Account for derived obligations to suggest ?Sized bound

Fix rust-lang#27964.

Author: estebank

src/librustc_trait_selection/traits/error_reporting/mod.rs

@@ -1696,14 +1696,14 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
         err: &mut DiagnosticBuilder<'tcx>,
         obligation: &PredicateObligation<'tcx>,
     ) {
-        let (pred, item_def_id, span) = match (obligation.predicate.kind(), &obligation.cause.code)
-        {
-            (
-                ty::PredicateKind::Trait(pred, _),
-                ObligationCauseCode::BindingObligation(item_def_id, span),
-            ) => (pred, item_def_id, span),
-            _ => return,
-        };
+        let (pred, item_def_id, span) =
+            match (obligation.predicate.kind(), &obligation.cause.code.peel_derives()) {
+                (
+                    ty::PredicateKind::Trait(pred, _),
+                    ObligationCauseCode::BindingObligation(item_def_id, span),
+                ) => (pred, item_def_id, span),
+                _ => return,
+            };
 
         let node = match (
             self.tcx.hir().get_if_local(*item_def_id),

src/test/ui/dst/dst-sized-trait-param.stderr

@@ -9,6 +9,10 @@ LL | impl Foo<[isize]> for usize { }
    |
    = help: the trait `std::marker::Sized` is not implemented for `[isize]`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | trait Foo<T: ?Sized> : Sized { fn take(self, x: &T) { } } // Note: T is sized
+   |            ^^^^^^^^
 
 error[E0277]: the size for values of type `[usize]` cannot be known at compilation time
   --> $DIR/dst-sized-trait-param.rs:10:6

src/test/ui/extern/extern-types-unsized.stderr

@@ -26,6 +26,10 @@ LL |     assert_sized::<Foo>();
    = help: within `Foo`, the trait `std::marker::Sized` is not implemented for `A`
    = 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 `Foo`
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | fn assert_sized<T: ?Sized>() { }
+   |                  ^^^^^^^^
 
 error[E0277]: the size for values of type `A` cannot be known at compilation time
   --> $DIR/extern-types-unsized.rs:28:5
@@ -39,6 +43,10 @@ LL |     assert_sized::<Bar<A>>();
    = help: within `Bar<A>`, the trait `std::marker::Sized` is not implemented for `A`
    = 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<A>`
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | fn assert_sized<T: ?Sized>() { }
+   |                  ^^^^^^^^
 
 error[E0277]: the size for values of type `A` cannot be known at compilation time
   --> $DIR/extern-types-unsized.rs:31:5
@@ -53,6 +61,10 @@ LL |     assert_sized::<Bar<Bar<A>>>();
    = 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<A>`
    = note: required because it appears within the type `Bar<Bar<A>>`
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | fn assert_sized<T: ?Sized>() { }
+   |                  ^^^^^^^^
 
 error: aborting due to 4 previous errors
 

src/test/ui/issues/issue-10412.stderr

@@ -57,6 +57,10 @@ LL | impl<'self> Serializable<str> for &'self str {
    |
    = help: the trait `std::marker::Sized` is not implemented for `str`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | trait Serializable<'self, T: ?Sized> {
+   |                            ^^^^^^^^
 
 error: aborting due to 9 previous errors
 

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

@@ -11,6 +11,10 @@ LL | impl<X: ?Sized> T2<X> for S4<X> {
    |
    = help: the trait `std::marker::Sized` is not implemented for `X`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | trait T2<Z: ?Sized> {
+   |           ^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/unsized3.stderr

@@ -48,6 +48,10 @@ LL |     f5(x1);
    = help: within `S<X>`, the trait `std::marker::Sized` is not implemented for `X`
    = 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>`
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | fn f5<Y: ?Sized>(x: &Y) {}
+   |        ^^^^^^^^
 
 error[E0277]: the size for values of type `X` cannot be known at compilation time
   --> $DIR/unsized3.rs:40:8
@@ -91,6 +95,10 @@ LL |     f5(&(32, *x1));
    = 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>)`
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | fn f5<Y: ?Sized>(x: &Y) {}
+   |        ^^^^^^^^
 
 error: aborting due to 6 previous errors
 

src/test/ui/unsized7.stderr

@@ -11,6 +11,10 @@ LL | impl<X: ?Sized + T> T1<X> for S3<X> {
    |
    = help: the trait `std::marker::Sized` is not implemented for `X`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | trait T1<Z: T + ?Sized> {
+   |               ^^^^^^^^
 
 error: aborting due to previous error