Cannot use macros when crate is renamed

[That3Percent]

When the futures crate is renamed:

futures03 = { version = "0.3.1", package = "futures" }

Macros like try_join cannot find the crate.
Eg:

async fn test() {
    futures03::try_join!()
}

fails with

failed to resolve: could not find `futures` in `{{root}}`

This impedes migration from version 0.1 to version 0.3, since one of the two futures crates must be renamed.

[cramertj]

This is known, but in general there's no way to solve this for procedural macros (at least that I know of). @dtolnay may know if there have been any recent innovations here?

[That3Percent]

@cramertj This crate claims to solve this problem, but I have not tried it myself - https://crates.io/crates/proc-macro-crate

[dtolnay]

I definitely do not recommend using proc-macro-crate. It's based on parsing a Cargo.toml at macro expansion time so it breaks non-Cargo builds.

I think this is possible to make work correctly but I don't know if anyone has done it yet. Conceivably the try_join! macro invoked by the user could be a macro_rules macro that passes a $crate token to an underlying procedural macro.

[cramertj]

@dtolnay I believe I tried that in the past and found that $crate passed to a procedural macro didn't resolve correctly. Could be worth trying again, though, as I don't remember precisely.

[taiki-e]

Conceivably the try_join! macro invoked by the user could be a macro_rules macro that passes a $crate token to an underlying procedural macro.

I think this probably works, but I found a way to support not only rename but also reexport, so I filed #2124.

EDIT: this seems to work after rust-lang/rust#57155