rust-clippy/book/src/development/common_tools_writing_lints.md

# Common tools for writing lints

You may need following tooltips to catch up with common operations.

- [Common tools for writing lints](#common-tools-for-writing-lints)
  - [Retrieving the type of expression](#retrieving-the-type-of-expression)
  - [Checking if an expr is calling a specific method](#checking-if-an-expr-is-calling-a-specific-method)
  - [Checking for a specific type](#checking-for-a-specific-type)
  - [Checking if a type implements a specific trait](#checking-if-a-type-implements-a-specific-trait)
  - [Checking if a type defines a specific method](#checking-if-a-type-defines-a-specific-method)
  - [Dealing with macros](#dealing-with-macros-and-expansions)

Useful Rustc dev guide links:
- [Stages of compilation](https://rustc-dev-guide.rust-lang.org/compiler-src.html#the-main-stages-of-compilation)
- [Diagnostic items](https://rustc-dev-guide.rust-lang.org/diagnostics/diagnostic-items.html)
- [Type checking](https://rustc-dev-guide.rust-lang.org/type-checking.html)
- [Ty module](https://rustc-dev-guide.rust-lang.org/ty.html)

## Retrieving the type of expression

Sometimes you may want to retrieve the type `Ty` of an expression `Expr`, for
example to answer following questions:

- which type does this expression correspond to (using its [`TyKind`][TyKind])?
- is it a sized type?
- is it a primitive type?
- does it implement a trait?

This operation is performed using the [`expr_ty()`][expr_ty] method from the
[`TypeckResults`][TypeckResults] struct, that gives you access to the underlying
structure [`Ty`][Ty].

Example of use:
```rust
impl LateLintPass<'_> for MyStructLint {
    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
        // Get type of `expr`
        let ty = cx.typeck_results().expr_ty(expr);
        // Match its kind to enter its type
        match ty.kind {
            ty::Adt(adt_def, _) if adt_def.is_struct() => println!("Our `expr` is a struct!"),
            _ => ()
        }
    }
}
```

Similarly, in [`TypeckResults`][TypeckResults] methods, you have the
[`pat_ty()`][pat_ty] method to retrieve a type from a pattern.

Two noticeable items here:
- `cx` is the lint context [`LateContext`][LateContext]. The two most useful
  data structures in this context are `tcx` and the `TypeckResults` returned by
  `LateContext::typeck_results`, allowing us to jump to type definitions and
  other compilation stages such as HIR.
- `typeck_results`'s return value is [`TypeckResults`][TypeckResults] and is
  created by type checking step, it includes useful information such as types of
  expressions, ways to resolve methods and so on.

## Checking if an expr is calling a specific method

Starting with an `expr`, you can check whether it is calling a specific method
`some_method`:

```rust
impl<'tcx> LateLintPass<'tcx> for MyStructLint {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
        // Check our expr is calling a method
        if let hir::ExprKind::MethodCall(path, _, _self_arg, ..) = &expr.kind
            // Check the name of this method is `some_method`
            && path.ident.name == sym!(some_method)
            // Optionally, check the type of the self argument.
            // - See "Checking for a specific type"
        {
                // ...
        }
    }
}
```

## Checking for a specific type

There are three ways to check if an expression type is a specific type we want
to check for. All of these methods only check for the base type, generic
arguments have to be checked separately.

```rust
use clippy_utils::ty::{is_type_diagnostic_item, is_type_lang_item};
use clippy_utils::{paths, match_def_path};
use rustc_span::symbol::sym;
use rustc_hir::LangItem;

impl LateLintPass<'_> for MyStructLint {
    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
        // Getting the expression type
        let ty = cx.typeck_results().expr_ty(expr);

        // 1. Using diagnostic items
        // The last argument is the diagnostic item to check for
        if is_type_diagnostic_item(cx, ty, sym::Option) {
            // The type is an `Option`
        }

        // 2. Using lang items
        if is_type_lang_item(cx, ty, LangItem::RangeFull) {
            // The type is a full range like `.drain(..)`
        }

        // 3. Using the type path
        // This method should be avoided if possible
        if match_def_path(cx, def_id, &paths::RESULT) {
            // The type is a `core::result::Result`
        }
    }
}
```

Prefer using diagnostic items and lang items where possible.

