fix: Expand proc-macros in workspace root, not package root

[Veykril]

Should fix #17748. The approach is generally not perfect though as rust-project.json projects don't benefit from this (still, nothing changes in that regard)

[Veykril]

@bors r+

[bors]

📌 Commit 9a47e6f has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 9a47e6f with merge 06a40a6...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 06a40a6 to master...

[max-sixty]

Thank you!

[max-sixty]

@Veykril thanks for giving this a go.

Unfortunately I don't think it solved the problem. The repro in #17748 still fails. I'm on v0.3.2096 which contains this commit.

If I augment this to the repro, on nightly rustc:

#[proc_macro]
pub fn test_each_file(_input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    dbg!(env!("CARGO_MANIFEST_DIR"));
    dbg!(std::env::var("CARGO_RUSTC_CURRENT_DIR"));

    let current_dir = dbg!(std::env::current_dir().unwrap());
    assert!(dbg!(Path::new("./ROOT.txt")).is_file());
    assert!(dbg!(Path::new("./ROOT.txt").canonicalize().unwrap()).is_file());
    proc_macro::TokenStream::new()
}

...then cargo build shows that it's using the workspace root as cwd:

[test_macro/src/lib.rs:5:5] env!("CARGO_MANIFEST_DIR") = "/Users/maximilian/workspace/test-each-file/test_macro"
[test_macro/src/lib.rs:6:5] std::env::var("CARGO_RUSTC_CURRENT_DIR") = Ok(
    "/Users/maximilian/workspace/test-each-file",
)
[test_macro/src/lib.rs:8:23] std::env::current_dir().unwrap() = "/Users/maximilian/workspace/test-each-file"
[test_macro/src/lib.rs:9:13] Path::new("./ROOT.txt").canonicalize().unwrap() = "/Users/maximilian/workspace/test-each-file/ROOT.txt"
[test_macro/src/lib.rs:10:13] Path::new("./ROOT.txt") = "./ROOT.txt"

But Rust Analyzer Language Server log doesn't find the newly added CARGO_RUSTC_CURRENT_DIR var and is still using the crate root:

[test_macro/src/lib.rs:5:5] env!("CARGO_MANIFEST_DIR") = "/Users/maximilian/workspace/test-each-file/test_macro"
[test_macro/src/lib.rs:6:5] std::env::var("CARGO_RUSTC_CURRENT_DIR") = Err(
    NotPresent,
)
[test_macro/src/lib.rs:8:23] std::env::current_dir().unwrap() = "/Users/maximilian/workspace/test-each-file/test_src"

(if you don't see this, I'll open a new issue, hope that's reasonable, thank you!)

[Veykril]

Oh I see the issue, the env var is set for the proc-macro crate, but we need to it to be set for the calling crate