rust-lang · rust-bors · Jan 29, 2026 · Jan 29, 2026
diff --git a/compiler/rustc_attr_parsing/src/target_checking.rs b/compiler/rustc_attr_parsing/src/target_checking.rs
@@ -205,7 +205,7 @@ pub(crate) fn allowed_targets_applied(
     ];
     const IMPL_LIKE: &[Target] =
         &[Target::Impl { of_trait: false }, Target::Impl { of_trait: true }];
-    const ADT_LIKE: &[Target] = &[Target::Struct, Target::Enum];
+    const ADT_LIKE: &[Target] = &[Target::Struct, Target::Enum, Target::Union];
 
     let mut added_fake_targets = Vec::new();
     filter_targets(

diff --git a/tests/ui/attributes/attr-on-mac-call.stderr b/tests/ui/attributes/attr-on-mac-call.stderr
@@ -55,7 +55,7 @@ LL |     #[deprecated]
    |     ^^^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[deprecated]` can be applied to associated consts, associated types, constants, crates, data types, enum variants, foreign statics, functions, inherent impl blocks, macro defs, modules, statics, struct fields, traits, type aliases, unions, and use statements
+   = help: `#[deprecated]` can be applied to associated consts, associated types, constants, crates, data types, enum variants, foreign statics, functions, inherent impl blocks, macro defs, modules, statics, struct fields, traits, type aliases, and use statements
 
 warning: `#[inline]` attribute cannot be used on macro calls
   --> $DIR/attr-on-mac-call.rs:24:5
@@ -136,7 +136,7 @@ LL |     #[deprecated]
    |     ^^^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[deprecated]` can be applied to associated consts, associated types, constants, crates, data types, enum variants, foreign statics, functions, inherent impl blocks, macro defs, modules, statics, struct fields, traits, type aliases, unions, and use statements
+   = help: `#[deprecated]` can be applied to associated consts, associated types, constants, crates, data types, enum variants, foreign statics, functions, inherent impl blocks, macro defs, modules, statics, struct fields, traits, type aliases, and use statements
 
 warning: `#[automatically_derived]` attribute cannot be used on macro calls
   --> $DIR/attr-on-mac-call.rs:51:5
@@ -163,7 +163,7 @@ LL |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 warning: `#[no_implicit_prelude]` attribute cannot be used on macro calls
   --> $DIR/attr-on-mac-call.rs:60:5

diff --git a/tests/ui/deprecation/deprecated-expr-precedence.stderr b/tests/ui/deprecation/deprecated-expr-precedence.stderr
@@ -5,7 +5,7 @@ LL |     #[deprecated] 0
    |     ^^^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[deprecated]` can be applied to associated consts, associated types, constants, crates, data types, enum variants, foreign statics, functions, inherent impl blocks, macro defs, modules, statics, struct fields, traits, type aliases, unions, and use statements
+   = help: `#[deprecated]` can be applied to associated consts, associated types, constants, crates, data types, enum variants, foreign statics, functions, inherent impl blocks, macro defs, modules, statics, struct fields, traits, type aliases, and use statements
    = note: requested on the command line with `-W unused-attributes`
 
 error[E0308]: mismatched types

diff --git a/tests/ui/deprecation/deprecation-sanity.stderr b/tests/ui/deprecation/deprecation-sanity.stderr
@@ -83,7 +83,7 @@ LL | #[deprecated = "hello"]
    | ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[deprecated]` can be applied to associated consts, associated types, constants, crates, data types, enum variants, foreign statics, functions, inherent impl blocks, macro defs, modules, statics, struct fields, traits, type aliases, unions, and use statements
+   = help: `#[deprecated]` can be applied to associated consts, associated types, constants, crates, data types, enum variants, foreign statics, functions, inherent impl blocks, macro defs, modules, statics, struct fields, traits, type aliases, and use statements
    = note: `#[deny(useless_deprecated)]` on by default
 
 error: aborting due to 10 previous errors

diff --git a/tests/ui/feature-gates/issue-43106-gating-of-builtin-attrs.stderr b/tests/ui/feature-gates/issue-43106-gating-of-builtin-attrs.stderr
@@ -1055,7 +1055,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 warning: `#[must_use]` attribute cannot be used on modules
   --> $DIR/issue-43106-gating-of-builtin-attrs.rs:766:17
@@ -1064,7 +1064,7 @@ LL |     mod inner { #![must_use] }
    |                 ^^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 warning: `#[must_use]` attribute cannot be used on type aliases
   --> $DIR/issue-43106-gating-of-builtin-attrs.rs:775:5
@@ -1073,7 +1073,7 @@ LL |     #[must_use] type T = S;
    |     ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 warning: `#[must_use]` attribute cannot be used on inherent impl blocks
   --> $DIR/issue-43106-gating-of-builtin-attrs.rs:780:5
@@ -1082,7 +1082,7 @@ LL |     #[must_use] impl S { }
    |     ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 warning: crate-level attribute should be an inner attribute: add an exclamation mark: `#![windows_subsystem]`
   --> $DIR/issue-43106-gating-of-builtin-attrs.rs:786:1
@@ -1635,7 +1635,7 @@ LL | #![must_use]
    | ^^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 warning: 175 warnings emitted
 
