Add new chapter: "Trait Checking"

Author: blyxyas

book/src/SUMMARY.md

@@ -15,6 +15,7 @@
     - [Adding Lints](development/adding_lints.md)
     - [Lint Passes](development/lint_passes.md)
     - [Type Checking](development/type_checking.md)
+    - [Trait Checking](development/trait_checking.md)
     - [Macro Expansions](development/macro_expansions.md)
     - [Common Tools](development/common_tools_writing_lints.md)
     - [Infrastructure](development/infrastructure/README.md)

book/src/development/trait_checking.md

@@ -0,0 +1,112 @@
+# Trait Checking
+
+Besides [type checking](type_checking.md), we might want to examine if
+a specific type `Ty` implements certain trait when implementing a lint.
+There are three approaches to achieve this, depending on if the target trait
+that we want to examine has a [diagnostic item][diagnostic_items],
+[lang item][lang_items], or neither.
+
+## Using Diagnostic Items
+
+As explained in the [Rust Compiler Development Guide][rustc_dev_guide], diagnostic items
+are introduced for identifying types via [Symbols][symbol].
+
+While the Rust Compiler Development Guide has [a section][using_diagnostic_items] on
+how to check for a specific trait on a type `Ty`, Clippy provides
+a helper function `is_trait_method`, which simplifies the process for us.
+
+For instance, if we want to examine whether an expression implements
+the `Iterator` trait, we could simply write the following code,
+providing the `LateContext` (`cx`), our expression at hand, and
+the symbol of the trait in question:
+
+```rust
+use clippy_utils::is_trait_method;
+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<'_>) {
+        if is_trait_method(cx, expr, sym::Iterator) {
+            println!("This expression implements `Iterator` trait!");
+        }
+    }
+}
+```
+
+> **Note**: Refer to [this index][symbol_index] for all the defined `Symbol`s.
+
+## Using Lang Items
+
+Besides diagnostic items, we can also use [`lang_items`][lang_items].
+Take a look at the documentation and we find that `LanguageItems` contains
+all language items both from the current crate or its
+dependencies.
+
+Using one of its `*_trait` method, we could obtain the [DefId] of any
+specific item, such as `Clone`, `Copy`, `Drop`, `Eq`, which are familiar
+to many Rustaceans.
+
+For instance, if we want to examine whether an expression `expr` implements
+`Drop` trait, we could access `LanguageItems` via our `LateContext`'s
+[TyCtxt], which provides a `lang_items` method that will return the id of
+`Drop` trait to us. Then, by calling Clippy utils function `implements_trait`
+we can check that the `Ty` of the `expr` implements the trait:
+
+```rust
+use clippy_utils::implements_trait;
+use rustc_hir::Expr;
+use rustc_lint::{LateContext, LateLintPass};
+
+impl LateLintPass<'_> for CheckDropTraitLint {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
+        let ty = cx.typeck_results().expr_ty(expr);
+        if cx.tcx.lang_items()
+            .drop_trait()
+            .map_or(false, |id| implements_trait(cx, ty, id, &[])) {
+                println!("`expr` implements `Drop` trait!");
+            }
+    }
+}
+```
+
+## Using Type Path
+
+If neither diagnostic item or lang item is available, we can use
+[`clippy_utils::paths`][paths] with the `match_trait_method` to determine trait
+implementation.
+
+> **Note**: This approach should be avoided if possible.
+
+Below, we check if the given `expr` implements `tokio`'s
+[`AsyncReadExt`][AsyncReadExt] trait:
+
+```rust
+use clippy_utils::{match_trait_method, paths};
+use rustc_hir::Expr;
+use rustc_lint::{LateContext, LateLintPass};
+
+impl LateLintPass<'_> for CheckTokioAsyncReadExtTrait {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
+        if match_trait_method(cx, expr, &paths::TOKIO_IO_ASYNCREADEXT) {
+            println!("`expr` implements `TOKIO_IO_ASYNCREADEXT` trait!");
+        }
+    }
+}
+```
+
+> **Note**: Even though all the `clippy_utils` methods we have seen in this
+> chapter takes `expr` as a parameter, these methods are actually using
+> each expression's `HirId` under the hood.
+
+[AsyncReadExt]: https://docs.rs/tokio/latest/tokio/io/trait.AsyncReadExt.html
+[DefId]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html
+[diagnostic_items]: https://rustc-dev-guide.rust-lang.org/diagnostics/diagnostic-items.html
+[lang_items]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/lang_items/struct.LanguageItems.html
+[paths]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_utils/src/paths.rs
+[rustc_dev_guide]: https://rustc-dev-guide.rust-lang.org/
+[symbol]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Symbol.html
+[symbol_index]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_span/symbol/sym/index.html
+[TyCtxt]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html
+[using_diagnostic_items]: https://rustc-dev-guide.rust-lang.org/diagnostics/diagnostic-items.html#using-diagnostic-items