Embed config-relative paths as a substring

[rhgndf]

Problem

Config env vars as far as I can tell cannot embed relative paths as a substring.

Proposed Solution

It would be useful to be able to set env vars like this for dependencies that need to compile C libraries.

[env]
CFLAGS = ["-I", { value = "include", relative = true }]

Notes

Useful if needed to modify how a library compiles for wasm32-unknown-unknown

[weihanglo]

Thanks for the report. It's a bit unclear what you want to achieve. Can you share a more complete example for your use case? Will environment variable expansion #10789 helps?

Besides, an unstable flag -C can change the current directory. Please have a look whether it is useful in this situation.

[rhgndf]

I think #10789 might work if it can expand env vars set within the cargo.toml.

For this feature request, it is to emulate setting the env var on the command line like CFLAGS="-I$(pwd)/include"

[weihanglo]

Pretty much understood. Just to me feel a bit awkward to have a nested { relative = "…" } inside an array. Personally I am looking forward to #10789 more.

@rustbot label -S-needs-design +S-needs-team-input +I-nominated-to-discuss -S-needs-info

[joshtriplett]

Slight bikeshed: this seems like it could be written more simply as { relative-path = "include" }.