Rollup merge of #112614 - lukas-code:apit-unsized-suggestion, r=compi…

…ler-errors

tweak suggestion for argument-position `impl ?Sized`

fixes this invalid suggestion:
```text
help: consider removing the `?Sized` bound to make the type parameter `Sized`
  |
1 - fn foo(_: impl ?Sized) {}
1 + fn foo(_: impl ) {}
  |
```

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -14,8 +14,8 @@ use rustc_errors::{Applicability, Diagnostic, DiagnosticArgValue, IntoDiagnostic
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
-use rustc_hir::WherePredicate;
-use rustc_span::Span;
+use rustc_hir::{PredicateOrigin, WherePredicate};
+use rustc_span::{BytePos, Span};
 use rustc_type_ir::sty::TyKind::*;
 
 impl<'tcx> IntoDiagnosticArg for Ty<'tcx> {
@@ -156,10 +156,11 @@ enum SuggestChangingConstraintsMessage<'a> {
     RestrictBoundFurther,
     RestrictType { ty: &'a str },
     RestrictTypeFurther { ty: &'a str },
-    RemovingQSized,
+    RemoveMaybeUnsized,
+    ReplaceMaybeUnsizedWithSized,
 }
 
-fn suggest_removing_unsized_bound(
+fn suggest_changing_unsized_bound(
     generics: &hir::Generics<'_>,
     suggestions: &mut Vec<(Span, String, SuggestChangingConstraintsMessage<'_>)>,
     param: &hir::GenericParam<'_>,
@@ -183,12 +184,25 @@ fn suggest_removing_unsized_bound(
             if poly.trait_ref.trait_def_id() != def_id {
                 continue;
             }
-            let sp = generics.span_for_bound_removal(where_pos, pos);
-            suggestions.push((
-                sp,
-                String::new(),
-                SuggestChangingConstraintsMessage::RemovingQSized,
-            ));
+            if predicate.origin == PredicateOrigin::ImplTrait && predicate.bounds.len() == 1 {
+                // For `impl ?Sized` with no other bounds, suggest `impl Sized` instead.
+                let bound_span = bound.span();
+                if bound_span.can_be_used_for_suggestions() {
+                    let question_span = bound_span.with_hi(bound_span.lo() + BytePos(1));
+                    suggestions.push((
+                        question_span,
+                        String::new(),
+                        SuggestChangingConstraintsMessage::ReplaceMaybeUnsizedWithSized,
+                    ));
+                }
+            } else {
+                let sp = generics.span_for_bound_removal(where_pos, pos);
+                suggestions.push((
+                    sp,
+                    String::new(),
+                    SuggestChangingConstraintsMessage::RemoveMaybeUnsized,
+                ));
+            }
         }
     }
 }
@@ -238,14 +252,11 @@ pub fn suggest_constraining_type_params<'a>(
         {
             let mut sized_constraints =
                 constraints.extract_if(|(_, def_id)| *def_id == tcx.lang_items().sized_trait());
-            if let Some((constraint, def_id)) = sized_constraints.next() {
+            if let Some((_, def_id)) = sized_constraints.next() {
                 applicability = Applicability::MaybeIncorrect;
 
-                err.span_label(
-                    param.span,
-                    format!("this type parameter needs to be `{}`", constraint),
-                );
-                suggest_removing_unsized_bound(generics, &mut suggestions, param, def_id);
+                err.span_label(param.span, "this type parameter needs to be `Sized`");
+                suggest_changing_unsized_bound(generics, &mut suggestions, param, def_id);
             }
         }
 
@@ -395,9 +406,12 @@ pub fn suggest_constraining_type_params<'a>(
             SuggestChangingConstraintsMessage::RestrictTypeFurther { ty } => {
                 Cow::from(format!("consider further restricting type parameter `{}`", ty))
             }
-            SuggestChangingConstraintsMessage::RemovingQSized => {
+            SuggestChangingConstraintsMessage::RemoveMaybeUnsized => {
                 Cow::from("consider removing the `?Sized` bound to make the type parameter `Sized`")
             }
+            SuggestChangingConstraintsMessage::ReplaceMaybeUnsizedWithSized => {
+                Cow::from("consider replacing `?Sized` with `Sized`")
+            }
         };
 
         err.span_suggestion_verbose(span, msg, suggestion, applicability);

tests/ui/const-generics/const-argument-if-length.full.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/const-argument-if-length.rs:15:12
    |
 LL | pub struct AtLeastByte<T: ?Sized> {
-   |                        - this type parameter needs to be `std::marker::Sized`
+   |                        - this type parameter needs to be `Sized`
 LL |     value: T,
    |            ^ doesn't have a size known at compile-time
    |

