Rollup merge of #58387 - alexreg:fix-trait-alias-2, r=centril

Disallow `auto` trait alias syntax

See https://github.com/rust-lang/rust/issues/41517#issuecomment-462567679.

r? @Centril

CC @topecongiro @nikomatsakis

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
+