Rollup merge of #33841 - LeoTestard:macro-sequence-lhs, r=pnkfelix

Reject a LHS formed of a single sequence TT during `macro_rules!` checking.

This was already rejected during expansion. Encountering malformed LHS or RHS during expansion is now considered a bug.

Follow up to #33689.

r? @pnkfelix

Note: this can break code that defines such macros but does not use them.

src/test/compile-fail/malformed_macro_lhs.rs

@@ -9,9 +9,9 @@
 // except according to those terms.
 
 macro_rules! my_precioooous {
-    $($t:tt)* => (1);
+    $($t:tt)* => (1); //~ ERROR invalid macro matcher
 }
 
 fn main() {
-    my_precioooous!(); //~ ERROR malformed macro lhs
+    my_precioooous!();
 }

src/test/run-pass/issue-21350.rs

@@ -1,19 +0,0 @@
-// 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.
-
-// Make sure that "bare sequences" don't ICE in follow checking
-
-// pretty-expanded FIXME #23616
-
-macro_rules! bare {
-    $($id:expr),+ => ( $($id)+ )
-}
-
-fn main() { }