Auto merge of #7928 - xFrednet:rust-90354-deploy-clippy-docs, r=flip1995

Reference `clippy_utils` docs on nightly-rustc and some other documentation updates

The `clippy_utils` crate is now part of the nightly-rustc documentation. See [**very beautiful documentation**](https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/). This PR references them in our documentation and updates some other documentation.

changelog: none

4 files changed, 80 insertions, 14 deletions

diff --git a/clippy_utils/src/source.rs b/clippy_utils/src/source.rs
index 789079510c5..7f68cc388eb 100644
--- a/clippy_utils/src/source.rs
+++ b/clippy_utils/src/source.rs
@@ -155,14 +155,22 @@ fn reindent_multiline_inner(s: &str, ignore_first: bool, indent: Option<usize>,
         .join("\n")
 }
 
-/// Converts a span to a code snippet if available, otherwise use default.
+/// Converts a span to a code snippet if available, otherwise returns the default.
 ///
 /// This is useful if you want to provide suggestions for your lint or more generally, if you want
-/// to convert a given `Span` to a `str`.
+/// to convert a given `Span` to a `str`. To create suggestions consider using
+/// [`snippet_with_applicability`] to ensure that the applicability stays correct.
 ///
 /// # Example
 /// ```rust,ignore
-/// snippet(cx, expr.span, "..")
+/// // Given two spans one for `value` and one for the `init` expression.
+/// let value = Vec::new();
+/// //  ^^^^^   ^^^^^^^^^^
+/// //  span1   span2
+///
+/// // The snipped call would return the corresponding code snippet
+/// snippet(cx, span1, "..") // -> "value"
+/// snippet(cx, span2, "..") // -> "Vec::new()"
 /// ```
 pub fn snippet<'a, T: LintContext>(cx: &T, span: Span, default: &'a str) -> Cow<'a, str> {
     snippet_opt(cx, span).map_or_else(|| Cow::Borrowed(default), From::from)
diff --git a/clippy_utils/src/ty.rs b/clippy_utils/src/ty.rs
index ca64ac7de3e..69715828270 100644
--- a/clippy_utils/src/ty.rs
+++ b/clippy_utils/src/ty.rs
@@ -114,7 +114,12 @@ pub fn has_iter_method(cx: &LateContext<'_>, probably_ref_ty: Ty<'_>) -> Option<
 
 /// Checks whether a type implements a trait.
 /// The function returns false in case the type contains an inference variable.
-/// See also [`get_trait_def_id`](super::get_trait_def_id).
+///
+/// See:
+/// * [`get_trait_def_id`](super::get_trait_def_id) to get a trait [`DefId`].
+/// * [Common tools for writing lints] for an example how to use this function and other options.
+///
+/// [Common tools for writing lints]: https://github.com/rust-lang/rust-clippy/blob/master/doc/common_tools_writing_lints.md#checking-if-a-type-implements-a-specific-trait
 pub fn implements_trait<'tcx>(
     cx: &LateContext<'tcx>,
     ty: Ty<'tcx>,
@@ -254,9 +259,17 @@ pub fn is_type_ref_to_diagnostic_item(cx: &LateContext<'_>, ty: Ty<'_>, diag_ite
     }
 }
 
-/// Checks if the type is equal to a diagnostic item
+/// Checks if the type is equal to a diagnostic item. To check if a type implements a
+/// trait marked with a diagnostic item use [`implements_trait`].
+///
+/// For a further exploitation what diagnostic items are see [diagnostic items] in
+/// rustc-dev-guide.
+///
+/// ---
 ///
 /// If you change the signature, remember to update the internal lint `MatchTypeOnDiagItem`
+///
+/// [Diagnostic Items]: https://rustc-dev-guide.rust-lang.org/diagnostics/diagnostic-items.html
 pub fn is_type_diagnostic_item(cx: &LateContext<'_>, ty: Ty<'_>, diag_item: Symbol) -> bool {
     match ty.kind() {
         ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(diag_item, adt.did),
diff --git a/doc/adding_lints.md b/doc/adding_lints.md
index bd32696d6db..ae2444f0171 100644
--- a/doc/adding_lints.md
+++ b/doc/adding_lints.md
@@ -634,7 +634,7 @@ in the following steps:
 Here are some pointers to things you are likely going to need for every lint:
 
 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
-  is already in here (`implements_trait`, `match_def_path`, `snippet`, etc)
+  is already in here ([`is_type_diagnostic_item`], [`implements_trait`], [`snippet`], etc)
 * [Clippy diagnostics][diagnostics]
 * [The `if_chain` macro][if_chain]
 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
@@ -660,7 +660,10 @@ documentation currently. This is unfortunate, but in most cases you can probably
 get away with copying things from existing similar lints. If you are stuck,
 don't hesitate to ask on [Zulip] or in the issue/PR.
 
