Tweak Sized obligation spans on fn argument impl Trait

Remove now unnecessary span label.

Author: estebank

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -266,8 +266,9 @@ pub fn suggest_constraining_type_params<'a>(
                 constraints.extract_if(|(_, def_id)| *def_id == tcx.lang_items().sized_trait());
             if let Some((_, def_id)) = sized_constraints.next() {
                 applicability = Applicability::MaybeIncorrect;
-
-                err.span_label(param.span, "this type parameter needs to be `Sized`");
+                if !param.name.ident().as_str().starts_with("impl ") {
+                    err.span_label(param.span, "this type parameter needs to be `Sized`");
+                }
                 suggest_changing_unsized_bound(generics, &mut suggestions, param, def_id);
             }
         }

tests/ui/trait-bounds/apit-unsized.stderr

@@ -2,10 +2,7 @@ error[E0277]: the size for values of type `impl Iterator<Item = i32> + ?Sized` c
   --> $DIR/apit-unsized.rs:1:11
    |
 LL | fn foo(_: impl Iterator<Item = i32> + ?Sized) {}
-   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |           |
-   |           doesn't have a size known at compile-time
-   |           this type parameter needs to be `Sized`
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: unsized fn params are gated as an unstable feature
 help: consider removing the `?Sized` bound to make the type parameter `Sized`
@@ -22,10 +19,7 @@ error[E0277]: the size for values of type `impl ?Sized` cannot be known at compi
   --> $DIR/apit-unsized.rs:2:11
    |
 LL | fn bar(_: impl ?Sized) {}
-   |           ^^^^^^^^^^^
-   |           |
-   |           doesn't have a size known at compile-time
-   |           this type parameter needs to be `Sized`
+   |           ^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: unsized fn params are gated as an unstable feature
 help: consider replacing `?Sized` with `Sized`