T-lang came back on the stabilization PR asking for CR to be disallowed
to leave room for all stray CRs to be rejected in the future.
At that point, the test can remain but the implementation can be
removed.
If that plan does not go through, we'll need to re-evaluate
- whether this is more lint-like and should defer to the calling tool
that is managing the frontmatter
- how much Rust should treat the frontmatter as Rust and apply the same
grammar restrictions of "no stray CR" (like raw string literals)
When working on the stabilization report,
I found it annoying to determine what cases were covered because areas
of the frontmatter feature were either not in the file name or in
inconsistent locations.
This moves the area of frontmatter to the start of the file name and the
moves to more specific the later in the file name so coverage is easier
to see.
The RFC only limits hyphens at the beginning of lines and not if they
are indented or embedded in other content.
Sticking to that approach was confirmed by the T-lang liason at
https://github.com/rust-lang/rust/issues/141367#issuecomment-3202217544
There is a regression in error message quality which I'm leaving for
someone if they feel this needs improving.
