Avoid an empty trait name in impl blocks.

`resolve_ident_in_lexical_scope` checks for an empty name. Why is this
necessary? Because `parse_item_impl` can produce an `impl` block with an
empty trait name in some cases. This is pretty gross and very
non-obvious.

This commit avoids the use of the empty trait name. In one case the
trait name is instead pulled from `TyKind::ImplTrait`, which prevents
the output for `tests/ui/impl-trait/extra-impl-in-trait-impl.rs` from
changing. In the other case we just fail the parse and don't try to
recover. I think losing error recovery in this obscure case is worth
the code cleanup.

This change affects `tests/ui/parser/impl-parsing.rs`, which is split in
two, and the obsolete `..` syntax cases are removed (they are tested
elsewhere).

compiler/rustc_parse/src/parser/item.rs

@@ -649,6 +649,7 @@ impl<'a> Parser<'a> {
                     other => {
                         if let TyKind::ImplTrait(_, bounds) = other
                             && let [bound] = bounds.as_slice()
+                            && let GenericBound::Trait(poly_trait_ref) = bound
                         {
                             // Suggest removing extra `impl` keyword:
                             // `impl<T: Default> impl Default for Wrapper<T>`
@@ -658,12 +659,12 @@ impl<'a> Parser<'a> {
                                 extra_impl_kw,
                                 impl_trait_span: ty_first.span,
                             });
+                            poly_trait_ref.trait_ref.path.clone()
                         } else {
-                            self.dcx().emit_err(errors::ExpectedTraitInTraitImplFoundType {
-                                span: ty_first.span,
-                            });
+                            return Err(self.dcx().create_err(
+                                errors::ExpectedTraitInTraitImplFoundType { span: ty_first.span },
+                            ));
                         }
-                        ast::Path::from_ident(Ident::new(kw::Empty, ty_first.span))
                     }
                 };
                 let trait_ref = TraitRef { path, ref_id: ty_first.id };

compiler/rustc_resolve/src/ident.rs

@@ -296,9 +296,6 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     ) -> Option<LexicalScopeBinding<'ra>> {
         assert!(ns == TypeNS || ns == ValueNS);
         let orig_ident = ident;
-        if ident.name == kw::Empty {
-            return Some(LexicalScopeBinding::Res(Res::Err));
-        }
         let (general_span, normalized_span) = if ident.name == kw::SelfUpper {
             // FIXME(jseyfried) improve `Self` hygiene
             let empty_span = ident.span.with_ctxt(SyntaxContext::root());

tests/ui/parser/impl-parsing-2.rs

@@ -0,0 +1,4 @@
+impl ! {} // OK
+
+default unsafe FAIL //~ ERROR expected item, found keyword `unsafe`
+//~^ ERROR `default` is not followed by an item

tests/ui/parser/impl-parsing-2.stderr

@@ -0,0 +1,18 @@
+error: `default` is not followed by an item
+  --> $DIR/impl-parsing-2.rs:3:1
+   |
+LL | default unsafe FAIL
+   | ^^^^^^^ the `default` qualifier
+   |
+   = note: only `fn`, `const`, `type`, or `impl` items may be prefixed by `default`
+
+error: expected item, found keyword `unsafe`
+  --> $DIR/impl-parsing-2.rs:3:9
+   |
+LL | default unsafe FAIL
+   |         ^^^^^^ expected item
+   |
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
+
+error: aborting due to 2 previous errors
+

tests/ui/parser/impl-parsing.rs

@@ -2,9 +2,4 @@ impl ! {} // OK
 impl ! where u8: Copy {} // OK
 
 impl Trait Type {} //~ ERROR missing `for` in a trait impl
-impl Trait .. {} //~ ERROR missing `for` in a trait impl
 impl ?Sized for Type {} //~ ERROR expected a trait, found type
-impl ?Sized for .. {} //~ ERROR expected a trait, found type
-
-default unsafe FAIL //~ ERROR expected item, found keyword `unsafe`
-//~^ ERROR `default` is not followed by an item

tests/ui/parser/impl-parsing.stderr

@@ -9,44 +9,11 @@ help: add `for` here
 LL | impl Trait for Type {}
    |            +++
 
-error: missing `for` in a trait impl
-  --> $DIR/impl-parsing.rs:5:11
-   |
-LL | impl Trait .. {}
-   |           ^
-   |
-help: add `for` here
-   |
-LL | impl Trait for .. {}
-   |            +++
-
 error: expected a trait, found type
-  --> $DIR/impl-parsing.rs:6:6
+  --> $DIR/impl-parsing.rs:5:6
    |
 LL | impl ?Sized for Type {}
    |      ^^^^^^
 
-error: expected a trait, found type
-  --> $DIR/impl-parsing.rs:7:6
-   |
-LL | impl ?Sized for .. {}
-   |      ^^^^^^
-
-error: `default` is not followed by an item
-  --> $DIR/impl-parsing.rs:9:1
-   |
-LL | default unsafe FAIL
-   | ^^^^^^^ the `default` qualifier
-   |
-   = note: only `fn`, `const`, `type`, or `impl` items may be prefixed by `default`
-
-error: expected item, found keyword `unsafe`
-  --> $DIR/impl-parsing.rs:9:9
-   |
-LL | default unsafe FAIL
-   |         ^^^^^^ expected item
-   |
-   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
-
-error: aborting due to 6 previous errors
+error: aborting due to 2 previous errors