migration lint for expr2024 for the edition 2024

[vincenzopalazzo]

This is adding a migration lint for the current (in the 2021 edition and previous)
to move expr to expr_2021 from expr

Issue #123742

I created also a repository to test out the migration https://github.com/vincenzopalazzo/expr2024-cargo-fix-migration

Co-Developed-by: @eholk

[rustbot]

r? @michaelwoerister

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[vincenzopalazzo]

r? @traviscross

[traviscross]

r? @eholk

[eholk]

This seems to be in a pretty decent shape, but I left a few comments.

[traviscross]

This needs further changes, e.g. to explain what this lint does and what people should know about it (e.g. that they probably want to switch back to expr from expr_2021), and to remove text from the explanation that is not correct for what this lint does.

[vincenzopalazzo]

@traviscross we update the docs with @eholk so should be good for another review

[eholk]

This incorrectly suggests changing the literal identifier expr in macros:

macro_rules! test {
    (expr) => {}
}

test!(expr);

which suggests:

help: to keep the existing behavior, use the `expr_2021` fragment specifier.
  |
LL |     (expr_2021) => {}
  |      ~~~~~~~~~

It'd be good to add a test case to make sure we don't do this.

Ah, it looks like such a tests exists: https://github.com/rust-lang/rust/pull/125627/files#diff-af322a90f8768b5f39a55aa9cbdce92b80b502f4e8b5d84c0959e93d73ddc7d4

Carry on.

[vincenzopalazzo]

@compiler-errors When you have time I would like to have another review on the parsing code to see if the code can have your bless 😸

[compiler-errors]

This looks fine to me for now. Please rebase to squash that fixup! commit, since bors doesn't autosquash.

[vincenzopalazzo]

This looks fine to me for now. Please rebase to squash that fixup! commit, since bors doesn't autosquash.

Concurrency problem, I did while you was writing the comment sorry 😸

[compiler-errors]

@bors r=compiler-errors,eholk rollup

[bors]

📌 Commit 568e78f has been approved by compiler-errors,eholk

It is now in the queue for this repository.