Ensure all precedence rules are covered.

[ehuss]

I think there are various bits of precedence rules that are not covered in the reference. expressions.md covers most of them, and patterns.md has one precedence rule noted.

I'm not sure what is missing, some that I can think of:

Attribute precedence. #[attr] x = 1; the attribute (I think) goes to the place expression, not the statement. RFC 16 says "Attributes bind tighter than any operator".

I think the way let _ = ||{}(); fails to parse isn't clear to me. The precedence rules just say "closures", but doesn't specify what that encompasses. See rust-lang/rust#55851.

impl precedence in types. This is briefly mentioned in rust-lang/rust#34511. I think there are other various precedence rules in type expressions.

I feel like there are various other precedence rules that are missing, but I can't think of any others right now. Perhaps at some point the gll grammar could inform all the rules?