-[utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_utils/src/lib.rs
+[utils]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/index.html
+[`is_type_diagnostic_item`]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/ty/fn.is_type_diagnostic_item.html
+[`implements_trait`]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/ty/fn.implements_trait.html
+[`snippet`]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/source/fn.snippet.html
 [if_chain]: https://docs.rs/if_chain/*/if_chain/
 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
diff --git a/doc/common_tools_writing_lints.md b/doc/common_tools_writing_lints.md
index 1a6b7c8cb47..552b9b2587b 100644
--- a/doc/common_tools_writing_lints.md
+++ b/doc/common_tools_writing_lints.md
@@ -4,7 +4,8 @@ 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 an expr is calling a specific method](#checking-if-an-expr-is-calling-a-specific-method)
+  - [Checking for a specific type](#checking-for-a-specific-type)
   - [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)
@@ -15,7 +16,7 @@ Useful Rustc dev guide links:
 - [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
+## 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:
 
@@ -54,7 +55,7 @@ Two noticeable items here:
   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
+## 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`:
 
@@ -63,9 +64,11 @@ 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;
+            if let hir::ExprKind::MethodCall(path, _, [_self_arg, ..], _) = &expr.kind;
             // Check the name of this method is `some_method`
             if path.ident.name == sym!(some_method);
+            // Optionally, check the type of the self argument.
+            // - See "Checking for a specific type"
             then {
                 // ...
             }
@@ -74,7 +77,45 @@ impl LateLintPass<'_> for MyStructLint {
 }
 ```
 
-# Checking if a type implements a specific trait
+## Checking for a specific type
+
+There are three ways to check if an expression type is a specific type we want to check for.
+All of these methods only check for the base type, generic arguments have to be checked separately.
+
+```rust
+use clippy_utils::ty::{is_type_diagnostic_item, is_type_lang_item};
+use clippy_utils::{paths, match_def_path};
+use rustc_span::symbol::sym;
+use rustc_hir::LangItem;
+
+impl LateLintPass<'_> for MyStructLint {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
+        // Getting the expression type
+        let ty = cx.typeck_results().expr_ty(expr);
+
+        // 1. Using diagnostic items
+        // The last argument is the diagnostic item to check for
+        if is_type_diagnostic_item(cx, ty, sym::Option) {
+            // The type is an `Option`
+        }
+
+        // 2. Using lang items
+        if is_type_lang_item(cx, ty, LangItem::RangeFull) {
+            // The type is a full range like `.drain(..)`
+        }
+
+        // 3. Using the type path
+        // This method should be avoided if possible
+        if match_def_path(cx, def_id, &paths::RESULT) {
+            // The type is a `core::result::Result`
+        }
+    }
+}
+```
+
+Prefer using diagnostic items and lang items where possible.
+
+## Checking if a type implements a specific trait
 
 There are three ways to do this, depending on if the target trait has a diagnostic item, lang item or neither.
 
@@ -102,6 +143,7 @@ impl LateLintPass<'_> for MyStructLint {
 
         // 3. Using the type path with the expression
         // we use `match_trait_method` function from Clippy's utils
+        // (This method should be avoided if possible)
         if match_trait_method(cx, expr, &paths::INTO) {
             // `expr` implements `Into` trait
         }
@@ -114,7 +156,7 @@ impl LateLintPass<'_> for MyStructLint {
 We access lang items through the type context `tcx`. `tcx` is of type [`TyCtxt`][TyCtxt] and is defined in the `rustc_middle` crate.
 A list of defined paths for Clippy can be found in [paths.rs][paths]
 
-# Checking if a type defines a specific method
+## Checking if a type defines a specific method
 
 To check if our type defines a method called `some_method`:
 
@@ -140,7 +182,7 @@ impl<'tcx> LateLintPass<'tcx> for MyTypeImpl {
 }
 ```
 
-# Dealing with macros
+## Dealing with macros
 
 There are several helpers in [`clippy_utils`][utils] to deal with macros: