book: encourage the use of clippy_utils::sym

Also, explain how new symbols can be added to this module in order to
compare names.

Author: samueltardieu

book/src/development/common_tools_writing_lints.md

@@ -68,7 +68,7 @@ impl<'tcx> LateLintPass<'tcx> for MyStructLint {
         // 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.as_str() == "some_method"
+            && path.ident.name == sym::some_method
             // Optionally, check the type of the self argument.
             // - See "Checking for a specific type"
         {
@@ -85,9 +85,8 @@ to check for. All of these methods only check for the base type, generic
 arguments have to be checked separately.
 
 ```rust
-use clippy_utils::paths;
+use clippy_utils::{paths, sym};
 use clippy_utils::res::MaybeDef;
-use rustc_span::symbol::sym;
 use rustc_hir::LangItem;
 
 impl LateLintPass<'_> for MyStructLint {
@@ -123,8 +122,8 @@ 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::sym;
 use clippy_utils::ty::implements_trait;
-use rustc_span::symbol::sym;
 
 impl LateLintPass<'_> for MyStructLint {
     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {

book/src/development/method_checking.md

@@ -15,15 +15,15 @@ the [`ExprKind`] that we can access from `expr.kind`:
 ```rust
 use rustc_hir as hir;
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_span::sym;
 use clippy_utils::res::{MaybeDef, MaybeTypeckRes};
+use clippy_utils::sym;
 
 impl<'tcx> LateLintPass<'tcx> for OurFancyMethodLint {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
         // Check our expr is calling a method with pattern matching
         if let hir::ExprKind::MethodCall(path, _, [self_arg, ..], _) = &expr.kind
             // Check if the name of this method is `our_fancy_method`
-            && path.ident.name.as_str() == "our_fancy_method"
+            && path.ident.name == sym::our_fancy_method
             // We can check the type of the self argument whenever necessary.
             // (It's necessary if we want to check that method is specifically belonging to a specific trait,
             // for example, a `map` method could belong to user-defined trait instead of to `Iterator`)
@@ -41,6 +41,10 @@ information on the pattern matching. As mentioned in [Define
 Lints](defining_lints.md#lint-types), the `methods` lint type is full of pattern
 matching with `MethodCall` in case the reader wishes to explore more.
 
+New symbols such as `our_fancy_method` need to be added to the `clippy_utils::sym` module.
+This module extends the list of symbols already provided by the compiler crates
+in `rustc_span::sym`.
+
 ## Checking if a `impl` block implements a method
 
 While sometimes we want to check whether a method is being called or not, other
@@ -56,11 +60,10 @@ Let us take a look at how we might check for the implementation of
 `our_fancy_method` on a type:
 
 ```rust
+use clippy_utils::{return_ty, sym};
 use clippy_utils::res::MaybeDef;
-use clippy_utils::return_ty;
 use rustc_hir::{ImplItem, ImplItemKind};
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_span::symbol::sym;
 
 impl<'tcx> LateLintPass<'tcx> for MyTypeImpl {
     fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {

book/src/development/trait_checking.md

@@ -17,10 +17,10 @@ providing the `LateContext` (`cx`), our expression at hand, and
 the symbol of the trait in question:
 
 ```rust
+use clippy_utils::sym;
 use clippy_utils::ty::implements_trait;
 use rustc_hir::Expr;
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_span::symbol::sym;
 
 impl LateLintPass<'_> for CheckIteratorTraitLint {
     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
@@ -124,8 +124,8 @@ The following code demonstrates how to do this:
 ```rust
 
 use rustc_middle::ty::Ty;
+use clippy_utils::sym;
 use clippy_utils::ty::implements_trait;
-use rustc_span::symbol::sym;
 
 let ty = todo!("Get the `Foo` type to check for a trait implementation");
 let borrow_id = cx.tcx.get_diagnostic_item(sym::Borrow).unwrap(); // avoid unwrap in real code