tests/ui/const-generics/const-argument-if-length.min.stderr

@@ -11,7 +11,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/const-argument-if-length.rs:15:12
    |
 LL | pub struct AtLeastByte<T: ?Sized> {
-   |                        - this type parameter needs to be `std::marker::Sized`
+   |                        - this type parameter needs to be `Sized`
 LL |     value: T,
    |            ^ doesn't have a size known at compile-time
    |

tests/ui/dst/dst-object-from-unsized-type.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/dst-object-from-unsized-type.rs:8:23
    |
 LL | fn test1<T: ?Sized + Foo>(t: &T) {
-   |          - this type parameter needs to be `std::marker::Sized`
+   |          - this type parameter needs to be `Sized`
 LL |     let u: &dyn Foo = t;
    |                       ^ doesn't have a size known at compile-time
    |
@@ -17,7 +17,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/dst-object-from-unsized-type.rs:13:23
    |
 LL | fn test2<T: ?Sized + Foo>(t: &T) {
-   |          - this type parameter needs to be `std::marker::Sized`
+   |          - this type parameter needs to be `Sized`
 LL |     let v: &dyn Foo = t as &dyn Foo;
    |                       ^ doesn't have a size known at compile-time
    |

tests/ui/generic-associated-types/issue-88287.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `A` cannot be known at compilation tim
   --> $DIR/issue-88287.rs:34:9
    |
 LL | type SearchFutureTy<'f, A, B: 'f>
-   |                         - this type parameter needs to be `std::marker::Sized`
+   |                         - this type parameter needs to be `Sized`
 ...
 LL |         async move { todo!() }
    |         ^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time

tests/ui/offset-of/offset-of-dst-field.stderr

@@ -70,7 +70,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/offset-of-dst-field.rs:50:5
    |
 LL | fn generic_with_maybe_sized<T: ?Sized>() -> usize {
-   |                             - this type parameter needs to be `std::marker::Sized`
+   |                             - this type parameter needs to be `Sized`
 LL |     offset_of!(Delta<T>, z)
    |     ^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |

tests/ui/packed/issue-27060-2.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/issue-27060-2.rs:3:11
    |
 LL | pub struct Bad<T: ?Sized> {
-   |                - this type parameter needs to be `std::marker::Sized`
+   |                - this type parameter needs to be `Sized`
 LL |     data: T,
    |           ^ doesn't have a size known at compile-time
    |

tests/ui/suggestions/adt-param-with-implicit-sized-bound.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/adt-param-with-implicit-sized-bound.rs:25:9
    |
 LL | struct Struct5<T: ?Sized>{
-   |                - this type parameter needs to be `std::marker::Sized`
+   |                - this type parameter needs to be `Sized`
 LL |     _t: X<T>,
    |         ^^^^ doesn't have a size known at compile-time
    |

tests/ui/suggestions/removal-of-multiline-trait-bound-in-where-clause.stderr

@@ -4,7 +4,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
 LL | fn foo<T>(foo: Wrapper<T>)
    |        -       ^^^^^^^^^^ doesn't have a size known at compile-time
    |        |
-   |        this type parameter needs to be `std::marker::Sized`
+   |        this type parameter needs to be `Sized`
    |
 note: required by a bound in `Wrapper`
   --> $DIR/removal-of-multiline-trait-bound-in-where-clause.rs:1:16
@@ -33,7 +33,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
 LL | fn bar<T>(foo: Wrapper<T>)
    |        -       ^^^^^^^^^^ doesn't have a size known at compile-time
    |        |
-   |        this type parameter needs to be `std::marker::Sized`
+   |        this type parameter needs to be `Sized`
    |
 note: required by a bound in `Wrapper`
   --> $DIR/removal-of-multiline-trait-bound-in-where-clause.rs:1:16
@@ -58,7 +58,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
 LL | fn qux<T>(foo: Wrapper<T>)
    |        -       ^^^^^^^^^^ doesn't have a size known at compile-time
    |        |
-   |        this type parameter needs to be `std::marker::Sized`
+   |        this type parameter needs to be `Sized`
    |
 note: required by a bound in `Wrapper`
   --> $DIR/removal-of-multiline-trait-bound-in-where-clause.rs:1:16

tests/ui/trait-bounds/apit-unsized.rs

@@ -0,0 +1,4 @@
+fn foo(_: impl Iterator<Item = i32> + ?Sized) {} //~ ERROR [E0277]
+fn bar(_: impl ?Sized) {} //~ ERROR [E0277]
+
+fn main() {}

