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# 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 an expression](#retrieving-the-type-of-an-expression)
- [Checking if an expression is calling a specific method](#checking-if-an-expr-is-calling-a-specific-method)
- [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)
Useful Rustc dev guide links:
- [Stages of compilation](https://rustc-dev-guide.rust-lang.org/compiler-src.html#the-main-stages-of-compilation)
- [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 an 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 [`TyS`][TyS].
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 LateLintPass<'_> for MyStructLint {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
if_chain! {
// Check our expr is calling a method
if let hir::ExprKind::MethodCall(path, _, _args, _) = &expr.kind;
// Check the name of this method is `some_method`
if path.ident.name == sym!(some_method);
then {
// ...
}
}
}
}
```
# Checking if a type implements a specific trait
There are two ways to do this, depending if the target trait is part of lang items.
```rust
use clippy_utils::{implements_trait, match_trait_method, paths};
impl LateLintPass<'_> for MyStructLint {
fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
// 1. Using expression and Clippy's convenient method
// we use `match_trait_method` function from Clippy's toolbox
if match_trait_method(cx, expr, &paths::INTO) {
// `expr` implements `Into` trait
}
// 2. Using type context `TyCtxt`
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 lang items, if the target trait is available there.
A list of defined paths for Clippy can be found in [paths.rs][paths]
We access lang items through the type context `tcx`. `tcx` is of type [`TyCtxt`][TyCtxt] and is defined in the `rustc_middle` crate.
# Checking if a type defines a specific method
To check if our type defines a method called `some_method`:
```rust
use clippy_utils::{is_type_diagnostic_item, return_ty};
impl<'tcx> LateLintPass<'tcx> for MyTypeImpl {
fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {
if_chain! {
// Check if item is a method/function
if let ImplItemKind::Fn(ref signature, _) = impl_item.kind;
// Check the method is named `some_method`
if impl_item.ident.name == sym!(some_method);
// We can also check it has a parameter `self`
if signature.decl.implicit_self.has_implicit_self();
// We can go further and even check if its return type is `String`
if is_type_diagnostic_item(cx, return_ty(cx, impl_item.hir_id), sym!(string_type));
then {
// ...
}
}
}
}
```
# Dealing with macros
There are several helpers in [`clippy_utils`][utils] to deal with macros:
- `in_macro()`: detect if the given span is expanded by a macro
You may want to use this for example to not start linting in any macro.
```rust
macro_rules! foo {
($param:expr) => {
match $param {
"bar" => println!("whatever"),
_ => ()
}
};
}
foo!("bar");
// if we lint the `match` of `foo` call and test its span
assert_eq!(in_macro(match_span), true);
```
- `in_external_macro()`: detect if the given span is from an external macro, defined in a foreign crate
You may want to use it for example to not start linting in macros from other crates
```rust
#[macro_use]
extern crate a_crate_with_macros;
// `foo` is defined in `a_crate_with_macros`
foo!("bar");
// if we lint the `match` of `foo` call and test its span
assert_eq!(in_external_macro(cx.sess(), match_span), true);
```
- `differing_macro_contexts()`: returns true if the two given spans are not from the same context
```rust
macro_rules! m {
($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!(differing_macro_contexts(x_is_some_span, x_unwrap_span), true);
```
[TyS]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyS.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]: ../clippy_utils/src/paths.rs
[utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_utils/src/lib.rs
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