Don't fatal when calling expect_one_of when recovering arg in parse_seq

[compiler-errors]

In parse_seq, when parsing a sequence of token-separated items, if we don't see a separator, we try to parse another item eagerly in order to give a good diagnostic and recover from a missing separator:

rust/compiler/rustc_parse/src/parser/mod.rs

Lines 900 to 901 in d1a0fa5

	// Attempt to keep parsing if it was an omitted separator.
	match f(self) {

If parsing the item itself calls expect_one_of, then we will fatal because of #58903:

rust/compiler/rustc_parse/src/parser/mod.rs

Lines 513 to 516 in d1a0fa5

	} else if self.token.kind != token::Eof
	&& self.last_unexpected_token_span == Some(self.token.span)
	{
	FatalError.raise();

For precise_capturing feature I implemented, we do end up calling expected_one_of:

rust/compiler/rustc_parse/src/parser/ty.rs

Lines 712 to 714 in d1a0fa5

	} else {
	self_.unexpected_any()
	}

This leads the compiler to fatal before having emitted the first error, leading to absolutely no useful information for the user about what happened in the parser.

This PR makes it so that we stop doing that.

Fixes #124195

[rustbot]

r? @lcnr

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[compiler-errors]

r? @fmease or @oli-obk :D

[oli-obk]

@bors r+ rollup

[bors]

📌 Commit 2ef1552 has been approved by oli-obk

It is now in the queue for this repository.