[feat] easy LLVM_PROFILE_FILE config for cargo test

[loynoir]

Problem

Below workspace config leads to /path/to/project/crates/hello/target/xxx under each member cargo toml dir, but not /path/to/project/target/xxx under top cargo toml dir.

/path/to/project/.cargo/config.toml

[env]
LLVM_PROFILE_FILE = "target/xxx"

Below workspace config leads to target/xxx under top cargo toml dir, but it is a hard coded config.

/path/to/project/.cargo/config.toml

[env]
LLVM_PROFILE_FILE = "/path/to/project/target/xxx"

Proposed Solution

/path/to/project/.cargo/config.toml

[env]
LLVM_PROFILE_FILE = "${CARGO_CONFIG_PROJECT_CWD}/target/xxx"

Or maybe just add a new field ONLY for cargo test, as LLVM_PROFILE_FILE is not used in cargo build

[unstable]
profile-env-llvm-profile-file = true

[profile.test]
LLVM_PROFILE_FILE = "${CARGO_CONFIG_PROJECT_CWD}/target/xxx"

Notes

related env design

https://code.visualstudio.com/remote/advancedcontainers/environment-variables

[epage]

You can use the relative field to make an env variable be prefixed with the parent of the config dir, see https://doc.rust-lang.org/cargo/reference/config.html#env

[weihanglo]

Variable expansion has been discussed in #10789, and has some relevance in #12193. However, as epage has mentioned, the relative field should do the trick for you, so going to close as resolved and favor #10789 for the expansion discussion.

If the relative field doesn't work, please comment below and let us know what went wrong.