Mention implicit T: Sized bound in E0277 errors

src/librustc_infer/traits/error_reporting/mod.rs

@@ -584,17 +584,29 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             ))
                         );
 
-                        let explanation =
-                            if obligation.cause.code == ObligationCauseCode::MainFunctionType {
-                                "consider using `()`, or a `Result`".to_owned()
-                            } else {
-                                format!(
-                                    "{}the trait `{}` is not implemented for `{}`",
-                                    pre_message,
-                                    trait_ref.print_only_trait_path(),
-                                    trait_ref.self_ty(),
-                                )
-                            };
+                        let explanation = if obligation.cause.code
+                            == ObligationCauseCode::MainFunctionType
+                        {
+                            "consider using `()`, or a `Result`".to_owned()
+                        } else if self.tcx.lang_items().sized_trait().map_or(false, |sized_id| {
+                            let self_ty = trait_ref.self_ty();
+                            sized_id == trait_ref.def_id()
+                                && matches!(self_ty.kind, ty::Param(_))
+                                && self_ty != self.tcx.types.self_param
+                        }) {
+                            // Detect type parameters with an implied `Sized` bound and explain
+                            // it instead of giving a generic message. This will be displayed
+                            // as a `help`.
+                            "type parameters have an implicit `Sized` requirement by default"
+                                .to_string()
+                        } else {
+                            format!(
+                                "{}the trait `{}` is not implemented for `{}`",
+                                pre_message,
+                                trait_ref.print_only_trait_path(),
+                                trait_ref.self_ty(),
+                            )
+                        };
 
                         if self.suggest_add_reference_to_arg(
                             &obligation,

src/librustc_infer/traits/error_reporting/suggestions.rs

@@ -28,9 +28,9 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         body_id: hir::HirId,
     ) {
         let self_ty = trait_ref.self_ty();
-        let (param_ty, projection) = match &self_ty.kind {
-            ty::Param(_) => (true, None),
-            ty::Projection(projection) => (false, Some(projection)),
+        let projection = match &self_ty.kind {
+            ty::Param(_) => None,
+            ty::Projection(projection) => Some(projection),
             _ => return,
         };
 
@@ -64,7 +64,9 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     generics,
                     kind: hir::TraitItemKind::Method(..),
                     ..
-                }) if param_ty && self_ty == self.tcx.types.self_param => {
+                }) if matches!(self_ty.kind, ty::Param(_))
+                    && self_ty == self.tcx.types.self_param =>
+                {
                     // Restricting `Self` for a single method.
                     suggest_restriction(&generics, "`Self`", err);
                     return;
@@ -138,7 +140,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 })
                 | hir::Node::TraitItem(hir::TraitItem { generics, span, .. })
                 | hir::Node::ImplItem(hir::ImplItem { generics, span, .. })
-                    if param_ty =>
+                    if matches!(self_ty.kind, ty::Param(_)) =>
                 {
                     // Missing generic type parameter bound.
                     let param_name = self_ty.to_string();

src/librustc_typeck/collect/type_of.rs

@@ -440,17 +440,13 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                         _ => None,
                     })
                     .collect();
-                let is_param = |ty: Ty<'_>| match ty.kind {
-                    ty::Param(_) => true,
-                    _ => false,
-                };
                 let bad_substs: Vec<_> = substs
                     .iter()
                     .enumerate()
                     .filter_map(|(i, k)| {
                         if let GenericArgKind::Type(ty) = k.unpack() { Some((i, ty)) } else { None }
                     })
-                    .filter(|(_, ty)| !is_param(ty))
+                    .filter(|(_, ty)| !matches!(ty.kind, ty::Param(_)))
                     .collect();
                 if !bad_substs.is_empty() {
                     let identity_substs = InternalSubsts::identity_for_item(self.tcx, self.def_id);

