Reserve guarded string literals (RFC 3593)

[pitaj]

Implementation for RFC 3593, including:

• lexer / parser changes
• diagnostics
• migration lint
• tests

This PR does not implement any special lexing of the string internals:

• strings preceded by one or more # are denied
• regardless of the number of trailing #
• string contents are lexed as if it was just a bare "string"

Tracking issue: #123735
RFC: rust-lang/rfcs#3593

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[mattheww]

In edition 2024 I'm seeing a panic inside cook_unicode if I give this an empty guarded string without enough closing hashes (eg #"" or ##""#).

[mattheww]

In edition 2024 if I give this a string like blah"xx" I get both a warning for an unknown prefix and a warning for an unprefixed guarded string literal.

I think it would be better if I got only the first warning.

[pitaj]

@mattheww good catch with the empty strings. I wasn't able to reproduce the "double-warning" issue with blah"xx", but just managed to do so with blah#"xx"#.

I think it's actually the correct behavior. When you fix the issue with blah #"xx"# as recommended, you'll end up getting the second error anyways. Better to get both at once, then you can make a more educated decision.

Regardless, that case will be exceedingly rare if it exists in the wild at all, so I'm not going to spend any time on it.

[mattheww]

My stress-tester is now giving an ICE for £#""# in editions older than 2024.