().last()` or
/// `.chars().next_back()` on a `str` to check if it ends with a given char.
///
/// **Why is this bad?** Readability, this can be written more concisely as
/// `_.ends_with(_)`.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// name.chars().last() == Some('_') || name.chars().next_back() == Some('-')
/// ```
declare_clippy_lint! {
    pub CHARS_LAST_CMP,
    style,
    "using `.chars().last()` or `.chars().next_back()` to check if a string ends with a char"
}

/// **What it does:** Checks for usage of `.as_ref()` or `.as_mut()` where the
/// types before and after the call are the same.
///
/// **Why is this bad?** The call is unnecessary.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// let x: &[i32] = &[1,2,3,4,5];
/// do_stuff(x.as_ref());
/// ```
/// The correct use would be:
/// ```rust
/// let x: &[i32] = &[1,2,3,4,5];
/// do_stuff(x);
/// ```
declare_clippy_lint! {
    pub USELESS_ASREF,
    complexity,
    "using `as_ref` where the types before and after the call are the same"
}


/// **What it does:** Checks for using `fold` when a more succinct alternative exists.
/// Specifically, this checks for `fold`s which could be replaced by `any`, `all`,
/// `sum` or `product`.
///
/// **Why is this bad?** Readability.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// let _ = (0..3).fold(false, |acc, x| acc || x > 2);
/// ```
/// This could be written as:
/// ```rust
/// let _ = (0..3).any(|x| x > 2);
/// ```
declare_clippy_lint! {
    pub UNNECESSARY_FOLD,
    style,
    "using `fold` when a more succinct alternative exists"
}


/// **What it does:** Checks for `filter_map` calls which could be replaced by `filter` or `map`.
/// More specifically it checks if the closure provided is only performing one of the
/// filter or map operations and suggests the appropriate option.
///
/// **Why is this bad?** Complexity. The intent is also clearer if only a single
/// operation is being performed.
///
/// **Known problems:** None
///
/// **Example:**
/// ```rust
/// let _ = (0..3).filter_map(|x| if x > 2 { Some(x) } else { None });
/// ```
/// As there is no transformation of the argument this could be written as:
/// ```rust
/// let _ = (0..3).filter(|&x| x > 2);
/// ```
///
/// ```rust
/// let _ = (0..4).filter_map(i32::checked_abs);
/// ```
/// As there is no conditional check on the argument this could be written as:
/// ```rust
/// let _ = (0..4).map(i32::checked_abs);
/// ```
declare_clippy_lint! {
    pub UNNECESSARY_FILTER_MAP,
    complexity,
    "using `filter_map` when a more succinct alternative exists"
}

impl LintPass for Pass {
    fn get_lints(&self) -> LintArray {
        lint_array!(
            OPTION_UNWRAP_USED,
            RESULT_UNWRAP_USED,
            SHOULD_IMPLEMENT_TRAIT,
            WRONG_SELF_CONVENTION,
            WRONG_PUB_SELF_CONVENTION,
            OK_EXPECT,
            OPTION_MAP_UNWRAP_OR,
            OPTION_MAP_UNWRAP_OR_ELSE,
            RESULT_MAP_UNWRAP_OR_ELSE,
            OPTION_MAP_OR_NONE,
            OR_FUN_CALL,
            EXPECT_FUN_CALL,
            CHARS_NEXT_CMP,
            CHARS_LAST_CMP,
            CLONE_ON_COPY,
            CLONE_ON_REF_PTR,
            CLONE_DOUBLE_REF,
            NEW_RET_NO_SELF,
            SINGLE_CHAR_PATTERN,
            SEARCH_IS_SOME,
            TEMPORARY_CSTRING_AS_PTR,
            FILTER_NEXT,
            FILTER_MAP,
            MAP_FLATTEN,
            ITER_NTH,
            ITER_SKIP_NEXT,
            GET_UNWRAP,
            STRING_EXTEND_CHARS,
            ITER_CLONED_COLLECT,
            USELESS_ASREF,
            UNNECESSARY_FOLD,
            UNNECESSARY_FILTER_MAP
        )
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
    #[allow(clippy::cyclomatic_complexity)]
    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr) {
        if in_macro(expr.span) {
            return;
        }

        match expr.node {
            hir::ExprKind::MethodCall(ref method_call, ref method_span, ref args) => {
                // Chain calls
                // GET_UNWRAP needs to be checked before general `UNWRAP` lints
                if let Some(arglists) = method_chain_args(expr, &["get", "unwrap"]) {
                    lint_get_unwrap(cx, expr, arglists[0], false);
                } else if let Some(arglists) = method_chain_args(expr, &["get_mut", "unwrap"]) {
                    lint_get_unwrap(cx, expr, arglists[0], true);
                } else if let Some(arglists) = method_chain_args(expr, &["unwrap"]) {
                    lint_unwrap(cx, expr, arglists[0]);
                } else if let Some(arglists) = method_chain_args(expr, &["ok", "expect"]) {
                    lint_ok_expect(cx, expr, arglists[0]);
                } else if let Some(arglists) = method_chain_args(expr, &["map", "unwrap_or"]) {
                    lint_map_unwrap_or(cx, expr, arglists[0], arglists[1]);
                } else if let Some(arglists) = method_chain_args(expr, &["map", "unwrap_or_else"]) {
                    lint_map_unwrap_or_else(cx, expr, arglists[0], arglists[1]);
                } else if let Some(arglists) = method_chain_args(expr, &["map_or"]) {
                    lint_map_or_none(cx, expr, arglists[0]);
                } else if let Some(arglists) = method_chain_args(expr, &["filter", "next"]) {
                    lint_filter_next(cx, expr, arglists[0]);
                } else if let Some(arglists) = method_chain_args(expr, &["filter", "map"]) {
                    lint_filter_map(cx, expr, arglists[0], arglists[1]);
                } else if let Some(arglists) = method_chain_args(expr, &["filter_map", "map"]) {
                    lint_filter_map_map(cx, expr, arglists[0], arglists[1]);
                } else if let Some(arglists) = method_chain_args(expr, &["filter", "flat_map"]) {
                    lint_filter_flat_map(cx, expr, arglists[0], arglists[1]);
                } else if let Some(arglists) = method_chain_args(expr, &["filter_map", "flat_map"]) {
                    lint_filter_map_flat_map(cx, expr, arglists[0], arglists[1]);
                } else if let Some(arglists) = method_chain_args(expr, &["map", "flatten"]) {
                    lint_map_flatten(cx, expr, arglists[0]);
                } else if let Some(arglists) = method_chain_args(expr, &["find", "is_some"]) {
                    lint_search_is_some(cx, expr, "find", arglists[0], arglists[1]);
                } else if let Some(arglists) = method_chain_args(expr, &["position", "is_some"]) {
                    lint_search_is_some(cx, expr, "position", arglists[0], arglists[1]);
                } else if let Some(arglists) = method_chain_args(expr, &["rposition", "is_some"]) {
                    lint_search_is_some(cx, expr, "rposition", arglists[0], arglists[1]);
                } else if let Some(arglists) = method_chain_args(expr, &["extend"]) {
                    lint_extend(cx, expr, arglists[0]);
                } else if let Some(arglists) = method_chain_args(expr, &["unwrap", "as_ptr"]) {
                    lint_cstring_as_ptr(cx, expr, &arglists[0][0], &arglists[1][0]);
                } else if let Some(arglists) = method_chain_args(expr, &["iter", "nth"]) {
                    lint_iter_nth(cx, expr, arglists[0], false);
                } else if let Some(arglists) = method_chain_args(expr, &["iter_mut", "nth"]) {
                    lint_iter_nth(cx, expr, arglists[0], true);
                } else if method_chain_args(expr, &["skip", "next"]).is_some() {
                    lint_iter_skip_next(cx, expr);
                } else if let Some(arglists) = method_chain_args(expr, &["cloned", "collect"]) {
                    lint_iter_cloned_collect(cx, expr, arglists[0]);
                } else if let Some(arglists) = method_chain_args(expr, &["as_ref"]) {
                    lint_asref(cx, expr, "as_ref", arglists[0]);
                } else if let Some(arglists) = method_chain_args(expr, &["as_mut"]) {
                    lint_asref(cx, expr, "as_mut", arglists[0]);
                } else if let Some(arglists) = method_chain_args(expr, &["fold"]) {
                    lint_unnecessary_fold(cx, expr, arglists[0]);
                } else if let Some(arglists) = method_chain_args(expr, &["filter_map"]) {
                    unnecessary_filter_map::lint(cx, expr, arglists[0]);
                }

