Auto merge of #22158 - Kimundi:the_lonely_uppercase_keyword, r=pnkfelix

It is only allowed in paths now, where it will either work inside a `trait`
or `impl` item, or not resolve outside of it.

[breaking-change]

Closes #22137

src/test/compile-fail/self_type_keyword.rs

@@ -0,0 +1,49 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct Self;
+//~^ ERROR expected identifier, found keyword `Self`
+
+struct Bar<'Self>;
+//~^ ERROR invalid lifetime name
+
+pub fn main() {
+    let Self = 5;
+    //~^ ERROR expected identifier, found keyword `Self`
+
+    match 15 {
+        Self => (),
+        //~^ ERROR expected identifier, found keyword `Self`
+        ref Self => (),
+        //~^ ERROR expected identifier, found keyword `Self`
+        mut Self => (),
+        //~^ ERROR expected identifier, found keyword `Self`
+        ref mut Self => (),
+        //~^ ERROR expected identifier, found keyword `Self`
+        Self!() => (),
+        //~^ ERROR expected identifier, found keyword `Self`
+        Foo { x: Self } => (),
+        //~^ ERROR expected identifier, found keyword `Self`
+        Foo { Self } => (),
+        //~^ ERROR expected identifier, found keyword `Self`
+    }
+}
+
+use self::Self as Foo;
+//~^ ERROR expected identifier, found keyword `Self`
+
+use std::option::Option as Self;
+//~^ ERROR expected identifier, found keyword `Self`
+
+extern crate Self;
+//~^ ERROR expected identifier, found keyword `Self`
+
+trait Self {}
+//~^ ERROR expected identifier, found keyword `Self`