Lint #[must_use] attributes applied to methods in trait impls

The `#[must_use]` attribute has no effect when applied to methods in
trait implementations. This case was not linted before.

Author: samueltardieu

Diff for: compiler/rustc_passes/src/check_attr.rs

@@ -1431,37 +1431,48 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
 
     /// Warns against some misuses of `#[must_use]`
     fn check_must_use(&self, hir_id: HirId, attr: &Attribute, target: Target) {
-        if !matches!(
+        if matches!(
             target,
             Target::Fn
                 | Target::Enum
                 | Target::Struct
                 | Target::Union
-                | Target::Method(_)
+                | Target::Method(MethodKind::Trait { body: false } | MethodKind::Inherent)
                 | Target::ForeignFn
                 // `impl Trait` in return position can trip
                 // `unused_must_use` if `Trait` is marked as
                 // `#[must_use]`
                 | Target::Trait
         ) {
-            let article = match target {
-                Target::ExternCrate
-                | Target::Enum
-                | Target::Impl
-                | Target::Expression
-                | Target::Arm
-                | Target::AssocConst
-                | Target::AssocTy => "an",
-                _ => "a",
-            };
+            return;
+        }
 
-            self.tcx.emit_node_span_lint(
-                UNUSED_ATTRIBUTES,
-                hir_id,
-                attr.span,
-                errors::MustUseNoEffect { article, target },
-            );
+        // `#[must_use]` can be applied to a trait method definition with a default body
+        if let Target::Method(MethodKind::Trait { body: true }) = target
+            && let parent_def_id = self.tcx.hir().get_parent_item(hir_id).def_id
+            && let containing_item = self.tcx.hir().expect_item(parent_def_id)
+            && let hir::ItemKind::Trait(..) = containing_item.kind
+        {
+            return;
         }
+
+        let article = match target {
+            Target::ExternCrate
+            | Target::Enum
+            | Target::Impl
+            | Target::Expression
+            | Target::Arm
+            | Target::AssocConst
+            | Target::AssocTy => "an",
+            _ => "a",
+        };
+
+        self.tcx.emit_node_span_lint(
+            UNUSED_ATTRIBUTES,
+            hir_id,
+            attr.span,
+            errors::MustUseNoEffect { article, target },
+        );
     }
 
     /// Checks if `#[must_not_suspend]` is applied to a struct, enum, union, or trait.

Diff for: tests/ui/lint/unused/unused_attributes-must_use.rs

@@ -79,6 +79,11 @@ trait Use {
 #[must_use] //~ ERROR `#[must_use]` has no effect
 impl Use for () {
     type AssocTy = ();
+
+    #[must_use] //~ ERROR `#[must_use]` has no effect
+    fn get_four(&self) -> usize {
+        4
+    }
 }
 
 #[must_use] //~ ERROR `#[must_use]` has no effect

Diff for: tests/ui/lint/unused/unused_attributes-must_use.stderr

@@ -76,43 +76,43 @@ LL | #[must_use]
    | ^^^^^^^^^^^
 
 error: `#[must_use]` has no effect when applied to a trait alias
-  --> $DIR/unused_attributes-must_use.rs:84:1
+  --> $DIR/unused_attributes-must_use.rs:89:1
    |
 LL | #[must_use]
    | ^^^^^^^^^^^
 
 error: `#[must_use]` has no effect when applied to a macro def
-  --> $DIR/unused_attributes-must_use.rs:87:1
+  --> $DIR/unused_attributes-must_use.rs:92:1
    |
 LL | #[must_use]
    | ^^^^^^^^^^^
 
 error: `#[must_use]` has no effect when applied to a statement
-  --> $DIR/unused_attributes-must_use.rs:95:5
+  --> $DIR/unused_attributes-must_use.rs:100:5
    |
 LL |     #[must_use]
    |     ^^^^^^^^^^^
 
 error: `#[must_use]` has no effect when applied to a closure
-  --> $DIR/unused_attributes-must_use.rs:99:13
+  --> $DIR/unused_attributes-must_use.rs:104:13
    |
 LL |     let x = #[must_use]
    |             ^^^^^^^^^^^
 
