Make #[cfg(version)] respect RUSTC_OVERRIDE_VERSION_STRING

[est31]

The #[cfg(version(...))] feature is currently under-tested. Part of it is the difficulty that it is hard to write a test that never changes, while the version of the Rust compiler indeed does change.

PR #81468 added the first and so far only test of #[cfg(version(...))]'s functionality (there is one other test for the syntax, that also acts as feature gate). But that test uses a proc macro that parses the version: the text of the test doesn't contain the actual #[cfg(version(...))].

This PR makes #[cfg(version(...))] respect RUSTC_OVERRIDE_VERSION_STRING, added by PR #124339, allowing us to virtually pin the rustc version and write tests from all directions against some specific version.

The PR also adds a functional test of #[cfg(version(...))] that leverages RUSTC_OVERRIDE_VERSION_STRING.

Pulled out of #141137.

Tracking issue: #64796

[rustbot]

r? @BoxyUwU

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[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

Some changes occurred in compiler/rustc_attr_data_structures

cc @jdonszelmann

[jieyouxu]

r? jieyouxu

[jieyouxu]

Thanks. Two things:

1. A naming nit.
2. Can you squash the commits into one? The second test commit doesn't need to be its own commit IMO.

[jieyouxu]

@rustbot author

[jieyouxu]

(Also, feel free to r? me for cfg_version PRs since I was already looking at the previous stabilization PR)

[est31]

@rustbot ready

[jieyouxu]

Thanks, feel free to r=me after the static naming nit.

EDIT: actually one sec

[jieyouxu]

Thanks

[jieyouxu]

@bors r+ rollup

[bors]

📌 Commit 8616237 has been approved by jieyouxu

It is now in the queue for this repository.

[bors]

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.