Tweak diagnostics for relaxed bounds in invalid positions

fmease · fmease · commit 690a8a6dfd7a · 2025-10-16T19:53:29.000+02:00
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
@@ -2103,33 +2103,33 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 }
             }
             RelaxedBoundPolicy::Forbidden(reason) => {
+                let gate = |context, subject| {
+                    if self.tcx.features().more_maybe_bounds() {
+                        return;
+                    }
+
+                    let mut diag = self.dcx().struct_span_err(
+                        span,
+                        format!("relaxed bounds are not permitted in {context}"),
+                    );
+                    if let Some(def_id) = trait_ref.trait_def_id()
+                        && self.tcx.is_lang_item(def_id, hir::LangItem::Sized)
+                    {
+                        diag.note(format!(
+                            "{subject} are not implicitly bounded by `Sized`, \
+                             so there is nothing to relax"
+                        ));
+                    }
+                    diag.emit();
+                };
+
                 match reason {
                     RelaxedBoundForbiddenReason::TraitObjectTy => {
-                        if self.tcx.features().more_maybe_bounds() {
-                            return;
-                        }
-
-                        self.dcx().span_err(
-                            span,
-                            "relaxed bounds are not permitted in trait object types",
-                        );
+                        gate("trait object types", "trait object types");
                         return;
                     }
                     RelaxedBoundForbiddenReason::SuperTrait => {
-                        if self.tcx.features().more_maybe_bounds() {
-                            return;
-                        }
-
-                        let mut diag = self.dcx().struct_span_err(
-                            span,
-                            "relaxed bounds are not permitted in supertrait bounds",
-                        );
-                        if let Some(def_id) = trait_ref.trait_def_id()
-                            && self.tcx.is_lang_item(def_id, hir::LangItem::Sized)
-                        {
-                            diag.note("traits are `?Sized` by default");
-                        }
-                        diag.emit();
+                        gate("supertrait bounds", "traits");
                         return;
                     }
                     RelaxedBoundForbiddenReason::AssocTyBounds
@@ -2142,7 +2142,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             .struct_span_err(span, "this relaxed bound is not permitted here")
             .with_note(
                 "in this context, relaxed bounds are only allowed on \
-                 type parameters defined by the closest item",
+                 type parameters defined on the closest item",
             )
             .emit();
     }
diff --git a/tests/ui/feature-gates/feature-gate-more-maybe-bounds.stderr b/tests/ui/feature-gates/feature-gate-more-maybe-bounds.stderr
@@ -10,7 +10,7 @@ error: this relaxed bound is not permitted here
 LL | trait Trait4 where Self: ?Trait1 {}
    |                          ^^^^^^^
    |
-   = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item
+   = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item
 
 error: relaxed bounds are not permitted in trait object types
   --> $DIR/feature-gate-more-maybe-bounds.rs:8:28
diff --git a/tests/ui/parser/trait-object-trait-parens.stderr b/tests/ui/parser/trait-object-trait-parens.stderr
@@ -3,18 +3,24 @@ error: relaxed bounds are not permitted in trait object types
    |
 LL |     let _: Box<(Obj) + (?Sized) + (for<'a> Trait<'a>)>;
    |                        ^^^^^^^^
+   |
+   = note: trait object types are not implicitly bounded by `Sized`, so there is nothing to relax
 
 error: relaxed bounds are not permitted in trait object types
   --> $DIR/trait-object-trait-parens.rs:13:16
    |
 LL |     let _: Box<?Sized + (for<'a> Trait<'a>) + (Obj)>;
    |                ^^^^^^
+   |
+   = note: trait object types are not implicitly bounded by `Sized`, so there is nothing to relax
 
 error: relaxed bounds are not permitted in trait object types
   --> $DIR/trait-object-trait-parens.rs:18:44
    |
 LL |     let _: Box<for<'a> Trait<'a> + (Obj) + (?Sized)>;
    |                                            ^^^^^^^^
+   |
+   = note: trait object types are not implicitly bounded by `Sized`, so there is nothing to relax
 
 warning: trait objects without an explicit `dyn` are deprecated
   --> $DIR/trait-object-trait-parens.rs:8:16
diff --git a/tests/ui/sized-hierarchy/default-supertrait.stderr b/tests/ui/sized-hierarchy/default-supertrait.stderr
@@ -4,7 +4,7 @@ error: relaxed bounds are not permitted in supertrait bounds
 LL | trait NegSized: ?Sized { }
    |                 ^^^^^^
    |
-   = note: traits are `?Sized` by default
+   = note: traits are not implicitly bounded by `Sized`, so there is nothing to relax
 
