1 # Common tools for writing lints
3 You may need following tooltips to catch up with common operations.
5 - [Common tools for writing lints](#common-tools-for-writing-lints)
6 - [Retrieving the type of an expression](#retrieving-the-type-of-an-expression)
7 - [Checking if an expression is calling a specific method](#checking-if-an-expr-is-calling-a-specific-method)
8 - [Checking if a type implements a specific trait](#checking-if-a-type-implements-a-specific-trait)
9 - [Checking if a type defines a method](#checking-if-a-type-defines-a-method)
10 - [Dealing with macros](#dealing-with-macros)
12 Useful Rustc dev guide links:
13 - [Stages of compilation](https://rustc-dev-guide.rust-lang.org/compiler-src.html#the-main-stages-of-compilation)
14 - [Type checking](https://rustc-dev-guide.rust-lang.org/type-checking.html)
15 - [Ty module](https://rustc-dev-guide.rust-lang.org/ty.html)
17 # Retrieving the type of an expression
19 Sometimes you may want to retrieve the type `Ty` of an expression `Expr`, for example to answer following questions:
21 - which type does this expression correspond to (using its [`TyKind`][TyKind])?
23 - is it a primitive type?
24 - does it implement a trait?
26 This operation is performed using the [`expr_ty()`][expr_ty] method from the [`TypeckTables`][TypeckTables] struct,
27 that gives you access to the underlying structure [`TyS`][TyS].
31 impl LateLintPass<'_, '_> for MyStructLint {
32 fn check_expr(&mut self, cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
34 let ty = cx.tables().expr_ty(expr);
35 // Match its kind to enter its type
37 ty::Adt(adt_def, _) if adt_def.is_struct() => println!("Our `expr` is a struct!"),
44 Similarly in [`TypeckTables`][TypeckTables] methods, you have the [`pat_ty()`][pat_ty] method
45 to retrieve a type from a pattern.
47 Two noticeable items here:
48 - `cx` is the lint context [`LateContext`][LateContext].
49 The two most useful data structures in this context are `tcx` and `tables`,
50 allowing us to jump to type definitions and other compilation stages such as HIR.
51 - `tables` is [`TypeckTables`][TypeckTables] and is created by type checking step,
52 it includes useful information such as types of expressions, ways to resolve methods and so on.
54 # Checking if an expr is calling a specific method
56 Starting with an `expr`, you can check whether it is calling a specific method `some_method`:
59 impl LateLintPass<'_, '_> for MyStructLint {
60 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr<'_>) {
62 // Check our expr is calling a method
63 if let hir::ExprKind::MethodCall(path, _, _args) = &expr.kind;
64 // Check the name of this method is `some_method`
65 if path.ident.name == sym!(some_method);
74 # Checking if a type implements a specific trait
76 There are two ways to do this, depending if the target trait is part of lang items.
79 use crate::utils::{implements_trait, match_trait_method, paths};
81 impl LateLintPass<'_, '_> for MyStructLint {
82 fn check_expr(&mut self, cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
83 // 1. Using expression and Clippy's convenient method
84 // we use `match_trait_method` function from Clippy's toolbox
85 if match_trait_method(cx, expr, &paths::INTO) {
86 // `expr` implements `Into` trait
89 // 2. Using type context `TyCtxt`
90 let ty = cx.tables().expr_ty(expr);
91 if cx.tcx.lang_items()
92 // we are looking for the `DefId` of `Drop` trait in lang items
94 // then we use it with our type `ty` by calling `implements_trait` from Clippy's utils
95 .map_or(false, |id| implements_trait(cx, ty, id, &[])) {
96 // `expr` implements `Drop` trait
102 > Prefer using lang items, if the target trait is available there.
104 A list of defined paths for Clippy can be found in [paths.rs][paths]
106 We access lang items through the type context `tcx`. `tcx` is of type [`TyCtxt`][TyCtxt] and is defined in the `rustc_middle` crate.
108 # Checking if a type defines a specific method
110 To check if our type defines a method called `some_method`:
113 use crate::utils::{is_type_diagnostic_item, return_ty};
115 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MyTypeImpl {
116 fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, impl_item: &'tcx ImplItem<'_>) {
118 // Check if item is a method/function
119 if let ImplItemKind::Fn(ref signature, _) = impl_item.kind;
120 // Check the method is named `some_method`
121 if impl_item.ident.name == sym!(some_method);
122 // We can also check it has a parameter `self`
123 if signature.decl.implicit_self.has_implicit_self();
124 // We can go further and even check if its return type is `String`
125 if is_type_diagnostic_item(cx, return_ty(cx, impl_item.hir_id), sym!(string_type));
134 # Dealing with macros
136 There are several helpers in Clippy's utils to deal with macros:
138 - `in_macro()`: detect if the given span is expanded by a macro
140 You may want to use this for example to not start linting in any macro.
146 "bar" => println!("whatever"),
154 // if we lint the `match` of `foo` call and test its span
155 assert_eq!(in_macro(match_span), true);
158 - `in_external_macro()`: detect if the given span is from an external macro, defined in a foreign crate
160 You may want to use it for example to not start linting in macros from other crates
164 extern crate a_crate_with_macros;
166 // `foo` is defined in `a_crate_with_macros`
169 // if we lint the `match` of `foo` call and test its span
170 assert_eq!(in_external_macro(cx.sess(), match_span), true);
173 - `differing_macro_contexts()`: returns true if the two given spans are not from the same context
177 ($a:expr, $b:expr) => {
184 let x: Option<u32> = Some(42);
187 // These spans are not from the same context
188 // x.is_some() is from inside the macro
189 // x.unwrap() is from outside the macro
190 assert_eq!(differing_macro_contexts(x_is_some_span, x_unwrap_span), true);
193 [TyS]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyS.html
194 [TyKind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.TyKind.html
195 [TypeckTables]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TypeckTables.html
196 [expr_ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TypeckTables.html#method.expr_ty
197 [LateContext]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/struct.LateContext.html
198 [TyCtxt]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html
199 [pat_ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TypeckTables.html#method.pat_ty
200 [paths]: ../clippy_lints/src/utils/paths.rs