Account for ?Sized type parameter bounds

Author: estebank

src/librustc/traits/error_reporting/suggestions.rs

@@ -145,12 +145,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     let param_name = self_ty.to_string();
                     let constraint = trait_ref.print_only_trait_path().to_string();
                     if suggest_constraining_type_param(
+                        self.tcx,
                         generics,
                         &mut err,
                         &param_name,
                         &constraint,
                         self.tcx.sess.source_map(),
                         *span,
+                        Some(trait_ref.def_id()),
                     ) {
                         return;
                     }
@@ -1652,18 +1654,26 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
 /// Suggest restricting a type param with a new bound.
 pub fn suggest_constraining_type_param(
+    tcx: TyCtxt<'_>,
     generics: &hir::Generics<'_>,
     err: &mut DiagnosticBuilder<'_>,
     param_name: &str,
     constraint: &str,
     source_map: &SourceMap,
     span: Span,
+    def_id: Option<DefId>,
 ) -> bool {
     let restrict_msg = "consider further restricting this bound";
     if let Some(param) =
         generics.params.iter().filter(|p| p.name.ident().as_str() == param_name).next()
     {
-        if param_name.starts_with("impl ") {
+        if def_id == tcx.lang_items().sized_trait() {
+            // Type parameters are already `Sized` by default.
+            err.span_label(
+                param.span,
+                &format!("this type parameter needs to be `{}`", constraint),
+            );
+        } else if param_name.starts_with("impl ") {
             // `impl Trait` in argument:
             // `fn foo(x: impl Trait) {}` → `fn foo(t: impl Trait + Trait2) {}`
             err.span_suggestion(

src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs

@@ -217,12 +217,14 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                         tcx.hir().get_generics(tcx.closure_base_def_id(self.mir_def_id))
                     {
                         suggest_constraining_type_param(
+                            tcx,
                             generics,
                             &mut err,
                             &param.name.as_str(),
                             "Copy",
                             tcx.sess.source_map(),
                             span,
+                            None,
                         );
                     }
                 }

src/test/ui/consts/too_generic_eval_ice.stderr

@@ -18,7 +18,7 @@ LL | pub struct Foo<A, B>(A, B);
    | --------------------------- required by `Foo`
 LL | 
 LL | impl<A, B> Foo<A, B> {
-   |      - help: consider restricting this bound: `A: std::marker::Sized`
+   |      - this type parameter needs to be `std::marker::Sized`
 ...
 LL |         [5; Self::HOST_SIZE] == [6; 0]
    |             ^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
@@ -33,7 +33,7 @@ LL | pub struct Foo<A, B>(A, B);
    | --------------------------- required by `Foo`
 LL | 
 LL | impl<A, B> Foo<A, B> {
-   |         - help: consider restricting this bound: `B: std::marker::Sized`
+   |         - this type parameter needs to be `std::marker::Sized`
 ...
 LL |         [5; Self::HOST_SIZE] == [6; 0]
    |             ^^^^^^^^^^^^^^^ doesn't have a size known at compile-time

src/test/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) {
-   |          -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |          - this type parameter needs to be `std::marker::Sized`
 LL |     let u: &dyn Foo = t;
    |                       ^ doesn't have a size known at compile-time
    |
@@ -14,7 +14,7 @@ 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) {
-   |          -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |          - this type parameter needs to be `std::marker::Sized`
 LL |     let v: &dyn Foo = t as &dyn Foo;
    |                       ^ doesn't have a size known at compile-time
    |

src/test/ui/issues/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:3:5
    |
 LL | pub struct Bad<T: ?Sized> {
-   |                -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |                - this type parameter needs to be `std::marker::Sized`
 LL |     data: T,
    |     ^^^^^^^ doesn't have a size known at compile-time
    |

src/test/ui/traits/trait-suggest-where-clause.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `U` cannot be known at compilation tim
   --> $DIR/trait-suggest-where-clause.rs:11:20
    |
 LL | fn check<T: Iterator, U: ?Sized>() {
-   |                       -- help: consider further restricting this bound: `U: std::marker::Sized +`
+   |                       - this type parameter needs to be `std::marker::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/trait-suggest-where-clause.rs:14:5
    |
 LL | fn check<T: Iterator, U: ?Sized>() {
-   |                       -- help: consider further restricting this bound: `U: std::marker::Sized +`
+   |                       - this type parameter needs to be `std::marker::Sized`
 ...
 LL |     mem::size_of::<Misc<U>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time