src/test/ui/consts/too_generic_eval_ice.stderr

@@ -23,7 +23,7 @@ LL | impl<A, B> Foo<A, B> {
 LL |         [5; Self::HOST_SIZE] == [6; 0]
    |             ^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `A`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
 
 error[E0277]: the size for values of type `B` cannot be known at compilation time
@@ -38,7 +38,7 @@ LL | impl<A, B> Foo<A, B> {
 LL |         [5; Self::HOST_SIZE] == [6; 0]
    |             ^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `B`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
 
 error: aborting due to 3 previous errors

src/test/ui/dst/dst-object-from-unsized-type.stderr

@@ -6,7 +6,7 @@ LL | fn test1<T: ?Sized + Foo>(t: &T) {
 LL |     let u: &dyn Foo = t;
    |                       ^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: required for the cast to the object type `dyn Foo`
 
@@ -18,7 +18,7 @@ LL | fn test2<T: ?Sized + Foo>(t: &T) {
 LL |     let v: &dyn Foo = t as &dyn Foo;
    |                       ^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: required for the cast to the object type `dyn Foo`
 

src/test/ui/issues/issue-27060-2.stderr

@@ -6,7 +6,7 @@ LL | pub struct Bad<T: ?Sized> {
 LL |     data: T,
    |     ^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: the last field of a packed struct may only have a dynamically sized type if it does not need drop to be run
 

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

@@ -12,7 +12,7 @@ LL |     mem::size_of::<U>();
 LL | pub const fn size_of<T>() -> usize {
    |                      - required by this bound in `std::mem::size_of`
    |
-   = help: the trait `std::marker::Sized` is not implemented for `U`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
 
 error[E0277]: the size for values of type `U` cannot be known at compilation time
@@ -29,7 +29,7 @@ LL |     mem::size_of::<Misc<U>>();
 LL | pub const fn size_of<T>() -> usize {
    |                      - required by this bound in `std::mem::size_of`
    |
-   = help: within `Misc<U>`, the trait `std::marker::Sized` is not implemented for `U`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: required because it appears within the type `Misc<U>`
 

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

@@ -6,7 +6,7 @@ LL | union Foo<T: ?Sized> {
 LL |     value: T,
    |     ^^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: no field of a union may have a dynamically sized type
 
@@ -18,7 +18,7 @@ LL | struct Foo2<T: ?Sized> {
 LL |     value: T,
    |     ^^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: only the last field of a struct may have a dynamically sized type
 
@@ -30,7 +30,7 @@ LL | enum Foo3<T: ?Sized> {
 LL |     Value(T),
    |           ^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: no field of an enum variant may have a dynamically sized type
 

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

@@ -8,7 +8,7 @@ LL | fn foo<T: ?Sized>() { bar::<T>() }
    |        |
    |        this type parameter needs to be `std::marker::Sized`
    |
-   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
 
 error: aborting due to previous error

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

@@ -9,7 +9,7 @@ LL | fn foo2<T: ?Sized>() { not_sized::<Foo<T>>() }
    |         |
    |         this type parameter needs to be `std::marker::Sized`
    |
-   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
 
 error: aborting due to previous error

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

@@ -7,7 +7,7 @@ LL |     // parameter
 LL |     VA(W),
    |        ^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `W`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: no field of an enum variant may have a dynamically sized type
 
@@ -20,7 +20,7 @@ LL | enum E<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized> {
 LL |     VB{x: X},
    |        ^^^^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `X`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: no field of an enum variant may have a dynamically sized type
 
@@ -33,7 +33,7 @@ LL | enum E<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized> {
 LL |     VC(isize, Y),
    |               ^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `Y`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: no field of an enum variant may have a dynamically sized type
 
