Rollup merge of #146422 - fmease:less-greedy-maybe-const-bounds, r=estebank

Less greedily parse `[const]` bounds

> [!IMPORTANT]
> If you're coming here from any beta backport nomination thread on Zulip, only the last commit is truly relevant (the first commit doesn't need to be backported, it only contains test modifications)!

Don't consider `[` to start a bound, only consider `[const]` in its entirety to do so. This drastically reduces (but doesn't eliminate!) the chance of *real* breakages. Like `const`, `~const` and `async` before, `[const]` unavoidably brings along theoretical breakages, see preexisting tests: `macro-const-trait-bound-theoretical-regression.rs` and `macro-async-trait-bound-theoretical-regression.rs`.

Side note: It's unfortunate that we have to do this but apart from the known fact that MBE hurts forward compatibility, the `[const]` syntax is simply a bit scuffed (also CC'ing https://github.com/rust-lang/rust/issues/146122, section (3)).

Fixes [after beta backport] rust-lang/rust#146417.

* 1st commit: Restore the original test intentions of several preexisting related tests that were unfortunately lost over time
  * I've added a bunch of SCREAMING comments to make it less likely to be lost again
  * CC PR rust-lang/rust#119099 which added most of these tests
  * CC [#144409 (comment)](https://github.com/rust-lang/rust/pull/144409#discussion_r2337587513) for further context (NB: It's not the only PR that negatively affected the test intention)
* 2nd commit: Actually address the regression

r? `@oli-obk` or anyone

1 files changed, 11 insertions, 8 deletions

diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 6168647183f..23aaafac934 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -92,10 +92,10 @@ fn can_continue_type_after_non_fn_ident(t: &Token) -> bool {
 }
 
 fn can_begin_dyn_bound_in_edition_2015(t: &Token) -> bool {
-    // `Not`, `Tilde` & `Const` are deliberately not part of this list to
+    // `!`, `const`, `[`, `async` are deliberately not part of this list to
     // contain the number of potential regressions esp. in MBE code.
-    // `Const` would regress `rfc-2632-const-trait-impl/mbe-dyn-const-2015.rs`.
-    // `Not` would regress `dyn!(...)` macro calls in Rust 2015.
+    // `const` and `[` would regress UI test `macro-dyn-const-2015.rs`.
+    // `!` would regress `dyn!(...)` macro calls in Rust 2015.
     t.is_path_start()
         || t.is_lifetime()
         || t == &TokenKind::Question
@@ -1015,12 +1015,18 @@ impl<'a> Parser<'a> {
             || self.check(exp!(Tilde))
             || self.check_keyword(exp!(For))
             || self.check(exp!(OpenParen))
-            || self.check(exp!(OpenBracket))
+            || self.can_begin_maybe_const_bound()
             || self.check_keyword(exp!(Const))
             || self.check_keyword(exp!(Async))
             || self.check_keyword(exp!(Use))
     }
 
+    fn can_begin_maybe_const_bound(&mut self) -> bool {
+        self.check(exp!(OpenBracket))
+            && self.look_ahead(1, |t| t.is_keyword(kw::Const))
+            && self.look_ahead(2, |t| *t == token::CloseBracket)
+    }
+
     /// Parse a bound.
     ///
     /// ```ebnf
@@ -1199,10 +1205,7 @@ impl<'a> Parser<'a> {
             let span = tilde.to(self.prev_token.span);
             self.psess.gated_spans.gate(sym::const_trait_impl, span);
             BoundConstness::Maybe(span)
-        } else if self.check(exp!(OpenBracket))
-            && self.look_ahead(1, |t| t.is_keyword(kw::Const))
-            && self.look_ahead(2, |t| *t == token::CloseBracket)
-        {
+        } else if self.can_begin_maybe_const_bound() {
             let start = self.token.span;
             self.bump();
             self.expect_keyword(exp!(Const)).unwrap();