Build proc-macro-test-impl out-of-tree

[fasterthanlime]

Building it in-place fails in rust CI because the source directory is read-only. This changes proc-macro-test's build script to first
copy imp under OUT_DIR (which is read-write).

It also prints stdout/stderr for the nested cargo invocation, should it fail. (I've seen failures in rust CI that I couldn't explain, and
when they take 25 minutes to reproduce, you want to have that info)

This change is tracked in:

• [Tracking issue] rust-analyzer in-tree build prerequisites #12818

Maintainer impact: none.

[fasterthanlime]

Tested locally with:

$ cd crates/proc-macro-test
$ sudo chown root:root -R ./imp
$ (cd imp && cargo build)
error: Permission denied (os error 13) at path "/home/amos/bearcove/rust-analyzer/crates/proc-macro-test/imp/targetC0YSqJ"
$ cargo clean -q && cargo test -v
(tests pass)

[Veykril]

@bors r+

[bors]

📌 Commit 898898d has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 898898d with merge 72aa862...

[fasterthanlime]

@Veykril this needs another r+, @lnicola caught some typos in comments and other commits snuck in

[lnicola]

@bors delegate+

In case you remove the temporary directory at the end (you might also need to chdir to the previous directory if you do that).

[bors]

✌️ @fasterthanlime can now approve this pull request

[fasterthanlime]

In case you remove the temporary directory at the end (you might also need to chdir to the previous directory if you do that).

It's under OUT_DIR, so cargo clean will take care of it. We also overwrite it on every build where proc-macro-test-impl changes. It's also very small.

I'm gonna invoke "we might want to investigate that dir after a build" to not have to add additional cleanups 😄

(If it was under std::env::temp_dir() we definitely would want to clean it up, that would add up)

[fasterthanlime]

@bors r+

[bors]

📌 Commit 844aa8b has been approved by fasterthanlime

It is now in the queue for this repository.

[bors]

⌛ Testing commit 844aa8b with merge 7dc36ee...

[bors]

☀️ Test successful - checks-actions
Approved by: fasterthanlime
Pushing 7dc36ee to master...