                lint_or_fun_call(cx, expr, *method_span, &method_call.ident.as_str(), args);
                lint_expect_fun_call(cx, expr, *method_span, &method_call.ident.as_str(), args);

                let self_ty = cx.tables.expr_ty_adjusted(&args[0]);
                if args.len() == 1 && method_call.ident.name == "clone" {
                    lint_clone_on_copy(cx, expr, &args[0], self_ty);
                    lint_clone_on_ref_ptr(cx, expr, &args[0]);
                }

                match self_ty.sty {
                    ty::Ref(_, ty, _) if ty.sty == ty::Str => for &(method, pos) in &PATTERN_METHODS {
                        if method_call.ident.name == method && args.len() > pos {
                            lint_single_char_pattern(cx, expr, &args[pos]);
                        }
                    },
                    _ => (),
                }
            },
            hir::ExprKind::Binary(op, ref lhs, ref rhs) if op.node == hir::BinOpKind::Eq || op.node == hir::BinOpKind::Ne => {
                let mut info = BinaryExprInfo {
                    expr,
                    chain: lhs,
                    other: rhs,
                    eq: op.node == hir::BinOpKind::Eq,
                };
                lint_binary_expr_with_method_call(cx, &mut info);
            },
            _ => (),
        }
    }

    fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, implitem: &'tcx hir::ImplItem) {
        if in_external_macro(cx.sess(), implitem.span) {
            return;
        }
        let name = implitem.ident.name;
        let parent = cx.tcx.hir.get_parent(implitem.id);
        let item = cx.tcx.hir.expect_item(parent);
        if_chain! {
            if let hir::ImplItemKind::Method(ref sig, id) = implitem.node;
            if let Some(first_arg_ty) = sig.decl.inputs.get(0);
            if let Some(first_arg) = iter_input_pats(&sig.decl, cx.tcx.hir.body(id)).next();
            if let hir::ItemKind::Impl(_, _, _, _, None, ref self_ty, _) = item.node;
            then {
                if cx.access_levels.is_exported(implitem.id) {
                // check missing trait implementations
                    for &(method_name, n_args, self_kind, out_type, trait_name) in &TRAIT_METHODS {
                        if name == method_name &&
                        sig.decl.inputs.len() == n_args &&
                        out_type.matches(cx, &sig.decl.output) &&
                        self_kind.matches(cx, first_arg_ty, first_arg, self_ty, false, &implitem.generics) {
                            span_lint(cx, SHOULD_IMPLEMENT_TRAIT, implitem.span, &format!(
                                "defining a method called `{}` on this type; consider implementing \
                                the `{}` trait or choosing a less ambiguous name", name, trait_name));
                        }
                    }
                }

                // check conventions w.r.t. conversion method names and predicates
                let def_id = cx.tcx.hir.local_def_id(item.id);
                let ty = cx.tcx.type_of(def_id);
                let is_copy = is_copy(cx, ty);
                for &(ref conv, self_kinds) in &CONVENTIONS {
                    if conv.check(&name.as_str()) {
                        if !self_kinds
                                .iter()
                                .any(|k| k.matches(cx, first_arg_ty, first_arg, self_ty, is_copy, &implitem.generics)) {
                            let lint = if item.vis.node.is_pub() {
                                WRONG_PUB_SELF_CONVENTION
                            } else {
                                WRONG_SELF_CONVENTION
                            };
                            span_lint(cx,
                                      lint,
                                      first_arg.pat.span,
                                      &format!("methods called `{}` usually take {}; consider choosing a less \
                                                ambiguous name",
                                               conv,
                                               &self_kinds.iter()
                                                          .map(|k| k.description())
                                                          .collect::<Vec<_>>()
                                                          .join(" or ")));
                        }

                        // Only check the first convention to match (CONVENTIONS should be listed from most to least specific)
                        break;
                    }
                }

                let ret_ty = return_ty(cx, implitem.id);
                if name == "new" &&
                   !ret_ty.walk().any(|t| same_tys(cx, t, ty)) {
                    span_lint(cx,
                              NEW_RET_NO_SELF,
                              implitem.span,
                              "methods called `new` usually return `Self`");
                }
            }
        }
    }
}

/// Checks for the `OR_FUN_CALL` lint.
fn lint_or_fun_call(cx: &LateContext<'_, '_>, expr: &hir::Expr, method_span: Span, name: &str, args: &[hir::Expr]) {
    /// Check for `unwrap_or(T::new())` or `unwrap_or(T::default())`.
    fn check_unwrap_or_default(
        cx: &LateContext<'_, '_>,
        name: &str,
        fun: &hir::Expr,
        self_expr: &hir::Expr,
        arg: &hir::Expr,
        or_has_args: bool,
        span: Span,
    ) -> bool {
        if or_has_args {
            return false;
        }

        if name == "unwrap_or" {
            if let hir::ExprKind::Path(ref qpath) = fun.node {
                let path = &*last_path_segment(qpath).ident.as_str();

                if ["default", "new"].contains(&path) {
                    let arg_ty = cx.tables.expr_ty(arg);
                    let default_trait_id = if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT) {
                        default_trait_id
                    } else {
                        return false;
                    };

                    if implements_trait(cx, arg_ty, default_trait_id, &[]) {
                        span_lint_and_sugg(
                            cx,
                            OR_FUN_CALL,
                            span,
                            &format!("use of `{}` followed by a call to `{}`", name, path),
                            "try this",
                            format!("{}.unwrap_or_default()", snippet(cx, self_expr.span, "_")),
                        );
                        return true;
                    }
                }
            }
        }

        false
    }

    /// Check for `*or(foo())`.
    #[allow(clippy::too_many_arguments)]
    fn check_general_case(
        cx: &LateContext<'_, '_>,
        name: &str,
        method_span: Span,
        fun_span: Span,
        self_expr: &hir::Expr,
        arg: &hir::Expr,
        or_has_args: bool,
        span: Span,
    ) {
        // (path, fn_has_argument, methods, suffix)
        let know_types: &[(&[_], _, &[_], _)] = &[
            (&paths::BTREEMAP_ENTRY, false, &["or_insert"], "with"),
            (&paths::HASHMAP_ENTRY, false, &["or_insert"], "with"),
            (&paths::OPTION, false, &["map_or", "ok_or", "or", "unwrap_or"], "else"),
            (&paths::RESULT, true, &["or", "unwrap_or"], "else"),
        ];

        // early check if the name is one we care about
        if know_types.iter().all(|k| !k.2.contains(&name)) {
            return;
        }

        // don't lint for constant values
        let owner_def = cx.tcx.hir.get_parent_did(arg.id);
        let promotable = cx.tcx.rvalue_promotable_map(owner_def).contains(&arg.hir_id.local_id);
        if promotable {
            return;
        }

        let self_ty = cx.tables.expr_ty(self_expr);

        let (fn_has_arguments, poss, suffix) = if let Some(&(_, fn_has_arguments, poss, suffix)) =
            know_types.iter().find(|&&i| match_type(cx, self_ty, i.0))
        {
            (fn_has_arguments, poss, suffix)
        } else {
            return;
        };

        if !poss.contains(&name) {
            return;
        }

        let sugg: Cow<'_, _> = match (fn_has_arguments, !or_has_args) {
            (true, _) => format!("|_| {}", snippet(cx, arg.span, "..")).into(),
            (false, false) => format!("|| {}", snippet(cx, arg.span, "..")).into(),
            (false, true) => snippet(cx, fun_span, ".."),
        };
        let span_replace_word = method_span.with_hi(span.hi());
        span_lint_and_sugg(
            cx,
            OR_FUN_CALL,
            span_replace_word,
            &format!("use of `{}` followed by a function call", name),
            "try this",
            format!("{}_{}({})", name, suffix, sugg),
        );
    }

    if args.len() == 2 {
        match args[1].node {
            hir::ExprKind::Call(ref fun, ref or_args) => {
                let or_has_args = !or_args.is_empty();
                if !check_unwrap_or_default(cx, name, fun, &args[0], &args[1], or_has_args, expr.span) {
                    check_general_case(cx, name, method_span, fun.span, &args[0], &args[1], or_has_args, expr.span);
                }
            },
            hir::ExprKind::MethodCall(_, span, ref or_args) => {
                check_general_case(cx, name, method_span, span, &args[0], &args[1], !or_args.is_empty(), expr.span)
            },
            _ => {},
        }
    }
}

