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

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.

library/portable-simd/crates/core_simd/src/masks.rs

@@ -401,7 +401,6 @@ where
     LaneCount<N>: SupportedLaneCount,
 {
     #[inline]
-    #[must_use = "method returns a defaulted mask with all elements set to false (0)"]
     fn default() -> Self {
         Self::splat(false)
     }
@@ -413,7 +412,6 @@ where
     LaneCount<N>: SupportedLaneCount,
 {
     #[inline]
-    #[must_use = "method returns a new bool and does not mutate the original value"]
     fn eq(&self, other: &Self) -> bool {
         self.0 == other.0
     }
@@ -425,7 +423,6 @@ where
     LaneCount<N>: SupportedLaneCount,
 {
     #[inline]
-    #[must_use = "method returns a new Ordering and does not mutate the original value"]
     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
         self.0.partial_cmp(&other.0)
     }
@@ -451,7 +448,6 @@ where
 {
     type Output = Self;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitand(self, rhs: Self) -> Self {
         Self(self.0 & rhs.0)
     }
@@ -464,7 +460,6 @@ where
 {
     type Output = Self;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitand(self, rhs: bool) -> Self {
         self & Self::splat(rhs)
     }
@@ -477,7 +472,6 @@ where
 {
     type Output = Mask<T, N>;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitand(self, rhs: Mask<T, N>) -> Mask<T, N> {
         Mask::splat(self) & rhs
     }
@@ -490,7 +484,6 @@ where
 {
     type Output = Self;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitor(self, rhs: Self) -> Self {
         Self(self.0 | rhs.0)
     }
@@ -503,7 +496,6 @@ where
 {
     type Output = Self;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitor(self, rhs: bool) -> Self {
         self | Self::splat(rhs)
     }
@@ -516,7 +508,6 @@ where
 {
     type Output = Mask<T, N>;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitor(self, rhs: Mask<T, N>) -> Mask<T, N> {
         Mask::splat(self) | rhs
     }
@@ -529,7 +520,6 @@ where
 {
     type Output = Self;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitxor(self, rhs: Self) -> Self::Output {
         Self(self.0 ^ rhs.0)
     }
@@ -542,7 +532,6 @@ where
 {
     type Output = Self;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitxor(self, rhs: bool) -> Self::Output {
         self ^ Self::splat(rhs)
     }
@@ -555,7 +544,6 @@ where
 {
     type Output = Mask<T, N>;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitxor(self, rhs: Mask<T, N>) -> Self::Output {
         Mask::splat(self) ^ rhs
     }
@@ -568,7 +556,6 @@ where
 {
     type Output = Mask<T, N>;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn not(self) -> Self::Output {
         Self(!self.0)
     }

library/portable-simd/crates/core_simd/src/masks/full_masks.rs

@@ -21,7 +21,6 @@ where
     LaneCount<N>: SupportedLaneCount,
 {
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn clone(&self) -> Self {
         *self
     }
@@ -252,7 +251,6 @@ where
 {
     type Output = Self;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitand(self, rhs: Self) -> Self {
         // Safety: `self` is an integer vector
         unsafe { Self(core::intrinsics::simd::simd_and(self.0, rhs.0)) }
@@ -266,7 +264,6 @@ where
 {
     type Output = Self;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitor(self, rhs: Self) -> Self {
         // Safety: `self` is an integer vector
         unsafe { Self(core::intrinsics::simd::simd_or(self.0, rhs.0)) }
@@ -280,7 +277,6 @@ where
 {
     type Output = Self;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn bitxor(self, rhs: Self) -> Self {
         // Safety: `self` is an integer vector
         unsafe { Self(core::intrinsics::simd::simd_xor(self.0, rhs.0)) }
@@ -294,7 +290,6 @@ where
 {
     type Output = Self;
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
     fn not(self) -> Self::Output {
         Self::splat(true) ^ self
     }

library/portable-simd/crates/core_simd/src/ops.rs

@@ -135,7 +135,6 @@ macro_rules! for_base_types {
                     type Output = $out;
 
                     #[inline]
-                    #[must_use = "operator returns a new vector without mutating the inputs"]
                     // TODO: only useful for int Div::div, but we hope that this
                     // will essentially always get inlined anyway.
                     #[track_caller]

library/portable-simd/crates/core_simd/src/ops/deref.rs

@@ -18,7 +18,6 @@ macro_rules! deref_lhs {
             type Output = Simd<T, N>;
 
             #[inline]
-            #[must_use = "operator returns a new vector without mutating the inputs"]
             fn $call(self, rhs: $simd) -> Self::Output {
                 (*self).$call(rhs)
             }
@@ -39,7 +38,6 @@ macro_rules! deref_rhs {
             type Output = Simd<T, N>;
 
             #[inline]
-            #[must_use = "operator returns a new vector without mutating the inputs"]
             fn $call(self, rhs: &$simd) -> Self::Output {
                 self.$call(*rhs)
             }
@@ -71,7 +69,6 @@ macro_rules! deref_ops {
                 type Output = $simd;
 
                 #[inline]
-                #[must_use = "operator returns a new vector without mutating the inputs"]
                 fn $call(self, rhs: &'rhs $simd) -> Self::Output {
                     (*self).$call(*rhs)
                 }

library/portable-simd/crates/core_simd/src/ops/unary.rs

@@ -11,7 +11,6 @@ macro_rules! neg {
             type Output = Self;
 
             #[inline]
-            #[must_use = "operator returns a new vector without mutating the input"]
             fn neg(self) -> Self::Output {
                 // Safety: `self` is a signed vector
                 unsafe { core::intrinsics::simd::simd_neg(self) }
@@ -46,7 +45,6 @@ macro_rules! not {
             type Output = Self;
 
             #[inline]
-            #[must_use = "operator returns a new vector without mutating the input"]
             fn not(self) -> Self::Output {
                 self ^ (Simd::splat(!(0 as $scalar)))
             }

library/portable-simd/crates/core_simd/src/simd/num/float.rs

@@ -371,7 +371,6 @@ macro_rules! impl_trait {
             }
 
             #[inline]
-            #[must_use = "method returns a new mask and does not mutate the original value"]
             fn is_normal(self) -> Self::Mask {
                 !(self.abs().simd_eq(Self::splat(0.0)) | self.is_nan() | self.is_subnormal() | self.is_infinite())
             }

src/tools/clippy/clippy_lints/src/infinite_iter.rs

@@ -94,7 +94,6 @@ impl Finiteness {
 }
 
 impl From<bool> for Finiteness {
-    #[must_use]
     fn from(b: bool) -> Self {
         if b { Infinite } else { Finite }
     }

src/tools/clippy/clippy_utils/src/consts.rs

@@ -351,21 +351,18 @@ pub enum FullInt {
 }
 
 impl PartialEq for FullInt {
-    #[must_use]
     fn eq(&self, other: &Self) -> bool {
         self.cmp(other) == Ordering::Equal
     }
 }
 
 impl PartialOrd for FullInt {
-    #[must_use]
     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
         Some(self.cmp(other))
     }
 }
 
 impl Ord for FullInt {
-    #[must_use]
     fn cmp(&self, other: &Self) -> Ordering {
         use FullInt::{S, U};
 

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