@@ -46,7 +46,7 @@ LL | enum E<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized> {
 LL |     VD{u: isize, x: Z},
    |                  ^^^^ doesn't have a size known at compile-time
    |
-   = help: the trait `std::marker::Sized` is not implemented for `Z`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: no field of an enum variant may have a dynamically sized type
 

src/test/ui/unsized/unsized-inherent-impl-self-type.stderr

@@ -9,7 +9,7 @@ LL | impl<X: ?Sized> S5<X> {
    |      |
    |      this type parameter needs to be `std::marker::Sized`
    |
-   = help: the trait `std::marker::Sized` is not implemented for `X`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
 
 error: aborting due to previous error

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

@@ -9,7 +9,7 @@ LL | fn foo2<T: ?Sized>() { not_sized::<Foo<T>>() }
    |         |
    |         this type parameter needs to be `std::marker::Sized`
    |
-   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
 
 error[E0277]: the size for values of type `T` cannot be known at compilation time
@@ -23,7 +23,7 @@ LL | fn bar2<T: ?Sized>() { is_sized::<Bar<T>>() }
    |         |
    |         this type parameter needs to be `std::marker::Sized`
    |
-   = help: within `Bar<T>`, the trait `std::marker::Sized` is not implemented for `T`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: required because it appears within the type `Bar<T>`
 

src/test/ui/unsized/unsized-trait-impl-self-type.stderr

@@ -9,7 +9,7 @@ LL | impl<X: ?Sized> T3<X> for S5<X> {
    |      |
    |      this type parameter needs to be `std::marker::Sized`
    |
-   = help: the trait `std::marker::Sized` is not implemented for `X`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
 
 error: aborting due to previous error

src/test/ui/unsized/unsized-trait-impl-trait-arg.stderr

@@ -6,7 +6,7 @@ LL | impl<X: ?Sized> T2<X> for S4<X> {
    |      |
    |      this type parameter needs to be `std::marker::Sized`
    |
-   = help: the trait `std::marker::Sized` is not implemented for `X`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
 
 error: aborting due to previous error

src/test/ui/unsized3.stderr

@@ -11,7 +11,7 @@ LL | fn f2<X>(x: &X) {
    |       |
    |       required by this bound in `f2`
    |
-   = help: the trait `std::marker::Sized` is not implemented for `X`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
 
 error[E0277]: the size for values of type `X` cannot be known at compilation time
@@ -27,7 +27,7 @@ LL | fn f4<X: T>(x: &X) {
    |       |
    |       required by this bound in `f4`
    |
-   = help: the trait `std::marker::Sized` is not implemented for `X`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
 
 error[E0277]: the size for values of type `X` cannot be known at compilation time
@@ -41,7 +41,7 @@ LL | fn f8<X: ?Sized>(x1: &S<X>, x2: &S<X>) {
 LL |     f5(x1);
    |        ^^ doesn't have a size known at compile-time
    |
-   = help: within `S<X>`, the trait `std::marker::Sized` is not implemented for `X`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: required because it appears within the type `S<X>`
 
@@ -53,7 +53,7 @@ LL | fn f9<X: ?Sized>(x1: Box<S<X>>) {
 LL |     f5(&(*x1, 34));
    |        ^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: within `S<X>`, the trait `std::marker::Sized` is not implemented for `X`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: required because it appears within the type `S<X>`
    = note: only the last element of a tuple may have a dynamically sized type
@@ -66,7 +66,7 @@ LL | fn f10<X: ?Sized>(x1: Box<S<X>>) {
 LL |     f5(&(32, *x1));
    |         ^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: within `({integer}, S<X>)`, the trait `std::marker::Sized` is not implemented for `X`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: required because it appears within the type `S<X>`
    = note: required because it appears within the type `({integer}, S<X>)`
@@ -83,7 +83,7 @@ LL | fn f10<X: ?Sized>(x1: Box<S<X>>) {
 LL |     f5(&(32, *x1));
    |        ^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: within `({integer}, S<X>)`, the trait `std::marker::Sized` is not implemented for `X`
+   = help: type parameters have an implicit `Sized` requirement by default
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: required because it appears within the type `S<X>`
    = note: required because it appears within the type `({integer}, S<X>)`