/// Checks for the `EXPECT_FUN_CALL` lint.
fn lint_expect_fun_call(cx: &LateContext<'_, '_>, expr: &hir::Expr, method_span: Span, name: &str, args: &[hir::Expr]) {
    fn extract_format_args(arg: &hir::Expr) -> Option<&hir::HirVec<hir::Expr>> {
        let arg  = match &arg.node {
            hir::ExprKind::AddrOf(_, expr)=> expr,
            hir::ExprKind::MethodCall(method_name, _, args)
                if method_name.ident.name == "as_str" ||
                   method_name.ident.name == "as_ref"
                => &args[0],
            _ => arg,
        };

        if let hir::ExprKind::Call(ref inner_fun, ref inner_args) = arg.node {
            if is_expn_of(inner_fun.span, "format").is_some() && inner_args.len() == 1 {
                if let hir::ExprKind::Call(_, ref format_args) = inner_args[0].node {
                    return Some(format_args);
                }
            }
        }

        None
    }

    fn generate_format_arg_snippet(cx: &LateContext<'_, '_>, a: &hir::Expr) -> String {
        if let hir::ExprKind::AddrOf(_, ref format_arg) = a.node {
            if let hir::ExprKind::Match(ref format_arg_expr, _, _) = format_arg.node {
                if let hir::ExprKind::Tup(ref format_arg_expr_tup) = format_arg_expr.node {
                    return snippet(cx, format_arg_expr_tup[0].span, "..").into_owned();
                }
            }
        };

        snippet(cx, a.span, "..").into_owned()
    }

    fn check_general_case(
        cx: &LateContext<'_, '_>,
        name: &str,
        method_span: Span,
        self_expr: &hir::Expr,
        arg: &hir::Expr,
        span: Span,
    ) {
        fn is_call(node: &hir::ExprKind) -> bool {
            match node {
                hir::ExprKind::AddrOf(_, expr) => {
                    is_call(&expr.node)
                },
                hir::ExprKind::Call(..)
                | hir::ExprKind::MethodCall(..)
                // These variants are debatable or require further examination
                | hir::ExprKind::If(..)
                | hir::ExprKind::Match(..)
                | hir::ExprKind::Block{ .. } => true,
                _ => false,
            }
        }

        if name != "expect" {
            return;
        }

        let self_type = cx.tables.expr_ty(self_expr);
        let known_types = &[&paths::OPTION, &paths::RESULT];

        // if not a known type, return early
        if known_types.iter().all(|&k| !match_type(cx, self_type, k)) {
            return;
        }

        if !is_call(&arg.node) {
            return;
        }

        let closure = if match_type(cx, self_type, &paths::OPTION) { "||" } else { "|_|" };
        let span_replace_word = method_span.with_hi(span.hi());

        if let Some(format_args) = extract_format_args(arg) {
            let args_len = format_args.len();
            let args: Vec<String> = format_args
                .into_iter()
                .take(args_len - 1)
                .map(|a| generate_format_arg_snippet(cx, a))
                .collect();

            let sugg = args.join(", ");

            span_lint_and_sugg(
                cx,
                EXPECT_FUN_CALL,
                span_replace_word,
                &format!("use of `{}` followed by a function call", name),
                "try this",
                format!("unwrap_or_else({} panic!({}))", closure, sugg),
            );

            return;
        }

        let sugg: Cow<'_, _> = snippet(cx, arg.span, "..");

        span_lint_and_sugg(
            cx,
            EXPECT_FUN_CALL,
            span_replace_word,
            &format!("use of `{}` followed by a function call", name),
            "try this",
            format!("unwrap_or_else({} {{ let msg = {}; panic!(msg) }}))", closure, sugg),
        );
    }

    if args.len() == 2 {
        match args[1].node {
            hir::ExprKind::Lit(_) => {},
            _ => check_general_case(cx, name, method_span, &args[0], &args[1], expr.span),
        }
    }
}

/// Checks for the `CLONE_ON_COPY` lint.
fn lint_clone_on_copy(cx: &LateContext<'_, '_>, expr: &hir::Expr, arg: &hir::Expr, arg_ty: Ty<'_>) {
    let ty = cx.tables.expr_ty(expr);
    if let ty::Ref(_, inner, _) = arg_ty.sty {
        if let ty::Ref(_, innermost, _) = inner.sty {
            span_lint_and_then(
                cx,
                CLONE_DOUBLE_REF,
                expr.span,
                "using `clone` on a double-reference; \
                 this will copy the reference instead of cloning the inner type",
                |db| if let Some(snip) = sugg::Sugg::hir_opt(cx, arg) {
                    let mut ty = innermost;
                    let mut n = 0;
                    while let ty::Ref(_, inner, _) = ty.sty {
                        ty = inner;
                        n += 1;
                    }
                    let refs: String = iter::repeat('&').take(n + 1).collect();
                    let derefs: String = iter::repeat('*').take(n).collect();
                    let explicit = format!("{}{}::clone({})", refs, ty, snip);
                    db.span_suggestion_with_applicability(
                        expr.span,
                        "try dereferencing it",
                        format!("{}({}{}).clone()", refs, derefs, snip.deref()),
                        Applicability::MaybeIncorrect,
                    );
                    db.span_suggestion_with_applicability(
                        expr.span,
                        "or try being explicit about what type to clone",
                        explicit,
                        Applicability::MaybeIncorrect,
                    );
                },
            );
            return; // don't report clone_on_copy
        }
    }

    if is_copy(cx, ty) {
        let snip;
        if let Some(snippet) = sugg::Sugg::hir_opt(cx, arg) {
            if let ty::Ref(..) = cx.tables.expr_ty(arg).sty {
                let parent = cx.tcx.hir.get_parent_node(expr.id);
                match cx.tcx.hir.get(parent) {
                    hir::Node::Expr(parent) => match parent.node {
                        // &*x is a nop, &x.clone() is not
                        hir::ExprKind::AddrOf(..) |
                        // (*x).func() is useless, x.clone().func() can work in case func borrows mutably
                        hir::ExprKind::MethodCall(..) => return,
                        _ => {},
                    }
                    hir::Node::Stmt(stmt) => {
                        if let hir::StmtKind::Decl(ref decl, _) = stmt.node {
                            if let hir::DeclKind::Local(ref loc) = decl.node {
                                if let hir::PatKind::Ref(..) = loc.pat.node {
                                    // let ref y = *x borrows x, let ref y = x.clone() does not
                                    return;
                                }
                            }
                        }
                    },
                    _ => {},
                }
                snip = Some(("try dereferencing it", format!("{}", snippet.deref())));
            } else {
                snip = Some(("try removing the `clone` call", format!("{}", snippet)));
            }
        } else {
            snip = None;
        }
        span_lint_and_then(cx, CLONE_ON_COPY, expr.span, "using `clone` on a `Copy` type", |db| {
            if let Some((text, snip)) = snip {
                db.span_suggestion_with_applicability(
                    expr.span,
                    text,
                    snip,
                    Applicability::Unspecified,
                );
            }
        });
    }
}

fn lint_clone_on_ref_ptr(cx: &LateContext<'_, '_>, expr: &hir::Expr, arg: &hir::Expr) {
    let obj_ty = walk_ptrs_ty(cx.tables.expr_ty(arg));

    if let ty::Adt(_, subst) = obj_ty.sty {
        let caller_type = if match_type(cx, obj_ty, &paths::RC) {
            "Rc"
        } else if match_type(cx, obj_ty, &paths::ARC) {
            "Arc"
        } else if match_type(cx, obj_ty, &paths::WEAK_RC) || match_type(cx, obj_ty, &paths::WEAK_ARC) {
            "Weak"
        } else {
            return;
        };

        span_lint_and_sugg(
            cx,
            CLONE_ON_REF_PTR,
            expr.span,
            "using '.clone()' on a ref-counted pointer",
            "try this",
            format!("{}::<{}>::clone(&{})", caller_type, subst.type_at(0), snippet(cx, arg.span, "_")),
        );
    }
}


fn lint_string_extend(cx: &LateContext<'_, '_>, expr: &hir::Expr, args: &[hir::Expr]) {
    let arg = &args[1];
    if let Some(arglists) = method_chain_args(arg, &["chars"]) {
        let target = &arglists[0][0];
        let self_ty = walk_ptrs_ty(cx.tables.expr_ty(target));
        let ref_str = if self_ty.sty == ty::Str {
            ""
        } else if match_type(cx, self_ty, &paths::STRING) {
            "&"
        } else {
            return;
        };

        span_lint_and_sugg(
            cx,
            STRING_EXTEND_CHARS,
            expr.span,
            "calling `.extend(_.chars())`",
            "try this",
            format!(
                "{}.push_str({}{})",
                snippet(cx, args[0].span, "_"),
                ref_str,
                snippet(cx, target.span, "_")
            ),
        );
    }
}

fn lint_extend(cx: &LateContext<'_, '_>, expr: &hir::Expr, args: &[hir::Expr]) {
    let obj_ty = walk_ptrs_ty(cx.tables.expr_ty(&args[0]));
    if match_type(cx, obj_ty, &paths::STRING) {
        lint_string_extend(cx, expr, args);
    }
}

fn lint_cstring_as_ptr(cx: &LateContext<'_, '_>, expr: &hir::Expr, new: &hir::Expr, unwrap: &hir::Expr) {
    if_chain! {
        if let hir::ExprKind::Call(ref fun, ref args) = new.node;
        if args.len() == 1;
        if let hir::ExprKind::Path(ref path) = fun.node;
        if let Def::Method(did) = cx.tables.qpath_def(path, fun.hir_id);
        if match_def_path(cx.tcx, did, &paths::CSTRING_NEW);
        then {
            span_lint_and_then(
                cx,
                TEMPORARY_CSTRING_AS_PTR,
                expr.span,
                "you are getting the inner pointer of a temporary `CString`",
                |db| {
                    db.note("that pointer will be invalid outside this expression");
                    db.span_help(unwrap.span, "assign the `CString` to a variable to extend its lifetime");
                });
        }
    }
}

fn lint_iter_cloned_collect(cx: &LateContext<'_, '_>, expr: &hir::Expr, iter_args: &[hir::Expr]) {
    if match_type(cx, cx.tables.expr_ty(expr), &paths::VEC)
        && derefs_to_slice(cx, &iter_args[0], cx.tables.expr_ty(&iter_args[0])).is_some()
    {
        span_lint(
            cx,
            ITER_CLONED_COLLECT,
            expr.span,
            "called `cloned().collect()` on a slice to create a `Vec`. Calling `to_vec()` is both faster and \
             more readable",
        );
    }
}

fn lint_unnecessary_fold(cx: &LateContext<'_, '_>, expr: &hir::Expr, fold_args: &[hir::Expr]) {
    fn check_fold_with_op(
        cx: &LateContext<'_, '_>,
        fold_args: &[hir::Expr],
        op: hir::BinOpKind,
        replacement_method_name: &str,
        replacement_has_args: bool) {

        if_chain! {
            // Extract the body of the closure passed to fold
            if let hir::ExprKind::Closure(_, _, body_id, _, _) = fold_args[2].node;
            let closure_body = cx.tcx.hir.body(body_id);
            let closure_expr = remove_blocks(&closure_body.value);

            // Check if the closure body is of the form `acc <op> some_expr(x)`
            if let hir::ExprKind::Binary(ref bin_op, ref left_expr, ref right_expr) = closure_expr.node;
            if bin_op.node == op;

            // Extract the names of the two arguments to the closure
            if let Some(first_arg_ident) = get_arg_name(&closure_body.arguments[0].pat);
            if let Some(second_arg_ident) = get_arg_name(&closure_body.arguments[1].pat);

            if match_var(&*left_expr, first_arg_ident);
            if replacement_has_args || match_var(&*right_expr, second_arg_ident);

            then {
                // Span containing `.fold(...)`
                let next_point = cx.sess().source_map().next_point(fold_args[0].span);
                let fold_span = next_point.with_hi(fold_args[2].span.hi() + BytePos(1));

                let sugg = if replacement_has_args {
                    format!(
                        ".{replacement}(|{s}| {r})",
                        replacement = replacement_method_name,
                        s = second_arg_ident,
                        r = snippet(cx, right_expr.span, "EXPR"),
                    )
                } else {
                    format!(
                        ".{replacement}()",
                        replacement = replacement_method_name,
                    )
                };

                span_lint_and_sugg(
                    cx,
                    UNNECESSARY_FOLD,
                    fold_span,
                    // TODO #2371 don't suggest e.g. .any(|x| f(x)) if we can suggest .any(f)
                    "this `.fold` can be written more succinctly using another method",
                    "try",
                    sugg,
                );
            }
        }
    }

    // Check that this is a call to Iterator::fold rather than just some function called fold
    if !match_trait_method(cx, expr, &paths::ITERATOR) {
        return;
    }

    assert!(fold_args.len() == 3,
        "Expected fold_args to have three entries - the receiver, the initial value and the closure");

    // Check if the first argument to .fold is a suitable literal
    match fold_args[1].node {
        hir::ExprKind::Lit(ref lit) => {
            match lit.node {
                ast::LitKind::Bool(false) => check_fold_with_op(
                    cx, fold_args, hir::BinOpKind::Or, "any", true
                ),
                ast::LitKind::Bool(true) => check_fold_with_op(
                    cx, fold_args, hir::BinOpKind::And, "all", true
                ),
                ast::LitKind::Int(0, _) => check_fold_with_op(
                    cx, fold_args, hir::BinOpKind::Add, "sum", false
                ),
                ast::LitKind::Int(1, _) => check_fold_with_op(
                    cx, fold_args, hir::BinOpKind::Mul, "product", false
                ),
                _ => return
            }
        }
        _ => return
    };
}

fn lint_iter_nth(cx: &LateContext<'_, '_>, expr: &hir::Expr, iter_args: &[hir::Expr], is_mut: bool) {
    let mut_str = if is_mut { "_mut" } else { "" };
    let caller_type = if derefs_to_slice(cx, &iter_args[0], cx.tables.expr_ty(&iter_args[0])).is_some() {
        "slice"
    } else if match_type(cx, cx.tables.expr_ty(&iter_args[0]), &paths::VEC) {
        "Vec"
    } else if match_type(cx, cx.tables.expr_ty(&iter_args[0]), &paths::VEC_DEQUE) {
        "VecDeque"
    } else {
        return; // caller is not a type that we want to lint
    };

    span_lint(
        cx,
        ITER_NTH,
        expr.span,
        &format!(
            "called `.iter{0}().nth()` on a {1}. Calling `.get{0}()` is both faster and more readable",
            mut_str,
            caller_type
        ),
    );
}

fn lint_get_unwrap(cx: &LateContext<'_, '_>, expr: &hir::Expr, get_args: &[hir::Expr], is_mut: bool) {
    // Note: we don't want to lint `get_mut().unwrap` for HashMap or BTreeMap,
    // because they do not implement `IndexMut`
    let expr_ty = cx.tables.expr_ty(&get_args[0]);
    let get_args_str = if get_args.len() > 1 {
        snippet(cx, get_args[1].span, "_")
    } else {
        return; // not linting on a .get().unwrap() chain or variant
    };
    let needs_ref;
    let caller_type = if derefs_to_slice(cx, &get_args[0], expr_ty).is_some() {
        needs_ref = get_args_str.parse::<usize>().is_ok();
        "slice"
    } else if match_type(cx, expr_ty, &paths::VEC) {
        needs_ref = get_args_str.parse::<usize>().is_ok();
        "Vec"
    } else if match_type(cx, expr_ty, &paths::VEC_DEQUE) {
        needs_ref = get_args_str.parse::<usize>().is_ok();
        "VecDeque"
    } else if !is_mut && match_type(cx, expr_ty, &paths::HASHMAP) {
        needs_ref = true;
        "HashMap"
    } else if !is_mut && match_type(cx, expr_ty, &paths::BTREEMAP) {
        needs_ref = true;
        "BTreeMap"
    } else {
        return; // caller is not a type that we want to lint
    };