 error: relaxed bounds are not permitted in supertrait bounds
   --> $DIR/default-supertrait.rs:13:21
diff --git a/tests/ui/trait-bounds/more_maybe_bounds.stderr b/tests/ui/trait-bounds/more_maybe_bounds.stderr
@@ -4,23 +4,23 @@ error: this relaxed bound is not permitted here
 LL | fn baz<T>() where T: Iterator<Item: ?Trait1> {}
    |                                     ^^^^^^^
    |
-   = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item
+   = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item
 
 error: this relaxed bound is not permitted here
   --> $DIR/more_maybe_bounds.rs:29:21
    |
 LL |     fn f() where T: ?Trait1 {}
    |                     ^^^^^^^
    |
-   = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item
+   = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item
 
 error: this relaxed bound is not permitted here
   --> $DIR/more_maybe_bounds.rs:35:34
    |
 LL | struct S2<T>(T) where for<'a> T: ?Trait5<'a>;
    |                                  ^^^^^^^^^^^
    |
-   = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item
+   = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item
 
 error: bound modifier `?` can only be applied to default traits like `Sized`
   --> $DIR/more_maybe_bounds.rs:17:20
diff --git a/tests/ui/traits/wf-object/only-maybe-bound.stderr b/tests/ui/traits/wf-object/only-maybe-bound.stderr
@@ -3,6 +3,8 @@ error: relaxed bounds are not permitted in trait object types
    |
 LL | type _0 = dyn ?Sized;
    |               ^^^^^^
+   |
+   = note: trait object types are not implicitly bounded by `Sized`, so there is nothing to relax
 
 error[E0224]: at least one trait is required for an object type
   --> $DIR/only-maybe-bound.rs:3:11
diff --git a/tests/ui/unsized/relaxed-bounds-invalid-places.stderr b/tests/ui/unsized/relaxed-bounds-invalid-places.stderr
@@ -4,73 +4,79 @@ error: this relaxed bound is not permitted here
 LL | struct S1<T>(T) where (T): ?Sized;
    |                            ^^^^^^
    |
-   = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item
+   = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item
 
 error: this relaxed bound is not permitted here
   --> $DIR/relaxed-bounds-invalid-places.rs:8:27
    |
 LL | struct S2<T>(T) where u8: ?Sized;
    |                           ^^^^^^
    |
-   = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item
+   = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item
 
 error: this relaxed bound is not permitted here
   --> $DIR/relaxed-bounds-invalid-places.rs:10:35
    |
 LL | struct S3<T>(T) where &'static T: ?Sized;
    |                                   ^^^^^^
    |
-   = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item
+   = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item
 
 error: this relaxed bound is not permitted here
   --> $DIR/relaxed-bounds-invalid-places.rs:14:34
    |
 LL | struct S4<T>(T) where for<'a> T: ?Trait<'a>;
    |                                  ^^^^^^^^^^
    |
-   = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item
+   = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item
 
 error: this relaxed bound is not permitted here
   --> $DIR/relaxed-bounds-invalid-places.rs:22:21
    |
 LL |     fn f() where T: ?Sized {}
    |                     ^^^^^^
    |
-   = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item
+   = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item
 
 error: this relaxed bound is not permitted here
   --> $DIR/relaxed-bounds-invalid-places.rs:27:41
    |
 LL | struct S6<T>(T) where T: Iterator<Item: ?Sized>;
    |                                         ^^^^^^
    |
-   = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item
+   = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item
 
 error: relaxed bounds are not permitted in supertrait bounds
   --> $DIR/relaxed-bounds-invalid-places.rs:29:11
    |
 LL | trait Tr: ?Sized {}
    |           ^^^^^^
    |
-   = note: traits are `?Sized` by default
+   = note: traits are not implicitly bounded by `Sized`, so there is nothing to relax
 
 error: relaxed bounds are not permitted in trait object types
   --> $DIR/relaxed-bounds-invalid-places.rs:33:20
    |
 LL | type O1 = dyn Tr + ?Sized;
    |                    ^^^^^^
+   |
+   = note: trait object types are not implicitly bounded by `Sized`, so there is nothing to relax
 
 error: relaxed bounds are not permitted in trait object types
   --> $DIR/relaxed-bounds-invalid-places.rs:34:15
    |
 LL | type O2 = dyn ?Sized + ?Sized + Tr;
    |               ^^^^^^
+   |
+   = note: trait object types are not implicitly bounded by `Sized`, so there is nothing to relax
 
 error: relaxed bounds are not permitted in trait object types
   --> $DIR/relaxed-bounds-invalid-places.rs:34:24
    |
 LL | type O2 = dyn ?Sized + ?Sized + Tr;
    |                        ^^^^^^
+   |
+   = note: trait object types are not implicitly bounded by `Sized`, so there is nothing to relax
 
 error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/relaxed-bounds-invalid-places.rs:14:34

