set current working directory for procedural macros

[matklad]

In

https://github.com/rust-analyzer/rust-analyzer/blob/8dc3a270f6a893a592db1a5fe3cf8752521987e1/crates/proc_macro_srv/src/cli.rs#L8-L24

nothing sets currend dir explicitly, it gets inherited. This is wrong, as proc-macros generally can look at pwd. Note that this is per-macro thing -- we run a single expander process, but each crates' macros want to see their own pwd.

I think we need to add current_dir to this struct:

https://github.com/rust-analyzer/rust-analyzer/blob/8dc3a270f6a893a592db1a5fe3cf8752521987e1/crates/proc_macro_api/src/msg.rs#L31-L52

and call std::env::set_env when serving a request.

[matklad]

cc intellij-rust/intellij-rust#8238

[bjorn3]

Cargo has changed the current working directory of rustc invocations in the past. (from package root to workspace root) I don't think there is any guarantee that it won't change it again in the future.

[matklad]

Yeah, to be clear:

• macros that look at pwd are technically borken, this isn't something guaranteed to work and they shouldn't do that
• macros do look at pwd sometimes, and it would be useful if that worked more often than not.