Adds help message in error for invalid impl for T syntax

src/libsyntax/parse/parser.rs

@@ -6717,7 +6717,15 @@ impl<'a> Parser<'a> {
         };
 
         // Parse both types and traits as a type, then reinterpret if necessary.
-        let ty_first = self.parse_ty()?;
+        let err_path = |span| ast::Path::from_ident(Ident::new(keywords::Invalid.name(), span));
+        let ty_first = if self.token.is_keyword(keywords::For) &&
+                          self.look_ahead(1, |t| t != &token::Lt) {
+            let span = self.prev_span.between(self.span);
+            self.struct_span_err(span, "missing trait in a trait impl").emit();
+            P(Ty { node: TyKind::Path(None, err_path(span)), span, id: ast::DUMMY_NODE_ID })
+        } else {
+            self.parse_ty()?
+        };
 
         // If `for` is missing we try to recover.
         let has_for = self.eat_keyword(keywords::For);
@@ -6726,7 +6734,7 @@ impl<'a> Parser<'a> {
         let ty_second = if self.token == token::DotDot {
             // We need to report this error after `cfg` expansion for compatibility reasons
             self.bump(); // `..`, do not add it to expected tokens
-            Some(P(Ty { node: TyKind::Err, span: self.prev_span, id: ast::DUMMY_NODE_ID }))
+            Some(DummyResult::raw_ty(self.prev_span, true))
         } else if has_for || self.token.can_begin_type() {
             Some(self.parse_ty()?)
         } else {
@@ -6756,7 +6764,7 @@ impl<'a> Parser<'a> {
                     TyKind::Path(None, path) => path,
                     _ => {
                         self.span_err(ty_first.span, "expected a trait, found type");
-                        ast::Path::from_ident(Ident::new(keywords::Invalid.name(), ty_first.span))
+                        err_path(ty_first.span)
                     }
                 };
                 let trait_ref = TraitRef { path, ref_id: ty_first.id };

src/test/ui/issues/issue-56031.rs

@@ -0,0 +1,6 @@
+struct T;
+
+impl for T {}
+//~^ ERROR missing trait in a trait impl
+
+fn main() {}

src/test/ui/issues/issue-56031.stderr

@@ -0,0 +1,8 @@
+error: missing trait in a trait impl
+  --> $DIR/issue-56031.rs:3:5
+   |
+LL | impl for T {}
+   |     ^
+
+error: aborting due to previous error
+