Original file line number	Diff line number	Diff line change
`@@ -10,7 +10,7 @@ error: this relaxed bound is not permitted here`
`10`	`10`	`LL \| trait Trait4 where Self: ?Trait1 {}`
`11`	`11`	`\| ^^^^^^^`
`12`	`12`	`\|`
`13`		`- = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item`
	`13`	`+ = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item`
`14`	`14`
`15`	`15`	`error: relaxed bounds are not permitted in trait object types`
`16`	`16`	`--> $DIR/feature-gate-more-maybe-bounds.rs:8:28`
Original file line number	Diff line number	Diff line change
`@@ -4,7 +4,7 @@ error: relaxed bounds are not permitted in supertrait bounds`
`4`	`4`	`LL \| trait NegSized: ?Sized { }`
`5`	`5`	`\| ^^^^^^`
`6`	`6`	`\|`
`7`		- = note: traits are `?Sized` by default
	`7`	+ = note: traits are not implicitly bounded by `Sized`, so there is nothing to relax
`8`	`8`
`9`	`9`	`error: relaxed bounds are not permitted in supertrait bounds`
`10`	`10`	`--> $DIR/default-supertrait.rs:13:21`
Original file line number	Diff line number	Diff line change
`@@ -4,23 +4,23 @@ error: this relaxed bound is not permitted here`
`4`	`4`	`LL \| fn baz<T>() where T: Iterator<Item: ?Trait1> {}`
`5`	`5`	`\| ^^^^^^^`
`6`	`6`	`\|`
`7`		`- = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item`
	`7`	`+ = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item`
`8`	`8`
`9`	`9`	`error: this relaxed bound is not permitted here`
`10`	`10`	`--> $DIR/more_maybe_bounds.rs:29:21`
`11`	`11`	`\|`
`12`	`12`	`LL \| fn f() where T: ?Trait1 {}`
`13`	`13`	`\| ^^^^^^^`
`14`	`14`	`\|`
`15`		`- = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item`
	`15`	`+ = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item`
`16`	`16`
`17`	`17`	`error: this relaxed bound is not permitted here`
`18`	`18`	`--> $DIR/more_maybe_bounds.rs:35:34`
`19`	`19`	`\|`
`20`	`20`	`LL \| struct S2<T>(T) where for<'a> T: ?Trait5<'a>;`
`21`	`21`	`\| ^^^^^^^^^^^`
`22`	`22`	`\|`
`23`		`- = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item`
	`23`	`+ = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item`
`24`	`24`
`25`	`25`	error: bound modifier `?` can only be applied to default traits like `Sized`
`26`	`26`	`--> $DIR/more_maybe_bounds.rs:17:20`
Original file line number	Diff line number	Diff line change
`@@ -3,6 +3,8 @@ error: relaxed bounds are not permitted in trait object types`
`3`	`3`	`\|`
`4`	`4`	`LL \| type _0 = dyn ?Sized;`
`5`	`5`	`\| ^^^^^^`
	`6`	`+ \|`
	`7`	+ = note: trait object types are not implicitly bounded by `Sized`, so there is nothing to relax
`6`	`8`
`7`	`9`	`error[E0224]: at least one trait is required for an object type`
`8`	`10`	`--> $DIR/only-maybe-bound.rs:3:11`
Original file line number	Diff line number	Diff line change
`@@ -4,73 +4,79 @@ error: this relaxed bound is not permitted here`
`4`	`4`	`LL \| struct S1<T>(T) where (T): ?Sized;`
`5`	`5`	`\| ^^^^^^`
`6`	`6`	`\|`
`7`		`- = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item`
	`7`	`+ = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item`
`8`	`8`
`9`	`9`	`error: this relaxed bound is not permitted here`
`10`	`10`	`--> $DIR/relaxed-bounds-invalid-places.rs:8:27`
`11`	`11`	`\|`
`12`	`12`	`LL \| struct S2<T>(T) where u8: ?Sized;`
`13`	`13`	`\| ^^^^^^`
`14`	`14`	`\|`