src/test/ui/union/union-sized-field.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/union-sized-field.rs:4:5
    |
 LL | union Foo<T: ?Sized> {
-   |           -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |           - this type parameter needs to be `std::marker::Sized`
 LL |     value: T,
    |     ^^^^^^^^ doesn't have a size known at compile-time
    |
@@ -14,7 +14,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/union-sized-field.rs:9:5
    |
 LL | struct Foo2<T: ?Sized> {
-   |             -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |             - this type parameter needs to be `std::marker::Sized`
 LL |     value: T,
    |     ^^^^^^^^ doesn't have a size known at compile-time
    |
@@ -26,7 +26,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/union-sized-field.rs:15:11
    |
 LL | enum Foo3<T: ?Sized> {
-   |           -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |           - this type parameter needs to be `std::marker::Sized`
 LL |     Value(T),
    |           ^ doesn't have a size known at compile-time
    |

src/test/ui/unsized/unsized-bare-typaram.stderr

@@ -4,9 +4,9 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
 LL | fn bar<T: Sized>() { }
    |    --- - required by this bound in `bar`
 LL | fn foo<T: ?Sized>() { bar::<T>() }
-   |        --                   ^ doesn't have a size known at compile-time
+   |        -                    ^ doesn't have a size known at compile-time
    |        |
-   |        help: consider further restricting this bound: `T: std::marker::Sized +`
+   |        this type parameter needs to be `std::marker::Sized`
    |
    = help: the trait `std::marker::Sized` is not implemented for `T`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

src/test/ui/unsized/unsized-enum.stderr

@@ -5,9 +5,9 @@ LL | enum Foo<U> { FooSome(U), FooNone }
    | ----------- required by `Foo`
 LL | fn foo1<T>() { not_sized::<Foo<T>>() } // Hunky dory.
 LL | fn foo2<T: ?Sized>() { not_sized::<Foo<T>>() }
-   |         --                         ^^^^^^ doesn't have a size known at compile-time
+   |         -                          ^^^^^^ doesn't have a size known at compile-time
    |         |
-   |         help: consider further restricting this bound: `T: std::marker::Sized +`
+   |         this type parameter needs to be `std::marker::Sized`
    |
    = help: the trait `std::marker::Sized` is not implemented for `T`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

src/test/ui/unsized/unsized-enum2.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `W` cannot be known at compilation tim
   --> $DIR/unsized-enum2.rs:23:8
    |
 LL | enum E<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized> {
-   |        -- help: consider further restricting this bound: `W: std::marker::Sized +`
+   |        - this type parameter needs to be `std::marker::Sized`
 LL |     // parameter
 LL |     VA(W),
    |        ^ doesn't have a size known at compile-time
@@ -15,7 +15,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
   --> $DIR/unsized-enum2.rs:25:8
    |
 LL | enum E<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized> {
-   |                   -- help: consider further restricting this bound: `X: std::marker::Sized +`
+   |                   - this type parameter needs to be `std::marker::Sized`
 ...
 LL |     VB{x: X},
    |        ^^^^ doesn't have a size known at compile-time
@@ -28,7 +28,7 @@ error[E0277]: the size for values of type `Y` cannot be known at compilation tim
   --> $DIR/unsized-enum2.rs:27:15
    |
 LL | enum E<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized> {
-   |                              -- help: consider further restricting this bound: `Y: std::marker::Sized +`
+   |                              - this type parameter needs to be `std::marker::Sized`
 ...
 LL |     VC(isize, Y),
    |               ^ doesn't have a size known at compile-time
@@ -41,7 +41,7 @@ error[E0277]: the size for values of type `Z` cannot be known at compilation tim
   --> $DIR/unsized-enum2.rs:29:18
    |
 LL | enum E<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized> {
-   |                                         -- help: consider further restricting this bound: `Z: std::marker::Sized +`
+   |                                         - this type parameter needs to be `std::marker::Sized`
 ...
 LL |     VD{u: isize, x: Z},
    |                  ^^^^ doesn't have a size known at compile-time