fix: don't panic when a lifetime is passed to an ident metavariable

[shulaoda]

Closes #22801

Passing a lifetime to a macro that expects an ident, like m!('a) for ($t:ident), panics with Next token must be ident. rustc just reports no rules expected 'a. The matcher consumes the ' before the ident match fails, and the consumed token is still bound to the fragment, so the expansion ends up holding a ' with nothing after it.

to_parser_input assumed a ' is always followed by an ident. It is the point where every expansion, including proc macro output we don't control, becomes a syntax tree, so it now pushes an ERROR token instead of panicking.

The expect_* helpers consume a token before checking what it is, so a failed match still advances past it. match_meta_var then binds everything consumed since the savepoint, which for m!('a) is the bare ' that later trips the panic. Returning Fragment::Empty on failure makes the caller take the push_missing branch it already has.

The same bug made @--1.0 expand to a stray - - when matched against $lit:literal, hence the snapshot change in minus_belongs_to_literal.

🤖 AI-assisted: understanding the issue, locating the cause, and writing the test.

[Veykril]

This isn't the right spot to fix this. We are calling this function with an invalid state, so we need to fix this up in the caller somewhere. It seems like we are incorrectly recovering somewhere in the fragment parsing causing us to end up with a lone '

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[shulaoda]

Thanks for pointing this out!