tests/ui/trait-bounds/apit-unsized.stderr

@@ -0,0 +1,41 @@
+error[E0277]: the size for values of type `impl Iterator<Item = i32> + ?Sized` cannot be known at compilation time
+  --> $DIR/apit-unsized.rs:1:8
+   |
+LL | fn foo(_: impl Iterator<Item = i32> + ?Sized) {}
+   |        ^  ---------------------------------- this type parameter needs to be `Sized`
+   |        |
+   |        doesn't have a size known at compile-time
+   |
+   = help: unsized fn params are gated as an unstable feature
+help: consider removing the `?Sized` bound to make the type parameter `Sized`
+   |
+LL - fn foo(_: impl Iterator<Item = i32> + ?Sized) {}
+LL + fn foo(_: impl Iterator<Item = i32>) {}
+   |
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL | fn foo(_: &impl Iterator<Item = i32> + ?Sized) {}
+   |           +
+
+error[E0277]: the size for values of type `impl ?Sized` cannot be known at compilation time
+  --> $DIR/apit-unsized.rs:2:8
+   |
+LL | fn bar(_: impl ?Sized) {}
+   |        ^  ----------- this type parameter needs to be `Sized`
+   |        |
+   |        doesn't have a size known at compile-time
+   |
+   = help: unsized fn params are gated as an unstable feature
+help: consider replacing `?Sized` with `Sized`
+   |
+LL - fn bar(_: impl ?Sized) {}
+LL + fn bar(_: impl Sized) {}
+   |
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL | fn bar(_: &impl ?Sized) {}
+   |           +
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/trait-bounds/unsized-bound.stderr

