Disallow auto trait alias syntax

src/libsyntax/parse/parser.rs

@@ -6534,8 +6534,14 @@ impl<'a> Parser<'a> {
             let bounds = self.parse_generic_bounds()?;
             tps.where_clause = self.parse_where_clause()?;
             self.expect(&token::Semi)?;
+            if is_auto == IsAuto::Yes {
+                let msg = "trait aliases cannot be `auto`";
+                self.struct_span_err(self.prev_span, msg)
+                    .span_label(self.prev_span, msg)
+                    .emit();
+            }
             if unsafety != Unsafety::Normal {
-                let msg = "trait aliases cannot be unsafe";
+                let msg = "trait aliases cannot be `unsafe`";
                 self.struct_span_err(self.prev_span, msg)
                     .span_label(self.prev_span, msg)
                     .emit();

src/test/ui/traits/trait-alias-syntax.rs

@@ -0,0 +1,7 @@
+#![feature(trait_alias)]
+
+trait Foo {}
+auto trait A = Foo; //~ ERROR trait aliases cannot be `auto`
+unsafe trait B = Foo; //~ ERROR trait aliases cannot be `unsafe`
+
+fn main() {}

src/test/ui/traits/trait-alias-syntax.stderr

@@ -0,0 +1,14 @@
+error: trait aliases cannot be `auto`
+  --> $DIR/trait-alias-syntax.rs:4:19
+   |
+LL | auto trait A = Foo; //~ ERROR trait aliases cannot be `auto`
+   |                   ^ trait aliases cannot be `auto`
+
+error: trait aliases cannot be `unsafe`
+  --> $DIR/trait-alias-syntax.rs:5:21
+   |
+LL | unsafe trait B = Foo; //~ ERROR trait aliases cannot be `unsafe`
+   |                     ^ trait aliases cannot be `unsafe`
+
+error: aborting due to 2 previous errors
+