Auto merge of rust-lang#10644 - blyxyas:book-method_checking, r=flip1995

Clippy Book Chapter Updates Reborn: Method Checking

This PR adds a new chapter to the book: "Method Checking". Some major re-phrasing was done and some changes in the code comments (apart from grammar and minor changes).

## Notes

- Requires rust-lang#10595 **and** rust-lang#10622 to be merged before this, or else several links will be broken
- To talk about the whole project, please use the tracking issue for the project rust-lang#10597 (It also contains a timeline, discussions and more information)

changelog: Add a new "Method Checking" chapter to the book.

r? `@flip1995`

book/src/SUMMARY.md

@@ -16,6 +16,7 @@
     - [Defining Lints](development/defining_lints.md)
     - [Lint Passes](development/lint_passes.md)
     - [Type Checking](development/type_checking.md)
+    - [Method Checking](development/method_checking.md)
     - [Macro Expansions](development/macro_expansions.md)
     - [Common Tools](development/common_tools_writing_lints.md)
     - [Infrastructure](development/infrastructure/README.md)

book/src/development/method_checking.md

@@ -0,0 +1,93 @@
+# Method Checking
+
+In some scenarios we might want to check for methods when developing
+a lint. There are two kinds of questions that we might be curious about:
+
+- Invocation: Does an expression call a specific method?
+- Definition: Does an `impl` define a method?
+
+## Checking if an `expr` is calling a specific method
+
+Suppose we have an `expr`, we can check whether it calls a specific
+method, e.g. `our_fancy_method`, by performing a pattern match on
+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::is_trait_method;
+
+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 == 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`)
+            // See the next section for more information.
+            && is_trait_method(cx, self_arg, sym::OurFancyTrait)
+        {
+            println!("`expr` is a method call for `our_fancy_method`");
+        }
+    }
+}
+```
+
+Take a closer look at the `ExprKind` enum variant [`MethodCall`] for more
+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.
+
+Additionally, we use the [`clippy_utils::sym!`][sym] macro to conveniently
+convert an input `our_fancy_method` into a `Symbol` and compare that symbol to
+the [`Ident`]'s name in the [`PathSegment`] in the [`MethodCall`].
+
+## Checking if a `impl` block implements a method
+
+While sometimes we want to check whether a method is being called or not, other
+times we want to know if our `Ty` defines a method.
+
+To check if our `impl` block defines a method `our_fancy_method`, we will
+utilize the [`check_impl_item`] method that is available in our beloved
+[`LateLintPass`] (for more information, refer to the ["Lint
+Passes"](lint_passes.md) chapter in the Clippy book). This method provides us
+with an [`ImplItem`] struct, which represents anything within an `impl` block.
+
+Let us take a look at how we might check for the implementation of
+`our_fancy_method` on a type:
+
+```rust
+use clippy_utils::ty::is_type_diagnostic_item;
+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<'_>) {
+        // Check if item is a method/function
+        if let ImplItemKind::Fn(ref signature, _) = impl_item.kind
+            // Check the method is named `our_fancy_method`
+            && impl_item.ident.name == sym!(our_fancy_method)
+            // We can also check it has a parameter `self`
+            && signature.decl.implicit_self.has_implicit_self()
+            // We can go even further and even check if its return type is `String`
+            && is_type_diagnostic_item(cx, return_ty(cx, impl_item.hir_id), sym::String)
+        {
+            println!("`our_fancy_method` is implemented!");
+        }
+    }
+}
+```
+
+[`check_impl_item`]: https://doc.rust-lang.org/stable/nightly-rustc/rustc_lint/trait.LateLintPass.html#method.check_impl_item
+[`ExprKind`]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_hir/hir/enum.ExprKind.html
+[`Ident`]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_span/symbol/struct.Ident.html
+[`ImplItem`]: https://doc.rust-lang.org/stable/nightly-rustc/rustc_hir/hir/struct.ImplItem.html
+[`LateLintPass`]: https://doc.rust-lang.org/stable/nightly-rustc/rustc_lint/trait.LateLintPass.html
+[`MethodCall`]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_hir/hir/enum.ExprKind.html#variant.MethodCall
+[`PathSegment`]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_hir/hir/struct.PathSegment.html
+[sym]: https://doc.rust-lang.org/stable/nightly-rustc/clippy_utils/macro.sym.html