Recover missing turbofish for lifetime arguments in expression position

[raushan728]

Fixes #162656.

rustc already suggests ::<> when a type/const generic argument is missing its turbofish (foo<Bar>()), but not for lifetime arguments - Struct<'a> { .. } previously misfired through label/char-literal recovery, producing a confusing cascade of unrelated errors instead of one actionable turbofish suggestion.

cc @estebank

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

r? @chenyukang

rustbot has assigned @chenyukang.
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[estebank]

I need to do a more in-depth review, as I have some mild concerns about increasing the size of the parser, but the results look reasonable so far. I'll look at this again later this week if the assigned reviewer doesn't manage to get the time before then.

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[raushan728]

@rustbot ready

[chenyukang]

r? @estebank since you already take review :)

[raushan728]

Funny story this idea actually came to me while I was half asleep last night 😄. Your comment about parser size kept nagging at me and it hit me that we didn't need to touch eat_label/parse_expr_labeled or add any new parser fields at all we could intercept the missing turbofish with a simple lookahead right where < is already being parsed as a binary operator before it ever falls into label parsing.

@estebank should be a much smaller ask for review now

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.