return_self_not_must_use document #[must_use] on the type

Inspired by a discussion in rust-lang/rust-clippy#8197

---

r? `@llogiq`

changelog: none

The lint is this on nightly, therefore no changelog entry for you xD

clippy_lints/src/return_self_not_must_use.rs

@@ -1,5 +1,6 @@
+use clippy_utils::diagnostics::span_lint_and_help;
 use clippy_utils::ty::is_must_use_ty;
-use clippy_utils::{diagnostics::span_lint, nth_arg, return_ty};
+use clippy_utils::{nth_arg, return_ty};
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::intravisit::FnKind;
 use rustc_hir::{Body, FnDecl, HirId, TraitItem, TraitItemKind};
@@ -13,25 +14,46 @@ declare_clippy_lint! {
     /// This lint warns when a method returning `Self` doesn't have the `#[must_use]` attribute.
     ///
     /// ### Why is this bad?
-    /// It prevents to "forget" to use the newly created value.
+    /// Methods returning `Self` often create new values, having the `#[must_use]` attribute
+    /// prevents users from "forgetting" to use the newly created value.
+    ///
+    /// The `#[must_use]` attribute can be added to the type itself to ensure that instances
+    /// are never forgotten. Functions returning a type marked with `#[must_use]` will not be
+    /// linted, as the usage is already enforced by the type attribute.
     ///
     /// ### Limitations
     /// This lint is only applied on methods taking a `self` argument. It would be mostly noise
     /// if it was added on constructors for example.
     ///
     /// ### Example
+    /// Missing attribute
     /// ```rust
     /// pub struct Bar;
-    ///
     /// impl Bar {
     ///     // Bad
     ///     pub fn bar(&self) -> Self {
     ///         Self
     ///     }
+    /// }
+    /// ```
     ///
-    ///     // Good
+    /// It's better to have the `#[must_use]` attribute on the method like this:
+    /// ```rust
+    /// pub struct Bar;
+    /// impl Bar {
     ///     #[must_use]
-    ///     pub fn foo(&self) -> Self {
+    ///     pub fn bar(&self) -> Self {
+    ///         Self
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// Or on the type definition like this:
+    /// ```rust
+    /// #[must_use]
+    /// pub struct Bar;
+    /// impl Bar {
+    ///     pub fn bar(&self) -> Self {
     ///         Self
     ///     }
     /// }
@@ -65,11 +87,13 @@ fn check_method(cx: &LateContext<'tcx>, decl: &'tcx FnDecl<'tcx>, fn_def: LocalD
         if !is_must_use_ty(cx, ret_ty);
 
         then {
-            span_lint(
+            span_lint_and_help(
                 cx,
                 RETURN_SELF_NOT_MUST_USE,
                 span,
                 "missing `#[must_use]` attribute on a method returning `Self`",
+                None,
+                "consider adding the `#[must_use]` attribute to the method or directly to the `Self` type"
             );
         }
     }

tests/ui/return_self_not_must_use.stderr

@@ -5,6 +5,7 @@ LL |     fn what(&self) -> Self;
    |     ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::return-self-not-must-use` implied by `-D warnings`
+   = help: consider adding the `#[must_use]` attribute to the method or directly to the `Self` type
 
 error: missing `#[must_use]` attribute on a method returning `Self`
   --> $DIR/return_self_not_must_use.rs:17:5
@@ -13,6 +14,8 @@ LL | /     pub fn foo(&self) -> Self {
 LL | |         Self
 LL | |     }
    | |_____^
+   |
+   = help: consider adding the `#[must_use]` attribute to the method or directly to the `Self` type
 
 error: missing `#[must_use]` attribute on a method returning `Self`
   --> $DIR/return_self_not_must_use.rs:20:5
@@ -21,6 +24,8 @@ LL | /     pub fn bar(self) -> Self {
 LL | |         self
 LL | |     }
    | |_____^
+   |
+   = help: consider adding the `#[must_use]` attribute to the method or directly to the `Self` type
 
 error: aborting due to 3 previous errors