rust-clippy/clippy_lints/src/eta_reduction.rs

use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
use clippy_utils::higher::VecArgs;
use clippy_utils::source::snippet_opt;
use clippy_utils::ty::{implements_trait, type_is_unsafe_function};
use clippy_utils::usage::UsedAfterExprVisitor;
use clippy_utils::{get_enclosing_loop_or_closure, higher};
use clippy_utils::{is_adjusted, iter_input_pats};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::{def_id, Expr, ExprKind, Param, PatKind, QPath};
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::lint::in_external_macro;
use rustc_middle::ty::{self, ClosureKind, Ty};
use rustc_session::{declare_lint_pass, declare_tool_lint};

declare_clippy_lint! {
    /// **What it does:** Checks for closures which just call another function where
    /// the function can be called directly. `unsafe` functions or calls where types
    /// get adjusted are ignored.
    ///
    /// **Why is this bad?** Needlessly creating a closure adds code for no benefit
    /// and gives the optimizer more work.
    ///
    /// **Known problems:** If creating the closure inside the closure has a side-
    /// effect then moving the closure creation out will change when that side-
    /// effect runs.
    /// See [#1439](https://github.com/rust-lang/rust-clippy/issues/1439) for more details.
    ///
    /// **Example:**
    /// ```rust,ignore
    /// // Bad
    /// xs.map(|x| foo(x))
    ///
    /// // Good
    /// xs.map(foo)
    /// ```
    /// where `foo(_)` is a plain function that takes the exact argument type of
    /// `x`.
    pub REDUNDANT_CLOSURE,
    style,
    "redundant closures, i.e., `|a| foo(a)` (which can be written as just `foo`)"
}

declare_clippy_lint! {
    /// **What it does:** Checks for closures which only invoke a method on the closure
    /// argument and can be replaced by referencing the method directly.
    ///
    /// **Why is this bad?** It's unnecessary to create the closure.
    ///
    /// **Known problems:** [#3071](https://github.com/rust-lang/rust-clippy/issues/3071),
    /// [#3942](https://github.com/rust-lang/rust-clippy/issues/3942),
    /// [#4002](https://github.com/rust-lang/rust-clippy/issues/4002)
    ///
    ///
    /// **Example:**
    /// ```rust,ignore
    /// Some('a').map(|s| s.to_uppercase());
    /// ```
    /// may be rewritten as
    /// ```rust,ignore
    /// Some('a').map(char::to_uppercase);
    /// ```
    pub REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
    pedantic,
    "redundant closures for method calls"
}

declare_lint_pass!(EtaReduction => [REDUNDANT_CLOSURE, REDUNDANT_CLOSURE_FOR_METHOD_CALLS]);

impl<'tcx> LateLintPass<'tcx> for EtaReduction {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        if in_external_macro(cx.sess(), expr.span) {
            return;
        }

        match expr.kind {
            ExprKind::Call(_, args) | ExprKind::MethodCall(_, _, args, _) => {
                for arg in args {
                    // skip `foo(macro!())`
                    if arg.span.ctxt() == expr.span.ctxt() {
                        check_closure(cx, arg);
                    }
                }
            },
            _ => (),
        }
    }
}

fn check_closure<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
    if let ExprKind::Closure(_, decl, eid, _, _) = expr.kind {
        let body = cx.tcx.hir().body(eid);
        let ex = &body.value;

        if ex.span.ctxt() != expr.span.ctxt() {
            if decl.inputs.is_empty() {
                if let Some(VecArgs::Vec(&[])) = higher::vec_macro(cx, ex) {
                    // replace `|| vec![]` with `Vec::new`
                    span_lint_and_sugg(
                        cx,
                        REDUNDANT_CLOSURE,
                        expr.span,
                        "redundant closure",
                        "replace the closure with `Vec::new`",
                        "std::vec::Vec::new".into(),
                        Applicability::MachineApplicable,
                    );
                }
            }
            // skip `foo(|| macro!())`
            return;
        }

        if_chain!(
            if let ExprKind::Call(caller, args) = ex.kind;

            if let ExprKind::Path(_) = caller.kind;

            // Not the same number of arguments, there is no way the closure is the same as the function return;
            if args.len() == decl.inputs.len();

            // Are the expression or the arguments type-adjusted? Then we need the closure
            if !(is_adjusted(cx, ex) || args.iter().any(|arg| is_adjusted(cx, arg)));

            let fn_ty = cx.typeck_results().expr_ty(caller);

            if matches!(fn_ty.kind(), ty::FnDef(_, _) | ty::FnPtr(_) | ty::Closure(_, _));

            if !type_is_unsafe_function(cx, fn_ty);

            if compare_inputs(&mut iter_input_pats(decl, body), &mut args.iter());

            then {
                span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure", |diag| {
                    if let Some(mut snippet) = snippet_opt(cx, caller.span) {
                        if_chain! {
                            if let ty::Closure(_, substs) = fn_ty.kind();
                            if let ClosureKind::FnMut = substs.as_closure().kind();
                            if UsedAfterExprVisitor::is_found(cx, caller)
                                || get_enclosing_loop_or_closure(cx.tcx, expr).is_some();

                            then {
                                // Mutable closure is used after current expr; we cannot consume it.
                                snippet = format!("&mut {}", snippet);
                            }
                        }
                        diag.span_suggestion(
                            expr.span,
                            "replace the closure with the function itself",
                            snippet,
                            Applicability::MachineApplicable,
                        );
                    }
                });
            }
        );

        if_chain!(
            if let ExprKind::MethodCall(path, _, args, _) = ex.kind;

            // Not the same number of arguments, there is no way the closure is the same as the function return;
            if args.len() == decl.inputs.len();

            // Are the expression or the arguments type-adjusted? Then we need the closure
            if !(is_adjusted(cx, ex) || args.iter().skip(1).any(|arg| is_adjusted(cx, arg)));

            let method_def_id = cx.typeck_results().type_dependent_def_id(ex.hir_id).unwrap();
            if !type_is_unsafe_function(cx, cx.tcx.type_of(method_def_id));

            if compare_inputs(&mut iter_input_pats(decl, body), &mut args.iter());

            if let Some(name) = get_ufcs_type_name(cx, method_def_id, &args[0]);

            then {
                span_lint_and_sugg(
                    cx,
                    REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
                    expr.span,
                    "redundant closure",
                    "replace the closure with the method itself",
                    format!("{}::{}", name, path.ident.name),
                    Applicability::MachineApplicable,
                );
            }
        );
    }
}

/// Tries to determine the type for universal function call to be used instead of the closure
fn get_ufcs_type_name(cx: &LateContext<'_>, method_def_id: def_id::DefId, self_arg: &Expr<'_>) -> Option<String> {
    let expected_type_of_self = &cx.tcx.fn_sig(method_def_id).inputs_and_output().skip_binder()[0];
    let actual_type_of_self = &cx.typeck_results().node_type(self_arg.hir_id);

    if let Some(trait_id) = cx.tcx.trait_of_item(method_def_id) {
        if match_borrow_depth(expected_type_of_self, actual_type_of_self)
            && implements_trait(cx, actual_type_of_self, trait_id, &[])
        {
            return Some(cx.tcx.def_path_str(trait_id));
        }
    }

    cx.tcx.impl_of_method(method_def_id).and_then(|_| {
        //a type may implicitly implement other type's methods (e.g. Deref)
        if match_types(expected_type_of_self, actual_type_of_self) {
            Some(get_type_name(cx, actual_type_of_self))
        } else {
            None
        }
    })
}

fn match_borrow_depth(lhs: Ty<'_>, rhs: Ty<'_>) -> bool {
    match (&lhs.kind(), &rhs.kind()) {
        (ty::Ref(_, t1, mut1), ty::Ref(_, t2, mut2)) => mut1 == mut2 && match_borrow_depth(t1, t2),
        (l, r) => !matches!((l, r), (ty::Ref(_, _, _), _) | (_, ty::Ref(_, _, _))),
    }
}

fn match_types(lhs: Ty<'_>, rhs: Ty<'_>) -> bool {
    match (&lhs.kind(), &rhs.kind()) {
        (ty::Bool, ty::Bool)
        | (ty::Char, ty::Char)
        | (ty::Int(_), ty::Int(_))
        | (ty::Uint(_), ty::Uint(_))
        | (ty::Str, ty::Str) => true,
        (ty::Ref(_, t1, mut1), ty::Ref(_, t2, mut2)) => mut1 == mut2 && match_types(t1, t2),
        (ty::Array(t1, _), ty::Array(t2, _)) | (ty::Slice(t1), ty::Slice(t2)) => match_types(t1, t2),
        (ty::Adt(def1, _), ty::Adt(def2, _)) => def1 == def2,
        (_, _) => false,
    }
}

fn get_type_name(cx: &LateContext<'_>, ty: Ty<'_>) -> String {
    match ty.kind() {
        ty::Adt(t, _) => cx.tcx.def_path_str(t.did),
        ty::Ref(_, r, _) => get_type_name(cx, r),
        _ => ty.to_string(),
    }
}

fn compare_inputs(
    closure_inputs: &mut dyn Iterator<Item = &Param<'_>>,
    call_args: &mut dyn Iterator<Item = &Expr<'_>>,
) -> bool {
    for (closure_input, function_arg) in closure_inputs.zip(call_args) {
        if let PatKind::Binding(_, _, ident, _) = closure_input.pat.kind {
            // XXXManishearth Should I be checking the binding mode here?
            if let ExprKind::Path(QPath::Resolved(None, p)) = function_arg.kind {
                if p.segments.len() != 1 {
                    // If it's a proper path, it can't be a local variable
                    return false;
                }
                if p.segments[0].ident.name != ident.name {
                    // The two idents should be the same
                    return false;
                }
            } else {
                return false;
            }
        } else {
            return false;
        }
    }
    true
}