Suggest ?Sized on type parameters

src/librustc/traits/error_reporting/mod.rs

@@ -1340,6 +1340,44 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 &obligation.cause.code,
                 &mut vec![],
             );
+            self.suggest_unsized_bound_if_applicable(err, obligation);
+        }
+    }
+
+    fn suggest_unsized_bound_if_applicable(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        obligation: &PredicateObligation<'tcx>,
+    ) {
+        if let (
+            ty::Predicate::Trait(pred, _),
+            ObligationCauseCode::BindingObligation(item_def_id, span),
+        ) = (&obligation.predicate, &obligation.cause.code)
+        {
+            if let (Some(generics), true) = (
+                self.tcx.hir().get_if_local(*item_def_id).as_ref().and_then(|n| n.generics()),
+                Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
+            ) {
+                for param in generics.params {
+                    if param.span == *span
+                        && !param.bounds.iter().any(|bound| {
+                            bound.trait_def_id() == self.tcx.lang_items().sized_trait()
+                        })
+                    {
+                        let (span, separator) = match param.bounds {
+                            [] => (span.shrink_to_hi(), ":"),
+                            [.., bound] => (bound.span().shrink_to_hi(), " + "),
+                        };
+                        err.span_suggestion(
+                            span,
+                            "consider relaxing the implicit `Sized` restriction",
+                            format!("{} ?Sized", separator),
+                            Applicability::MachineApplicable,
+                        );
+                        return;
+                    }
+                }
+            }
         }
     }
 

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

@@ -2,7 +2,9 @@ error[E0277]: the size for values of type `A` cannot be known at compilation tim
   --> $DIR/extern-types-unsized.rs:22:20
    |
 LL | fn assert_sized<T>() { }
-   |    ------------ - required by this bound in `assert_sized`
+   |    ------------ -- help: consider relaxing the implicit `Sized` restriction: `: ?Sized`
+   |                 |
+   |                 required by this bound in `assert_sized`
 ...
 LL |     assert_sized::<A>();
    |                    ^ doesn't have a size known at compile-time

src/test/ui/str/str-mut-idx.stderr

@@ -2,7 +2,9 @@ error[E0277]: the size for values of type `str` cannot be known at compilation t
   --> $DIR/str-mut-idx.rs:4:15
    |
 LL | fn bot<T>() -> T { loop {} }
-   |    --- - required by this bound in `bot`
+   |    --- -- help: consider relaxing the implicit `Sized` restriction: `: ?Sized`
+   |        |
+   |        required by this bound in `bot`
 ...
 LL |     s[1..2] = bot();
    |               ^^^ doesn't have a size known at compile-time

src/test/ui/unsized3.stderr

@@ -1,8 +1,6 @@
 error[E0277]: the size for values of type `X` cannot be known at compilation time
   --> $DIR/unsized3.rs:7:13
    |
-LL | fn f1<X: ?Sized>(x: &X) {
-   |       -- help: consider further restricting this bound: `X: std::marker::Sized +`
 LL |     f2::<X>(x);
    |             ^ doesn't have a size known at compile-time
 ...
@@ -11,12 +9,18 @@ LL | fn f2<X>(x: &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 further restricting this bound
+   |
+LL | fn f1<X: std::marker::Sized +  ?Sized>(x: &X) {
+   |       ^^^^^^^^^^^^^^^^^^^^^^^
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | fn f2<X: ?Sized>(x: &X) {
+   |        ^^^^^^^^
 
 error[E0277]: the size for values of type `X` cannot be known at compilation time
   --> $DIR/unsized3.rs:18:13
    |
-LL | fn f3<X: ?Sized + T>(x: &X) {
-   |       -- help: consider further restricting this bound: `X: std::marker::Sized +`
 LL |     f4::<X>(x);
    |             ^ doesn't have a size known at compile-time
 ...
@@ -25,6 +29,14 @@ LL | fn f4<X: T>(x: &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 further restricting this bound
+   |
+LL | fn f3<X: std::marker::Sized +  ?Sized + T>(x: &X) {
+   |       ^^^^^^^^^^^^^^^^^^^^^^^
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | fn f4<X: T +  ?Sized>(x: &X) {
+   |            ^^^^^^^^^
 
 error[E0277]: the size for values of type `X` cannot be known at compilation time
   --> $DIR/unsized3.rs:33:8