Rollup merge of #112614 - lukas-code:apit-unsized-suggestion, r=compiler-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 ) {}
  |
```

Author: GuillaumeGomez

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() {}