    let mut_str = if is_mut { "_mut" } else { "" };
    let borrow_str = if !needs_ref { "" } else if is_mut { "&mut " } else { "&" };
    span_lint_and_sugg(
        cx,
        GET_UNWRAP,
        expr.span,
        &format!(
            "called `.get{0}().unwrap()` on a {1}. Using `[]` is more clear and more concise",
            mut_str,
            caller_type
        ),
        "try this",
        format!(
            "{}{}[{}]",
            borrow_str,
            snippet(cx, get_args[0].span, "_"),
            get_args_str
        ),
    );
}

fn lint_iter_skip_next(cx: &LateContext<'_, '_>, expr: &hir::Expr) {
    // lint if caller of skip is an Iterator
    if match_trait_method(cx, expr, &paths::ITERATOR) {
        span_lint(
            cx,
            ITER_SKIP_NEXT,
            expr.span,
            "called `skip(x).next()` on an iterator. This is more succinctly expressed by calling `nth(x)`",
        );
    }
}

fn derefs_to_slice(cx: &LateContext<'_, '_>, expr: &hir::Expr, ty: Ty<'_>) -> Option<sugg::Sugg<'static>> {
    fn may_slice(cx: &LateContext<'_, '_>, ty: Ty<'_>) -> bool {
        match ty.sty {
            ty::Slice(_) => true,
            ty::Adt(def, _) if def.is_box() => may_slice(cx, ty.boxed_ty()),
            ty::Adt(..) => match_type(cx, ty, &paths::VEC),
            ty::Array(_, size) => size.assert_usize(cx.tcx).expect("array length") < 32,
            ty::Ref(_, inner, _) => may_slice(cx, inner),
            _ => false,
        }
    }

    if let hir::ExprKind::MethodCall(ref path, _, ref args) = expr.node {
        if path.ident.name == "iter" && may_slice(cx, cx.tables.expr_ty(&args[0])) {
            sugg::Sugg::hir_opt(cx, &args[0]).map(|sugg| sugg.addr())
        } else {
            None
        }
    } else {
        match ty.sty {
            ty::Slice(_) => sugg::Sugg::hir_opt(cx, expr),
            ty::Adt(def, _) if def.is_box() && may_slice(cx, ty.boxed_ty()) => sugg::Sugg::hir_opt(cx, expr),
            ty::Ref(_, inner, _) => if may_slice(cx, inner) {
                sugg::Sugg::hir_opt(cx, expr)
            } else {
                None
            },
            _ => None,
        }
    }
}

/// lint use of `unwrap()` for `Option`s and `Result`s
fn lint_unwrap(cx: &LateContext<'_, '_>, expr: &hir::Expr, unwrap_args: &[hir::Expr]) {
    let obj_ty = walk_ptrs_ty(cx.tables.expr_ty(&unwrap_args[0]));

    let mess = if match_type(cx, obj_ty, &paths::OPTION) {
        Some((OPTION_UNWRAP_USED, "an Option", "None"))
    } else if match_type(cx, obj_ty, &paths::RESULT) {
        Some((RESULT_UNWRAP_USED, "a Result", "Err"))
    } else {
        None
    };

    if let Some((lint, kind, none_value)) = mess {
        span_lint(
            cx,
            lint,
            expr.span,
            &format!(
                "used unwrap() on {} value. If you don't want to handle the {} case gracefully, consider \
                 using expect() to provide a better panic \
                 message",
                kind,
                none_value
            ),
        );
    }
}

/// lint use of `ok().expect()` for `Result`s
fn lint_ok_expect(cx: &LateContext<'_, '_>, expr: &hir::Expr, ok_args: &[hir::Expr]) {
    // lint if the caller of `ok()` is a `Result`
    if match_type(cx, cx.tables.expr_ty(&ok_args[0]), &paths::RESULT) {
        let result_type = cx.tables.expr_ty(&ok_args[0]);
        if let Some(error_type) = get_error_type(cx, result_type) {
            if has_debug_impl(error_type, cx) {
                span_lint(
                    cx,
                    OK_EXPECT,
                    expr.span,
                    "called `ok().expect()` on a Result value. You can call `expect` directly on the `Result`",
                );
            }
        }
    }
}

/// lint use of `map().unwrap_or()` for `Option`s
fn lint_map_unwrap_or(cx: &LateContext<'_, '_>, expr: &hir::Expr, map_args: &[hir::Expr], unwrap_args: &[hir::Expr]) {
    // lint if the caller of `map()` is an `Option`
    if match_type(cx, cx.tables.expr_ty(&map_args[0]), &paths::OPTION) {
        // get snippets for args to map() and unwrap_or()
        let map_snippet = snippet(cx, map_args[1].span, "..");
        let unwrap_snippet = snippet(cx, unwrap_args[1].span, "..");
        // lint message
        // comparing the snippet from source to raw text ("None") below is safe
        // because we already have checked the type.
        let arg = if unwrap_snippet == "None" {
            "None"
        } else {
            "a"
        };
        let suggest = if unwrap_snippet == "None" {
            "and_then(f)"
        } else {
            "map_or(a, f)"
        };
        let msg = &format!(
            "called `map(f).unwrap_or({})` on an Option value. \
             This can be done more directly by calling `{}` instead",
            arg,
            suggest
        );
        // lint, with note if neither arg is > 1 line and both map() and
        // unwrap_or() have the same span
        let multiline = map_snippet.lines().count() > 1 || unwrap_snippet.lines().count() > 1;
        let same_span = map_args[1].span.ctxt() == unwrap_args[1].span.ctxt();
        if same_span && !multiline {
            let suggest = if unwrap_snippet == "None" {
                format!("and_then({})", map_snippet)
            } else {
                format!("map_or({}, {})", unwrap_snippet, map_snippet)
            };
            let note = format!(
                "replace `map({}).unwrap_or({})` with `{}`",
                map_snippet,
                unwrap_snippet,
                suggest
            );
            span_note_and_lint(cx, OPTION_MAP_UNWRAP_OR, expr.span, msg, expr.span, &note);
        } else if same_span && multiline {
            span_lint(cx, OPTION_MAP_UNWRAP_OR, expr.span, msg);
        };
    }
}

/// lint use of `map().flatten()` for `Iterators`
fn lint_map_flatten<'a, 'tcx>(
    cx: &LateContext<'a, 'tcx>,
    expr: &'tcx hir::Expr,
    map_args: &'tcx [hir::Expr],
) {
    // lint if caller of `.map().flatten()` is an Iterator
    if match_trait_method(cx, expr, &paths::ITERATOR) {
        let msg = "called `map(..).flatten()` on an `Iterator`. \
                   This is more succinctly expressed by calling `.flat_map(..)`";
        let self_snippet = snippet(cx, map_args[0].span, "..");
        let func_snippet = snippet(cx, map_args[1].span, "..");
        let hint = format!("{0}.flat_map({1})", self_snippet, func_snippet);
        span_lint_and_then(cx, MAP_FLATTEN, expr.span, msg, |db| {
            db.span_suggestion_with_applicability(
                expr.span,
                "try using flat_map instead",
                hint,
                Applicability::MachineApplicable,
            );
        });
    }
}

/// lint use of `map().unwrap_or_else()` for `Option`s and `Result`s
fn lint_map_unwrap_or_else<'a, 'tcx>(
    cx: &LateContext<'a, 'tcx>,
    expr: &'tcx hir::Expr,
    map_args: &'tcx [hir::Expr],
    unwrap_args: &'tcx [hir::Expr],
) {
    // lint if the caller of `map()` is an `Option`
    let is_option = match_type(cx, cx.tables.expr_ty(&map_args[0]), &paths::OPTION);
    let is_result = match_type(cx, cx.tables.expr_ty(&map_args[0]), &paths::RESULT);
    if is_option || is_result {
        // lint message
        let msg = if is_option {
            "called `map(f).unwrap_or_else(g)` on an Option value. This can be done more directly by calling \
             `map_or_else(g, f)` instead"
        } else {
            "called `map(f).unwrap_or_else(g)` on a Result value. This can be done more directly by calling \
             `ok().map_or_else(g, f)` instead"
        };
        // get snippets for args to map() and unwrap_or_else()
        let map_snippet = snippet(cx, map_args[1].span, "..");
        let unwrap_snippet = snippet(cx, unwrap_args[1].span, "..");
        // lint, with note if neither arg is > 1 line and both map() and
        // unwrap_or_else() have the same span
        let multiline = map_snippet.lines().count() > 1 || unwrap_snippet.lines().count() > 1;
        let same_span = map_args[1].span.ctxt() == unwrap_args[1].span.ctxt();
        if same_span && !multiline {
            span_note_and_lint(
                cx,
                if is_option {
                    OPTION_MAP_UNWRAP_OR_ELSE
                } else {
                    RESULT_MAP_UNWRAP_OR_ELSE
                },
                expr.span,
                msg,
                expr.span,
                &format!(
                    "replace `map({0}).unwrap_or_else({1})` with `{2}map_or_else({1}, {0})`",
                    map_snippet,
                    unwrap_snippet,
                    if is_result { "ok()." } else { "" }
                ),
            );
        } else if same_span && multiline {
            span_lint(
                cx,
                if is_option {
                    OPTION_MAP_UNWRAP_OR_ELSE
                } else {
                    RESULT_MAP_UNWRAP_OR_ELSE
                },
                expr.span,
                msg,
            );
        };
    }
}