 error: `#[must_use]` has no effect when applied to an match arm
-  --> $DIR/unused_attributes-must_use.rs:121:9
+  --> $DIR/unused_attributes-must_use.rs:126:9
    |
 LL |         #[must_use]
    |         ^^^^^^^^^^^
 
 error: `#[must_use]` has no effect when applied to a struct field
-  --> $DIR/unused_attributes-must_use.rs:129:28
+  --> $DIR/unused_attributes-must_use.rs:134:28
    |
 LL |     let s = PatternField { #[must_use]  foo: 123 };
    |                            ^^^^^^^^^^^
 
 error: `#[must_use]` has no effect when applied to a pattern field
-  --> $DIR/unused_attributes-must_use.rs:130:24
+  --> $DIR/unused_attributes-must_use.rs:135:24
    |
 LL |     let PatternField { #[must_use] foo } = s;
    |                        ^^^^^^^^^^^
@@ -129,14 +129,20 @@ error: `#[must_use]` has no effect when applied to an associated type
 LL |     #[must_use]
    |     ^^^^^^^^^^^
 
+error: `#[must_use]` has no effect when applied to a provided trait method
+  --> $DIR/unused_attributes-must_use.rs:83:5
+   |
+LL |     #[must_use]
+   |     ^^^^^^^^^^^
+
 error: `#[must_use]` has no effect when applied to a foreign static item
   --> $DIR/unused_attributes-must_use.rs:50:5
    |
 LL |     #[must_use]
    |     ^^^^^^^^^^^
 
 error: unused `X` that must be used
-  --> $DIR/unused_attributes-must_use.rs:103:5
+  --> $DIR/unused_attributes-must_use.rs:108:5
    |
 LL |     X;
    |     ^
@@ -152,7 +158,7 @@ LL |     let _ = X;
    |     +++++++
 
 error: unused `Y` that must be used
-  --> $DIR/unused_attributes-must_use.rs:104:5
+  --> $DIR/unused_attributes-must_use.rs:109:5
    |
 LL |     Y::Z;
    |     ^^^^
@@ -163,7 +169,7 @@ LL |     let _ = Y::Z;
    |     +++++++
 
 error: unused `U` that must be used
-  --> $DIR/unused_attributes-must_use.rs:105:5
+  --> $DIR/unused_attributes-must_use.rs:110:5
    |
 LL |     U { unit: () };
    |     ^^^^^^^^^^^^^^
@@ -174,7 +180,7 @@ LL |     let _ = U { unit: () };
    |     +++++++
 
 error: unused return value of `U::method` that must be used
-  --> $DIR/unused_attributes-must_use.rs:106:5
+  --> $DIR/unused_attributes-must_use.rs:111:5
    |
 LL |     U::method();
    |     ^^^^^^^^^^^
@@ -185,7 +191,7 @@ LL |     let _ = U::method();
    |     +++++++
 
 error: unused return value of `foo` that must be used
-  --> $DIR/unused_attributes-must_use.rs:107:5
+  --> $DIR/unused_attributes-must_use.rs:112:5
    |
 LL |     foo();
    |     ^^^^^
@@ -196,7 +202,7 @@ LL |     let _ = foo();
    |     +++++++
 
 error: unused return value of `foreign_foo` that must be used
-  --> $DIR/unused_attributes-must_use.rs:110:9
+  --> $DIR/unused_attributes-must_use.rs:115:9
    |
 LL |         foreign_foo();
    |         ^^^^^^^^^^^^^
@@ -207,7 +213,7 @@ LL |         let _ = foreign_foo();
    |         +++++++
 
 error: unused return value of `Use::get_four` that must be used
-  --> $DIR/unused_attributes-must_use.rs:118:5
+  --> $DIR/unused_attributes-must_use.rs:123:5
    |
 LL |     ().get_four();
    |     ^^^^^^^^^^^^^
@@ -217,5 +223,5 @@ help: use `let _ = ...` to ignore the resulting value
 LL |     let _ = ().get_four();
    |     +++++++
 
-error: aborting due to 28 previous errors
+error: aborting due to 29 previous errors