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, 9 insertions, 8 deletions

diff --git a/tests/ui/async-await/async-fn/macro-async-trait-bound-theoretical-regression.rs b/tests/ui/async-await/async-fn/macro-async-trait-bound-theoretical-regression.rs
index ea67831b68e..a13a255d536 100644
--- a/tests/ui/async-await/async-fn/macro-async-trait-bound-theoretical-regression.rs
+++ b/tests/ui/async-await/async-fn/macro-async-trait-bound-theoretical-regression.rs
@@ -1,21 +1,22 @@
 // Demonstrates and records a theoretical regressions / breaking changes caused by the
 // introduction of async trait bounds.
 
-// Setting the edition to 2018 since we don't regress `demo! { dyn async }` in Rust <2018.
+// Setting the edition to >2015 since we didn't regress `demo! { dyn async }` in Rust 2015.
 //@ edition:2018
 
 macro_rules! demo {
-    ($ty:ty) => { compile_error!("ty"); };
+    ($ty:ty) => { compile_error!("ty"); }; // KEEP THIS RULE FIRST AND AS IS!
     //~^ ERROR ty
     //~| ERROR ty
-    (impl $c:ident Trait) => {};
-    (dyn $c:ident Trait) => {};
+
+    // DON'T MODIFY THE MATCHERS BELOW UNLESS THE ASYNC TRAIT MODIFIER SYNTAX CHANGES!
+
+    (impl $c:ident Trait) => { /* KEEP THIS EMPTY! */ };
+    (dyn $c:ident Trait) => { /* KEEP THIS EMPTY! */ };
 }
 
-demo! { impl async Trait }
-//~^ ERROR `async` trait bounds are unstable
+demo! { impl async Trait } //~ ERROR `async` trait bounds are unstable
 
-demo! { dyn async Trait }
-//~^ ERROR `async` trait bounds are unstable
+demo! { dyn async Trait } //~ ERROR `async` trait bounds are unstable
 
 fn main() {}