/// lint use of `_.map_or(None, _)` for `Option`s
fn lint_map_or_none<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr, map_or_args: &'tcx [hir::Expr]) {
    if match_type(cx, cx.tables.expr_ty(&map_or_args[0]), &paths::OPTION) {
        // check if the first non-self argument to map_or() is None
        let map_or_arg_is_none = if let hir::ExprKind::Path(ref qpath) = map_or_args[1].node {
            match_qpath(qpath, &paths::OPTION_NONE)
        } else {
            false
        };

        if map_or_arg_is_none {
            // lint message
            let msg = "called `map_or(None, f)` on an Option value. This can be done more directly by calling \
                       `and_then(f)` instead";
            let map_or_self_snippet = snippet(cx, map_or_args[0].span, "..");
            let map_or_func_snippet = snippet(cx, map_or_args[2].span, "..");
            let hint = format!("{0}.and_then({1})", map_or_self_snippet, map_or_func_snippet);
            span_lint_and_then(cx, OPTION_MAP_OR_NONE, expr.span, msg, |db| {
                db.span_suggestion_with_applicability(
                    expr.span,
                    "try using and_then instead",
                    hint,
                    Applicability::MachineApplicable, // snippet
                );
            });
        }
    }
}

/// lint use of `filter().next()` for `Iterators`
fn lint_filter_next<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr, filter_args: &'tcx [hir::Expr]) {
    // lint if caller of `.filter().next()` is an Iterator
    if match_trait_method(cx, expr, &paths::ITERATOR) {
        let msg = "called `filter(p).next()` on an `Iterator`. This is more succinctly expressed by calling \
                   `.find(p)` instead.";
        let filter_snippet = snippet(cx, filter_args[1].span, "..");
        if filter_snippet.lines().count() <= 1 {
            // add note if not multi-line
            span_note_and_lint(
                cx,
                FILTER_NEXT,
                expr.span,
                msg,
                expr.span,
                &format!("replace `filter({0}).next()` with `find({0})`", filter_snippet),
            );
        } else {
            span_lint(cx, FILTER_NEXT, expr.span, msg);
        }
    }
}

/// lint use of `filter().map()` for `Iterators`
fn lint_filter_map<'a, 'tcx>(
    cx: &LateContext<'a, 'tcx>,
    expr: &'tcx hir::Expr,
    _filter_args: &'tcx [hir::Expr],
    _map_args: &'tcx [hir::Expr],
) {
    // lint if caller of `.filter().map()` is an Iterator
    if match_trait_method(cx, expr, &paths::ITERATOR) {
        let msg = "called `filter(p).map(q)` on an `Iterator`. \
                   This is more succinctly expressed by calling `.filter_map(..)` instead.";
        span_lint(cx, FILTER_MAP, expr.span, msg);
    }
}

/// lint use of `filter().map()` for `Iterators`
fn lint_filter_map_map<'a, 'tcx>(
    cx: &LateContext<'a, 'tcx>,
    expr: &'tcx hir::Expr,
    _filter_args: &'tcx [hir::Expr],
    _map_args: &'tcx [hir::Expr],
) {
    // lint if caller of `.filter().map()` is an Iterator
    if match_trait_method(cx, expr, &paths::ITERATOR) {
        let msg = "called `filter_map(p).map(q)` on an `Iterator`. \
                   This is more succinctly expressed by only calling `.filter_map(..)` instead.";
        span_lint(cx, FILTER_MAP, expr.span, msg);
    }
}

/// lint use of `filter().flat_map()` for `Iterators`
fn lint_filter_flat_map<'a, 'tcx>(
    cx: &LateContext<'a, 'tcx>,
    expr: &'tcx hir::Expr,
    _filter_args: &'tcx [hir::Expr],
    _map_args: &'tcx [hir::Expr],
) {
    // lint if caller of `.filter().flat_map()` is an Iterator
    if match_trait_method(cx, expr, &paths::ITERATOR) {
        let msg = "called `filter(p).flat_map(q)` on an `Iterator`. \
                   This is more succinctly expressed by calling `.flat_map(..)` \
                   and filtering by returning an empty Iterator.";
        span_lint(cx, FILTER_MAP, expr.span, msg);
    }
}

/// lint use of `filter_map().flat_map()` for `Iterators`
fn lint_filter_map_flat_map<'a, 'tcx>(
    cx: &LateContext<'a, 'tcx>,
    expr: &'tcx hir::Expr,
    _filter_args: &'tcx [hir::Expr],
    _map_args: &'tcx [hir::Expr],
) {
    // lint if caller of `.filter_map().flat_map()` is an Iterator
    if match_trait_method(cx, expr, &paths::ITERATOR) {
        let msg = "called `filter_map(p).flat_map(q)` on an `Iterator`. \
                   This is more succinctly expressed by calling `.flat_map(..)` \
                   and filtering by returning an empty Iterator.";
        span_lint(cx, FILTER_MAP, expr.span, msg);
    }
}

/// lint searching an Iterator followed by `is_some()`
fn lint_search_is_some<'a, 'tcx>(
    cx: &LateContext<'a, 'tcx>,
    expr: &'tcx hir::Expr,
    search_method: &str,
    search_args: &'tcx [hir::Expr],
    is_some_args: &'tcx [hir::Expr],
) {
    // lint if caller of search is an Iterator
    if match_trait_method(cx, &is_some_args[0], &paths::ITERATOR) {
        let msg = format!(
            "called `is_some()` after searching an `Iterator` with {}. This is more succinctly \
             expressed by calling `any()`.",
            search_method
        );
        let search_snippet = snippet(cx, search_args[1].span, "..");
        if search_snippet.lines().count() <= 1 {
            // add note if not multi-line
            span_note_and_lint(
                cx,
                SEARCH_IS_SOME,
                expr.span,
                &msg,
                expr.span,
                &format!("replace `{0}({1}).is_some()` with `any({1})`", search_method, search_snippet),
            );
        } else {
            span_lint(cx, SEARCH_IS_SOME, expr.span, &msg);
        }
    }
}

/// Used for `lint_binary_expr_with_method_call`.
#[derive(Copy, Clone)]
struct BinaryExprInfo<'a> {
    expr: &'a hir::Expr,
    chain: &'a hir::Expr,
    other: &'a hir::Expr,
    eq: bool,
}

/// Checks for the `CHARS_NEXT_CMP` and `CHARS_LAST_CMP` lints.
fn lint_binary_expr_with_method_call(cx: &LateContext<'_, '_>, info: &mut BinaryExprInfo<'_>) {
    macro_rules! lint_with_both_lhs_and_rhs {
        ($func:ident, $cx:expr, $info:ident) => {
            if !$func($cx, $info) {
                ::std::mem::swap(&mut $info.chain, &mut $info.other);
                if $func($cx, $info) {
                    return;
                }
            }
        }
    }

    lint_with_both_lhs_and_rhs!(lint_chars_next_cmp, cx, info);
    lint_with_both_lhs_and_rhs!(lint_chars_last_cmp, cx, info);
    lint_with_both_lhs_and_rhs!(lint_chars_next_cmp_with_unwrap, cx, info);
    lint_with_both_lhs_and_rhs!(lint_chars_last_cmp_with_unwrap, cx, info);
}

