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, 20 insertions, 10 deletions

diff --git a/tests/ui/traits/const-traits/macro-const-trait-bound-theoretical-regression.rs b/tests/ui/traits/const-traits/macro-const-trait-bound-theoretical-regression.rs
index 35e964eacec..3f70ffa7d10 100644
--- a/tests/ui/traits/const-traits/macro-const-trait-bound-theoretical-regression.rs
+++ b/tests/ui/traits/const-traits/macro-const-trait-bound-theoretical-regression.rs
@@ -1,22 +1,32 @@
 // Demonstrates and records a theoretical regressions / breaking changes caused by the
-// introduction of const trait bounds.
+// introduction of `const` and `[const]` trait bounds.
 
-// Setting the edition to 2018 since we don't regress `demo! { dyn const }` in Rust <2018.
+// Setting the edition to >2015 since we didn't regress `demo! { dyn const }` in Rust 2015.
+// See also test `traits/const-traits/macro-dyn-const-2015.rs`.
 //@ edition:2018
 
 trait Trait {}
 
 macro_rules! demo {
-    (impl $c:ident Trait) => { impl $c Trait {} };
-    //~^ ERROR inherent
-    //~| WARN trait objects without an explicit `dyn` are deprecated
-    //~| WARN this is accepted in the current edition
-    (dyn $c:ident Trait) => { dyn $c Trait {} }; //~ ERROR macro expansion
+    ($ty:ty) => { compile_error!("ty"); }; // KEEP THIS RULE FIRST AND AS IS!
+    //~^ ERROR ty
+    //~| ERROR ty
+    //~| ERROR ty
+    //~| ERROR ty
+
+    // DON'T MODIFY THE MATCHERS BELOW UNLESS THE CONST TRAIT MODIFIER SYNTAX CHANGES!
+
+    (impl $c:ident Trait) => { /* KEEP THIS EMPTY! */ };
+    (dyn $c:ident Trait) => { /* KEEP THIS EMPTY! */ };
+
+    (impl [const] Trait) => { /* KEEP THIS EMPTY! */ };
+    (dyn [const] Trait) => { /* KEEP THIS EMPTY! */ };
 }
 
-demo! { impl const Trait }
-//~^ ERROR const trait impls are experimental
+demo! { impl const Trait } //~ ERROR const trait impls are experimental
+demo! { dyn const Trait } //~ ERROR const trait impls are experimental
 
-demo! { dyn const Trait }
+demo! { impl [const] Trait } //~ ERROR const trait impls are experimental
+demo! { dyn [const] Trait } //~ ERROR const trait impls are experimental
 
 fn main() {}