diff --git a/tests/ui/lint/fn_must_use.stderr b/tests/ui/lint/fn_must_use.stderr
@@ -5,7 +5,7 @@ LL |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, foreign functions, functions, inherent methods, provided trait methods, required trait methods, traits, and unions
+   = help: `#[must_use]` can be applied to data types, foreign functions, functions, inherent methods, provided trait methods, required trait methods, and traits
    = note: requested on the command line with `-W unused-attributes`
 
 warning: unused return value of `need_to_use_this_value` that must be used

diff --git a/tests/ui/lint/must_not_suspend/other_items.stderr b/tests/ui/lint/must_not_suspend/other_items.stderr
@@ -4,7 +4,7 @@ error: `#[must_not_suspend]` attribute cannot be used on modules
 LL | #[must_not_suspend]
    | ^^^^^^^^^^^^^^^^^^^
    |
-   = help: `#[must_not_suspend]` can be applied to data types, traits, and unions
+   = help: `#[must_not_suspend]` can be applied to data types and traits
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/lint/must_not_suspend/return.stderr b/tests/ui/lint/must_not_suspend/return.stderr
@@ -4,7 +4,7 @@ error: `#[must_not_suspend]` attribute cannot be used on functions
 LL | #[must_not_suspend]
    | ^^^^^^^^^^^^^^^^^^^
    |
-   = help: `#[must_not_suspend]` can be applied to data types, traits, and unions
+   = help: `#[must_not_suspend]` can be applied to data types and traits
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/lint/unused/unused_attributes-must_use.stderr b/tests/ui/lint/unused/unused_attributes-must_use.stderr
@@ -5,7 +5,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 note: the lint level is defined here
   --> $DIR/unused_attributes-must_use.rs:4:9
    |
@@ -19,7 +19,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on modules
   --> $DIR/unused_attributes-must_use.rs:11:1
@@ -28,7 +28,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on use statements
   --> $DIR/unused_attributes-must_use.rs:15:1
@@ -37,7 +37,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on constants
   --> $DIR/unused_attributes-must_use.rs:19:1
@@ -46,7 +46,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on statics
   --> $DIR/unused_attributes-must_use.rs:22:1
@@ -55,7 +55,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on inherent impl blocks
   --> $DIR/unused_attributes-must_use.rs:40:1
@@ -64,7 +64,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on foreign modules
   --> $DIR/unused_attributes-must_use.rs:55:1
@@ -73,7 +73,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on foreign statics
   --> $DIR/unused_attributes-must_use.rs:59:5
@@ -82,7 +82,7 @@ LL |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on type aliases
   --> $DIR/unused_attributes-must_use.rs:73:1
@@ -91,7 +91,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on type parameters
   --> $DIR/unused_attributes-must_use.rs:77:8
@@ -100,7 +100,7 @@ LL | fn qux<#[must_use] T>(_: T) {}
    |        ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on associated consts
   --> $DIR/unused_attributes-must_use.rs:82:5
@@ -109,7 +109,7 @@ LL |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on associated types
   --> $DIR/unused_attributes-must_use.rs:85:5
@@ -118,7 +118,7 @@ LL |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on trait impl blocks
   --> $DIR/unused_attributes-must_use.rs:95:1
@@ -127,7 +127,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on trait methods in impl blocks
   --> $DIR/unused_attributes-must_use.rs:100:5
@@ -136,7 +136,7 @@ LL |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, foreign functions, functions, inherent methods, provided trait methods, required trait methods, traits, and unions
+   = help: `#[must_use]` can be applied to data types, foreign functions, functions, inherent methods, provided trait methods, required trait methods, and traits
 
 error: `#[must_use]` attribute cannot be used on trait aliases
   --> $DIR/unused_attributes-must_use.rs:107:1
@@ -145,7 +145,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on macro defs
   --> $DIR/unused_attributes-must_use.rs:111:1
@@ -154,7 +154,7 @@ LL | #[must_use]
    | ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on statements
   --> $DIR/unused_attributes-must_use.rs:120:5
@@ -163,7 +163,7 @@ LL |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on closures
   --> $DIR/unused_attributes-must_use.rs:125:13
@@ -172,7 +172,7 @@ LL |     let x = #[must_use]
    |             ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, foreign functions, functions, methods, traits, and unions
+   = help: `#[must_use]` can be applied to data types, foreign functions, functions, methods, and traits
 
 error: `#[must_use]` attribute cannot be used on match arms
   --> $DIR/unused_attributes-must_use.rs:148:9
@@ -181,7 +181,7 @@ LL |         #[must_use]
    |         ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on struct fields
   --> $DIR/unused_attributes-must_use.rs:157:28
@@ -190,7 +190,7 @@ LL |     let s = PatternField { #[must_use]  foo: 123 };
    |                            ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: `#[must_use]` attribute cannot be used on pattern fields
   --> $DIR/unused_attributes-must_use.rs:159:24
@@ -199,7 +199,7 @@ LL |     let PatternField { #[must_use] foo } = s;
    |                        ^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = help: `#[must_use]` can be applied to data types, functions, traits, and unions
+   = help: `#[must_use]` can be applied to data types, functions, and traits
 
 error: unused `X` that must be used
   --> $DIR/unused_attributes-must_use.rs:130:5