## Checking if a type implements a specific trait

There are three ways to do this, depending on if the target trait has a
diagnostic item, lang item or neither.

```rust
use clippy_utils::ty::implements_trait;
use clippy_utils::is_trait_method;
use rustc_span::symbol::sym;

impl LateLintPass<'_> for MyStructLint {
    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
        // 1. Using diagnostic items with the expression
        // we use `is_trait_method` function from Clippy's utils
        if is_trait_method(cx, expr, sym::Iterator) {
            // method call in `expr` belongs to `Iterator` trait
        }

        // 2. Using lang items with the expression type
        let ty = cx.typeck_results().expr_ty(expr);
        if cx.tcx.lang_items()
            // we are looking for the `DefId` of `Drop` trait in lang items
            .drop_trait()
            // then we use it with our type `ty` by calling `implements_trait` from Clippy's utils
            .map_or(false, |id| implements_trait(cx, ty, id, &[])) {
                // `expr` implements `Drop` trait
            }
    }
}
```

> Prefer using diagnostic and lang items, if the target trait has one.

We access lang items through the type context `tcx`. `tcx` is of type
[`TyCtxt`][TyCtxt] and is defined in the `rustc_middle` crate. A list of defined
paths for Clippy can be found in [paths.rs][paths]

## Checking if a type defines a specific method

To check if our type defines a method called `some_method`:

```rust
use clippy_utils::ty::is_type_diagnostic_item;
use clippy_utils::return_ty;

impl<'tcx> LateLintPass<'tcx> for MyTypeImpl {
    fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {
        // Check if item is a method/function
        if let ImplItemKind::Fn(ref signature, _) = impl_item.kind
            // Check the method is named `some_method`
            && impl_item.ident.name == sym!(some_method)
            // We can also check it has a parameter `self`
            && signature.decl.implicit_self.has_implicit_self()
            // We can go further and even check if its return type is `String`
            && is_type_diagnostic_item(cx, return_ty(cx, impl_item.hir_id), sym!(string_type))
        {
            // ...
        }
    }
}
```

## Dealing with macros and expansions

Keep in mind that macros are already expanded and desugaring is already applied
to the code representation that you are working with in Clippy. This
unfortunately causes a lot of false positives because macro expansions are
"invisible" unless you actively check for them. Generally speaking, code with
macro expansions should just be ignored by Clippy because that code can be
dynamic in ways that are difficult or impossible to see. Use the following
functions to deal with macros:

- `span.from_expansion()`: detects if a span is from macro expansion or
  desugaring. Checking this is a common first step in a lint.

   ```rust,ignore
   if expr.span.from_expansion() {
       // just forget it
       return;
   }
   ```

- `span.ctxt()`: the span's context represents whether it is from expansion, and
  if so, which macro call expanded it. It is sometimes useful to check if the
  context of two spans are equal.

  ```rust,ignore
  // expands to `1 + 0`, but don't lint
  1 + mac!()
  ```
  ```rust,ignore
  if left.span.ctxt() != right.span.ctxt() {
      // the coder most likely cannot modify this expression
      return;
  }
  ```
  > Note: Code that is not from expansion is in the "root" context. So any spans
  > where `from_expansion` returns `true` can be assumed to have the same
  > context. And so just using `span.from_expansion()` is often good enough.


- `in_external_macro(span)`: detect if the given span is from a macro defined in
  a foreign crate. If you want the lint to work with macro-generated code, this
  is the next line of defense to avoid macros not defined in the current crate.
  It doesn't make sense to lint code that the coder can't change.

  You may want to use it for example to not start linting in macros from other
  crates

  ```rust
  use rustc_middle::lint::in_external_macro;

  use a_crate_with_macros::foo;

  // `foo` is defined in `a_crate_with_macros`
  foo!("bar");

  // if we lint the `match` of `foo` call and test its span
  assert_eq!(in_external_macro(cx.sess(), match_span), true);
  ```

- `span.ctxt()`: the span's context represents whether it is from expansion, and
  if so, what expanded it

  One thing `SpanContext` is useful for is to check if two spans are in the same
  context. For example, in `a == b`, `a` and `b` have the same context. In a
  `macro_rules!` with `a == $b`, `$b` is expanded to some expression with a
  different context from `a`.

   ```rust,ignore
   macro_rules! m {
       ($a:expr, $b:expr) => {
           if $a.is_some() {
               $b;
           }
       }
   }

   let x: Option<u32> = Some(42);
   m!(x, x.unwrap());

   // These spans are not from the same context
   // x.is_some() is from inside the macro
   // x.unwrap() is from outside the macro
   assert_eq!(x_is_some_span.ctxt(), x_unwrap_span.ctxt());
   ```

[Ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html
[TyKind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.TyKind.html
[TypeckResults]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TypeckResults.html
[expr_ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TypeckResults.html#method.expr_ty
[LateContext]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/struct.LateContext.html
[TyCtxt]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html
[pat_ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TypeckResults.html#method.pat_ty
[paths]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/paths/index.html