RFC 3101 reserved prefixes

[mattheww]

RFC 3101 (rust-lang/rfcs#3101) reserves lexical forms like the following:

• ident#foo
• ident"foo"
• ident'f'
• ident#123

This isn't covered in the FLS.

They're reserved in the 2021 edition and later, and the FLS currently describes the 2021 edition.

rust-lang/rust#126452 extended the RFC to reserving forms like

• 'ident#foo

Here, "reserves" means that it's a lexing-time error if any of those forms appear.

These are covered in the Reference under lex.token.reserved-prefix. That models them as separate forms of token, using grammar productions which don't overlap with the productions for accepted tokens.

There is some discussion of this approach in rust-lang/reference#1128.

[mattheww]

The related "Reserved guards" (lex.token.reserved-guards in the Reference) are reserved only in the 2024 edition, so it's expected that the FLS doesn't cover them.