/// Wrapper fn for `CHARS_NEXT_CMP` and `CHARS_NEXT_CMP` lints.
fn lint_chars_cmp(
    cx: &LateContext<'_, '_>,
    info: &BinaryExprInfo<'_>,
    chain_methods: &[&str],
    lint: &'static Lint,
    suggest: &str,
) -> bool {
    if_chain! {
        if let Some(args) = method_chain_args(info.chain, chain_methods);
        if let hir::ExprKind::Call(ref fun, ref arg_char) = info.other.node;
        if arg_char.len() == 1;
        if let hir::ExprKind::Path(ref qpath) = fun.node;
        if let Some(segment) = single_segment_path(qpath);
        if segment.ident.name == "Some";
        then {
            let self_ty = walk_ptrs_ty(cx.tables.expr_ty_adjusted(&args[0][0]));

            if self_ty.sty != ty::Str {
                return false;
            }

            span_lint_and_sugg(cx,
                               lint,
                               info.expr.span,
                               &format!("you should use the `{}` method", suggest),
                               "like this",
                               format!("{}{}.{}({})",
                                       if info.eq { "" } else { "!" },
                                       snippet(cx, args[0][0].span, "_"),
                                       suggest,
                                       snippet(cx, arg_char[0].span, "_")));

            return true;
        }
    }

    false
}

/// Checks for the `CHARS_NEXT_CMP` lint.
fn lint_chars_next_cmp<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, info: &BinaryExprInfo<'_>) -> bool {
    lint_chars_cmp(cx, info, &["chars", "next"], CHARS_NEXT_CMP, "starts_with")
}

/// Checks for the `CHARS_LAST_CMP` lint.
fn lint_chars_last_cmp<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, info: &BinaryExprInfo<'_>) -> bool {
    if lint_chars_cmp(cx, info, &["chars", "last"], CHARS_NEXT_CMP, "ends_with") {
        true
    } else {
        lint_chars_cmp(cx, info, &["chars", "next_back"], CHARS_NEXT_CMP, "ends_with")
    }
}

/// Wrapper fn for `CHARS_NEXT_CMP` and `CHARS_LAST_CMP` lints with `unwrap()`.
fn lint_chars_cmp_with_unwrap<'a, 'tcx>(
    cx: &LateContext<'a, 'tcx>,
    info: &BinaryExprInfo<'_>,
    chain_methods: &[&str],
    lint: &'static Lint,
    suggest: &str,
) -> bool {
    if_chain! {
        if let Some(args) = method_chain_args(info.chain, chain_methods);
        if let hir::ExprKind::Lit(ref lit) = info.other.node;
        if let ast::LitKind::Char(c) = lit.node;
        then {
            span_lint_and_sugg(
                cx,
                lint,
                info.expr.span,
                &format!("you should use the `{}` method", suggest),
                "like this",
                format!("{}{}.{}('{}')",
                        if info.eq { "" } else { "!" },
                        snippet(cx, args[0][0].span, "_"),
                        suggest,
                        c)
            );

            return true;
        }
    }

    false
}

/// Checks for the `CHARS_NEXT_CMP` lint with `unwrap()`.
fn lint_chars_next_cmp_with_unwrap<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, info: &BinaryExprInfo<'_>) -> bool {
    lint_chars_cmp_with_unwrap(cx, info, &["chars", "next", "unwrap"], CHARS_NEXT_CMP, "starts_with")
}

/// Checks for the `CHARS_LAST_CMP` lint with `unwrap()`.
fn lint_chars_last_cmp_with_unwrap<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, info: &BinaryExprInfo<'_>) -> bool {
    if lint_chars_cmp_with_unwrap(cx, info, &["chars", "last", "unwrap"], CHARS_LAST_CMP, "ends_with") {
        true
    } else {
        lint_chars_cmp_with_unwrap(cx, info, &["chars", "next_back", "unwrap"], CHARS_LAST_CMP, "ends_with")
    }
}

/// lint for length-1 `str`s for methods in `PATTERN_METHODS`
fn lint_single_char_pattern<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, _expr: &'tcx hir::Expr, arg: &'tcx hir::Expr) {
    if_chain! {
        if let hir::ExprKind::Lit(lit) = &arg.node;
        if let ast::LitKind::Str(r, _) = lit.node;
        if r.as_str().len() == 1;
        then {
            let snip = snippet(cx, arg.span, "..");
            let hint = format!("'{}'", &snip[1..snip.len() - 1]);
            span_lint_and_sugg(
                cx,
                SINGLE_CHAR_PATTERN,
                arg.span,
                "single-character string constant used as pattern",
                "try using a char instead",
                hint,
            );
        }
    }
}

/// Checks for the `USELESS_ASREF` lint.
fn lint_asref(cx: &LateContext<'_, '_>, expr: &hir::Expr, call_name: &str, as_ref_args: &[hir::Expr]) {
    // when we get here, we've already checked that the call name is "as_ref" or "as_mut"
    // check if the call is to the actual `AsRef` or `AsMut` trait
    if match_trait_method(cx, expr, &paths::ASREF_TRAIT) || match_trait_method(cx, expr, &paths::ASMUT_TRAIT) {
        // check if the type after `as_ref` or `as_mut` is the same as before
        let recvr = &as_ref_args[0];
        let rcv_ty = cx.tables.expr_ty(recvr);
        let res_ty = cx.tables.expr_ty(expr);
        let (base_res_ty, res_depth) = walk_ptrs_ty_depth(res_ty);
        let (base_rcv_ty, rcv_depth) = walk_ptrs_ty_depth(rcv_ty);
        if base_rcv_ty == base_res_ty && rcv_depth >= res_depth {
            span_lint_and_sugg(
                cx,
                USELESS_ASREF,
                expr.span,
                &format!("this call to `{}` does nothing", call_name),
                "try this",
                snippet(cx, recvr.span, "_").into_owned(),
            );
        }
    }
}

/// Given a `Result<T, E>` type, return its error type (`E`).
fn get_error_type<'a>(cx: &LateContext<'_, '_>, ty: Ty<'a>) -> Option<Ty<'a>> {
    if let ty::Adt(_, substs) = ty.sty {
        if match_type(cx, ty, &paths::RESULT) {
            substs.types().nth(1)
        } else {
            None
        }
    } else {
        None
    }
}

/// This checks whether a given type is known to implement Debug.
fn has_debug_impl<'a, 'b>(ty: Ty<'a>, cx: &LateContext<'b, 'a>) -> bool {
    match cx.tcx.lang_items().debug_trait() {
        Some(debug) => implements_trait(cx, ty, debug, &[]),
        None => false,
    }
}

enum Convention {
    Eq(&'static str),
    StartsWith(&'static str),
}

#[rustfmt::skip]
const CONVENTIONS: [(Convention, &[SelfKind]); 7] = [
    (Convention::Eq("new"), &[SelfKind::No]),
    (Convention::StartsWith("as_"), &[SelfKind::Ref, SelfKind::RefMut]),
    (Convention::StartsWith("from_"), &[SelfKind::No]),
    (Convention::StartsWith("into_"), &[SelfKind::Value]),
    (Convention::StartsWith("is_"), &[SelfKind::Ref, SelfKind::No]),
    (Convention::Eq("to_mut"), &[SelfKind::RefMut]),
    (Convention::StartsWith("to_"), &[SelfKind::Ref]),
];