@@ -4,7 +4,7 @@ error[E0277]: the size for values of type `B` cannot be known at compilation tim
 LL | impl<A, B> Trait<(A, B)> for (A, B) where A: ?Sized, B: ?Sized, {}
    |         -                    ^^^^^^ doesn't have a size known at compile-time
    |         |
-   |         this type parameter needs to be `std::marker::Sized`
+   |         this type parameter needs to be `Sized`
    |
    = note: required because it appears within the type `(A, B)`
 note: required by a bound in `Trait`
@@ -28,7 +28,7 @@ error[E0277]: the size for values of type `A` cannot be known at compilation tim
 LL | impl<A, B> Trait<(A, B)> for (A, B) where A: ?Sized, B: ?Sized, {}
    |      -                       ^^^^^^ doesn't have a size known at compile-time
    |      |
-   |      this type parameter needs to be `std::marker::Sized`
+   |      this type parameter needs to be `Sized`
    |
    = note: only the last element of a tuple may have a dynamically sized type
 help: consider removing the `?Sized` bound to make the type parameter `Sized`
@@ -43,7 +43,7 @@ error[E0277]: the size for values of type `C` cannot be known at compilation tim
 LL | impl<A, B: ?Sized, C: ?Sized> Trait<(A, B, C)> for (A, B, C) where A: ?Sized, {}
    |                    -                               ^^^^^^^^^ doesn't have a size known at compile-time
    |                    |
-   |                    this type parameter needs to be `std::marker::Sized`
+   |                    this type parameter needs to be `Sized`
    |
    = note: required because it appears within the type `(A, B, C)`
 note: required by a bound in `Trait`
@@ -65,9 +65,7 @@ error[E0277]: the size for values of type `A` cannot be known at compilation tim
   --> $DIR/unsized-bound.rs:5:52
    |
 LL | impl<A, B: ?Sized, C: ?Sized> Trait<(A, B, C)> for (A, B, C) where A: ?Sized, {}
-   |      -                                             ^^^^^^^^^ doesn't have a size known at compile-time
-   |      |
-   |      this type parameter needs to be `std::marker::Sized`
+   |      - this type parameter needs to be `Sized`     ^^^^^^^^^ doesn't have a size known at compile-time
    |
    = note: only the last element of a tuple may have a dynamically sized type
 help: consider removing the `?Sized` bound to make the type parameter `Sized`
@@ -80,9 +78,7 @@ error[E0277]: the size for values of type `B` cannot be known at compilation tim
   --> $DIR/unsized-bound.rs:5:52
    |
 LL | impl<A, B: ?Sized, C: ?Sized> Trait<(A, B, C)> for (A, B, C) where A: ?Sized, {}
-   |         -                                          ^^^^^^^^^ doesn't have a size known at compile-time
-   |         |
-   |         this type parameter needs to be `std::marker::Sized`
+   |         - this type parameter needs to be `Sized`  ^^^^^^^^^ doesn't have a size known at compile-time
    |
    = note: only the last element of a tuple may have a dynamically sized type
 help: consider removing the `?Sized` bound to make the type parameter `Sized`
@@ -97,7 +93,7 @@ error[E0277]: the size for values of type `B` cannot be known at compilation tim
 LL | impl<A: ?Sized, B: ?Sized> Trait2<(A, B)> for (A, B) {}
    |                 -                             ^^^^^^ doesn't have a size known at compile-time
    |                 |
-   |                 this type parameter needs to be `std::marker::Sized`
+   |                 this type parameter needs to be `Sized`
    |
    = note: required because it appears within the type `(A, B)`
 note: required by a bound in `Trait2`
@@ -121,7 +117,7 @@ error[E0277]: the size for values of type `A` cannot be known at compilation tim
 LL | impl<A: ?Sized, B: ?Sized> Trait2<(A, B)> for (A, B) {}
    |      -                                        ^^^^^^ doesn't have a size known at compile-time
    |      |
-   |      this type parameter needs to be `std::marker::Sized`
+   |      this type parameter needs to be `Sized`
    |
    = note: only the last element of a tuple may have a dynamically sized type
 help: consider removing the `?Sized` bound to make the type parameter `Sized`
@@ -136,7 +132,7 @@ error[E0277]: the size for values of type `A` cannot be known at compilation tim
 LL | impl<A> Trait3<A> for A where A: ?Sized {}
    |      -                ^ doesn't have a size known at compile-time
    |      |
-   |      this type parameter needs to be `std::marker::Sized`
+   |      this type parameter needs to be `Sized`
    |
 note: required by a bound in `Trait3`
   --> $DIR/unsized-bound.rs:13:14
@@ -159,7 +155,7 @@ error[E0277]: the size for values of type `A` cannot be known at compilation tim
 LL | impl<A: ?Sized> Trait4<A> for A {}
    |      -                        ^ doesn't have a size known at compile-time
    |      |
-   |      this type parameter needs to be `std::marker::Sized`
+   |      this type parameter needs to be `Sized`
    |
 note: required by a bound in `Trait4`
   --> $DIR/unsized-bound.rs:16:14
@@ -182,7 +178,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
 LL | impl<X, Y> Trait5<X, Y> for X where X: ?Sized {}
    |      -                      ^ doesn't have a size known at compile-time
    |      |
-   |      this type parameter needs to be `std::marker::Sized`
+   |      this type parameter needs to be `Sized`
    |
 note: required by a bound in `Trait5`
   --> $DIR/unsized-bound.rs:19:14
@@ -205,7 +201,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
 LL | impl<X: ?Sized, Y> Trait6<X, Y> for X {}
    |      -                              ^ doesn't have a size known at compile-time
    |      |
-   |      this type parameter needs to be `std::marker::Sized`
+   |      this type parameter needs to be `Sized`
    |
 note: required by a bound in `Trait6`
   --> $DIR/unsized-bound.rs:22:14
@@ -228,7 +224,7 @@ error[E0277]: the size for values of type `Y` cannot be known at compilation tim
 LL | impl<X, Y> Trait7<X, Y> for X where Y: ?Sized {}
    |         -  ^^^^^^^^^^^^ doesn't have a size known at compile-time
    |         |
-   |         this type parameter needs to be `std::marker::Sized`
+   |         this type parameter needs to be `Sized`
    |
 note: required by a bound in `Trait7`
   --> $DIR/unsized-bound.rs:25:17
@@ -251,7 +247,7 @@ error[E0277]: the size for values of type `Y` cannot be known at compilation tim
 LL | impl<X, Y: ?Sized> Trait8<X, Y> for X {}
    |         -          ^^^^^^^^^^^^ doesn't have a size known at compile-time
    |         |
-   |         this type parameter needs to be `std::marker::Sized`
+   |         this type parameter needs to be `Sized`
    |
 note: required by a bound in `Trait8`
   --> $DIR/unsized-bound.rs:28:17

tests/ui/traits/suggest-where-clause.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `U` cannot be known at compilation tim
   --> $DIR/suggest-where-clause.rs:7:20
    |
 LL | fn check<T: Iterator, U: ?Sized>() {
-   |                       - this type parameter needs to be `std::marker::Sized`
+   |                       - this type parameter needs to be `Sized`
 LL |     // suggest a where-clause, if needed
 LL |     mem::size_of::<U>();
    |                    ^ doesn't have a size known at compile-time
@@ -19,7 +19,7 @@ error[E0277]: the size for values of type `U` cannot be known at compilation tim
   --> $DIR/suggest-where-clause.rs:10:20
    |
 LL | fn check<T: Iterator, U: ?Sized>() {
-   |                       - this type parameter needs to be `std::marker::Sized`
+   |                       - this type parameter needs to be `Sized`
 ...
 LL |     mem::size_of::<Misc<U>>();
    |                    ^^^^^^^ doesn't have a size known at compile-time