`15`		`- = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item`
	`15`	`+ = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item`
`16`	`16`
`17`	`17`	`error: this relaxed bound is not permitted here`
`18`	`18`	`--> $DIR/relaxed-bounds-invalid-places.rs:10:35`
`19`	`19`	`\|`
`20`	`20`	`LL \| struct S3<T>(T) where &'static T: ?Sized;`
`21`	`21`	`\| ^^^^^^`
`22`	`22`	`\|`
`23`		`- = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item`
	`23`	`+ = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item`
`24`	`24`
`25`	`25`	`error: this relaxed bound is not permitted here`
`26`	`26`	`--> $DIR/relaxed-bounds-invalid-places.rs:14:34`
`27`	`27`	`\|`
`28`	`28`	`LL \| struct S4<T>(T) where for<'a> T: ?Trait<'a>;`
`29`	`29`	`\| ^^^^^^^^^^`
`30`	`30`	`\|`
`31`		`- = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item`
	`31`	`+ = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item`
`32`	`32`
`33`	`33`	`error: this relaxed bound is not permitted here`
`34`	`34`	`--> $DIR/relaxed-bounds-invalid-places.rs:22:21`
`35`	`35`	`\|`
`36`	`36`	`LL \| fn f() where T: ?Sized {}`
`37`	`37`	`\| ^^^^^^`
`38`	`38`	`\|`
`39`		`- = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item`
	`39`	`+ = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item`
`40`	`40`
`41`	`41`	`error: this relaxed bound is not permitted here`
`42`	`42`	`--> $DIR/relaxed-bounds-invalid-places.rs:27:41`
`43`	`43`	`\|`
`44`	`44`	`LL \| struct S6<T>(T) where T: Iterator<Item: ?Sized>;`
`45`	`45`	`\| ^^^^^^`
`46`	`46`	`\|`
`47`		`- = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item`
	`47`	`+ = note: in this context, relaxed bounds are only allowed on type parameters defined on the closest item`
`48`	`48`
`49`	`49`	`error: relaxed bounds are not permitted in supertrait bounds`
`50`	`50`	`--> $DIR/relaxed-bounds-invalid-places.rs:29:11`
`51`	`51`	`\|`
`52`	`52`	`LL \| trait Tr: ?Sized {}`
`53`	`53`	`\| ^^^^^^`
`54`	`54`	`\|`
`55`		- = note: traits are `?Sized` by default
	`55`	+ = note: traits are not implicitly bounded by `Sized`, so there is nothing to relax
`56`	`56`
`57`	`57`	`error: relaxed bounds are not permitted in trait object types`
`58`	`58`	`--> $DIR/relaxed-bounds-invalid-places.rs:33:20`
`59`	`59`	`\|`
`60`	`60`	`LL \| type O1 = dyn Tr + ?Sized;`
`61`	`61`	`\| ^^^^^^`
	`62`	`+ \|`
	`63`	+ = note: trait object types are not implicitly bounded by `Sized`, so there is nothing to relax
`62`	`64`
`63`	`65`	`error: relaxed bounds are not permitted in trait object types`
`64`	`66`	`--> $DIR/relaxed-bounds-invalid-places.rs:34:15`
`65`	`67`	`\|`
`66`	`68`	`LL \| type O2 = dyn ?Sized + ?Sized + Tr;`
`67`	`69`	`\| ^^^^^^`
	`70`	`+ \|`
	`71`	+ = note: trait object types are not implicitly bounded by `Sized`, so there is nothing to relax
`68`	`72`
`69`	`73`	`error: relaxed bounds are not permitted in trait object types`
`70`	`74`	`--> $DIR/relaxed-bounds-invalid-places.rs:34:24`
`71`	`75`	`\|`
`72`	`76`	`LL \| type O2 = dyn ?Sized + ?Sized + Tr;`
`73`	`77`	`\| ^^^^^^`
	`78`	`+ \|`
	`79`	+ = note: trait object types are not implicitly bounded by `Sized`, so there is nothing to relax
`74`	`80`
`75`	`81`	error: bound modifier `?` can only be applied to `Sized`
`76`	`82`	`--> $DIR/relaxed-bounds-invalid-places.rs:14:34`