#[rustfmt::skip]
const TRAIT_METHODS: [(&str, usize, SelfKind, OutType, &str); 30] = [
    ("add", 2, SelfKind::Value, OutType::Any, "std::ops::Add"),
    ("as_mut", 1, SelfKind::RefMut, OutType::Ref, "std::convert::AsMut"),
    ("as_ref", 1, SelfKind::Ref, OutType::Ref, "std::convert::AsRef"),
    ("bitand", 2, SelfKind::Value, OutType::Any, "std::ops::BitAnd"),
    ("bitor", 2, SelfKind::Value, OutType::Any, "std::ops::BitOr"),
    ("bitxor", 2, SelfKind::Value, OutType::Any, "std::ops::BitXor"),
    ("borrow", 1, SelfKind::Ref, OutType::Ref, "std::borrow::Borrow"),
    ("borrow_mut", 1, SelfKind::RefMut, OutType::Ref, "std::borrow::BorrowMut"),
    ("clone", 1, SelfKind::Ref, OutType::Any, "std::clone::Clone"),
    ("cmp", 2, SelfKind::Ref, OutType::Any, "std::cmp::Ord"),
    ("default", 0, SelfKind::No, OutType::Any, "std::default::Default"),
    ("deref", 1, SelfKind::Ref, OutType::Ref, "std::ops::Deref"),
    ("deref_mut", 1, SelfKind::RefMut, OutType::Ref, "std::ops::DerefMut"),
    ("div", 2, SelfKind::Value, OutType::Any, "std::ops::Div"),
    ("drop", 1, SelfKind::RefMut, OutType::Unit, "std::ops::Drop"),
    ("eq", 2, SelfKind::Ref, OutType::Bool, "std::cmp::PartialEq"),
    ("from_iter", 1, SelfKind::No, OutType::Any, "std::iter::FromIterator"),
    ("from_str", 1, SelfKind::No, OutType::Any, "std::str::FromStr"),
    ("hash", 2, SelfKind::Ref, OutType::Unit, "std::hash::Hash"),
    ("index", 2, SelfKind::Ref, OutType::Ref, "std::ops::Index"),
    ("index_mut", 2, SelfKind::RefMut, OutType::Ref, "std::ops::IndexMut"),
    ("into_iter", 1, SelfKind::Value, OutType::Any, "std::iter::IntoIterator"),
    ("mul", 2, SelfKind::Value, OutType::Any, "std::ops::Mul"),
    ("neg", 1, SelfKind::Value, OutType::Any, "std::ops::Neg"),
    ("next", 1, SelfKind::RefMut, OutType::Any, "std::iter::Iterator"),
    ("not", 1, SelfKind::Value, OutType::Any, "std::ops::Not"),
    ("rem", 2, SelfKind::Value, OutType::Any, "std::ops::Rem"),
    ("shl", 2, SelfKind::Value, OutType::Any, "std::ops::Shl"),
    ("shr", 2, SelfKind::Value, OutType::Any, "std::ops::Shr"),
    ("sub", 2, SelfKind::Value, OutType::Any, "std::ops::Sub"),
];

#[rustfmt::skip]
const PATTERN_METHODS: [(&str, usize); 17] = [
    ("contains", 1),
    ("starts_with", 1),
    ("ends_with", 1),
    ("find", 1),
    ("rfind", 1),
    ("split", 1),
    ("rsplit", 1),
    ("split_terminator", 1),
    ("rsplit_terminator", 1),
    ("splitn", 2),
    ("rsplitn", 2),
    ("matches", 1),
    ("rmatches", 1),
    ("match_indices", 1),
    ("rmatch_indices", 1),
    ("trim_left_matches", 1),
    ("trim_right_matches", 1),
];


#[derive(Clone, Copy, PartialEq, Debug)]
enum SelfKind {
    Value,
    Ref,
    RefMut,
    No,
}

impl SelfKind {
    fn matches(
        self,
        cx: &LateContext<'_, '_>,
        ty: &hir::Ty,
        arg: &hir::Arg,
        self_ty: &hir::Ty,
        allow_value_for_ref: bool,
        generics: &hir::Generics,
    ) -> bool {
        // Self types in the HIR are desugared to explicit self types. So it will
        // always be `self:
        // SomeType`,
        // where SomeType can be `Self` or an explicit impl self type (e.g. `Foo` if
        // the impl is on `Foo`)
        // Thus, we only need to test equality against the impl self type or if it is
        // an explicit
        // `Self`. Furthermore, the only possible types for `self: ` are `&Self`,
        // `Self`, `&mut Self`,
        // and `Box<Self>`, including the equivalent types with `Foo`.

        let is_actually_self = |ty| is_self_ty(ty) || SpanlessEq::new(cx).eq_ty(ty, self_ty);
        if is_self(arg) {
            match self {
                SelfKind::Value => is_actually_self(ty),
                SelfKind::Ref | SelfKind::RefMut => {
                    if allow_value_for_ref && is_actually_self(ty) {
                        return true;
                    }
                    match ty.node {
                        hir::TyKind::Rptr(_, ref mt_ty) => {
                            let mutability_match = if self == SelfKind::Ref {
                                mt_ty.mutbl == hir::MutImmutable
                            } else {
                                mt_ty.mutbl == hir::MutMutable
                            };
                            is_actually_self(&mt_ty.ty) && mutability_match
                        },
                        _ => false,
                    }
                },
                _ => false,
            }
        } else {
            match self {
                SelfKind::Value => false,
                SelfKind::Ref => is_as_ref_or_mut_trait(ty, self_ty, generics, &paths::ASREF_TRAIT),
                SelfKind::RefMut => is_as_ref_or_mut_trait(ty, self_ty, generics, &paths::ASMUT_TRAIT),
                SelfKind::No => true,
            }
        }
    }

    fn description(self) -> &'static str {
        match self {
            SelfKind::Value => "self by value",
            SelfKind::Ref => "self by reference",
            SelfKind::RefMut => "self by mutable reference",
            SelfKind::No => "no self",
        }
    }
}

fn is_as_ref_or_mut_trait(ty: &hir::Ty, self_ty: &hir::Ty, generics: &hir::Generics, name: &[&str]) -> bool {
    single_segment_ty(ty).map_or(false, |seg| {
        generics.params.iter().any(|param| match param.kind {
            hir::GenericParamKind::Type { .. } => {
                param.name.ident().name == seg.ident.name && param.bounds.iter().any(|bound| {
                    if let hir::GenericBound::Trait(ref ptr, ..) = *bound {
                        let path = &ptr.trait_ref.path;
                        match_path(path, name) && path.segments.last().map_or(false, |s| {
                            if let Some(ref params) = s.args {
                                if params.parenthesized {
                                    false
                                } else {
                                    // FIXME(flip1995): messy, improve if there is a better option
                                    // in the compiler
                                    let types: Vec<_> = params.args.iter().filter_map(|arg| match arg {
                                        hir::GenericArg::Type(ty) => Some(ty),
                                        _ => None,
                                    }).collect();
                                    types.len() == 1
                                        && (is_self_ty(&types[0]) || is_ty(&*types[0], self_ty))
                                }
                            } else {
                                false
                            }
                        })
                    } else {
                        false
                    }
                })
            },
            _ => false,
        })
    })
}

fn is_ty(ty: &hir::Ty, self_ty: &hir::Ty) -> bool {
    match (&ty.node, &self_ty.node) {
        (
            &hir::TyKind::Path(hir::QPath::Resolved(_, ref ty_path)),
            &hir::TyKind::Path(hir::QPath::Resolved(_, ref self_ty_path)),
        ) => ty_path
            .segments
            .iter()
            .map(|seg| seg.ident.name)
            .eq(self_ty_path.segments.iter().map(|seg| seg.ident.name)),
        _ => false,
    }
}

fn single_segment_ty(ty: &hir::Ty) -> Option<&hir::PathSegment> {
    if let hir::TyKind::Path(ref path) = ty.node {
        single_segment_path(path)
    } else {
        None
    }
}

impl Convention {
    fn check(&self, other: &str) -> bool {
        match *self {
            Convention::Eq(this) => this == other,
            Convention::StartsWith(this) => other.starts_with(this) && this != other,
        }
    }
}

impl fmt::Display for Convention {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
        match *self {
            Convention::Eq(this) => this.fmt(f),
            Convention::StartsWith(this) => this.fmt(f).and_then(|_| '*'.fmt(f)),
        }
    }
}

#[derive(Clone, Copy)]
enum OutType {
    Unit,
    Bool,
    Any,
    Ref,
}

impl OutType {
    fn matches(self, cx: &LateContext<'_, '_>, ty: &hir::FunctionRetTy) -> bool {
        let is_unit = |ty: &hir::Ty| SpanlessEq::new(cx).eq_ty_kind(&ty.node, &hir::TyKind::Tup(vec![].into()));
        match (self, ty) {
            (OutType::Unit, &hir::DefaultReturn(_)) => true,
            (OutType::Unit, &hir::Return(ref ty)) if is_unit(ty) => true,
            (OutType::Bool, &hir::Return(ref ty)) if is_bool(ty) => true,
            (OutType::Any, &hir::Return(ref ty)) if !is_unit(ty) => true,
            (OutType::Ref, &hir::Return(ref ty)) => matches!(ty.node, hir::TyKind::Rptr(_, _)),
            _ => false,
        }
    }
}

fn is_bool(ty: &hir::Ty) -> bool {
    if let hir::TyKind::Path(ref p) = ty.node {
        match_qpath(p, &["bool"])
    } else {
        false
    }
}