`cargo --config 'k=v' <plugin>` isn't passed down to the plugin

[sunshowers]

Problem

If someone runs the command:

cargo --config 'k=v' <plugin>

then the config isn't passed down to the plugin.

This was originally reported as nextest-rs/nextest#498.

Steps

1. Install nextest: https://nexte.st/book/pre-built-binaries.html
2. Run cargo --config 'build.target-dir="foo"' nextest run
3. See that the target dir is still target/.

(Note that cargo nextest run --config 'build.target-dir="foo"' works correctly, because nextest passes through Cargo configs.)

Possible Solution(s)

Best solution might be to pass configs down as an environment variable that plugins can read.

Notes

No response

Version

cargo 1.63.0 (fd9c4297c 2022-07-01)
release: 1.63.0
commit-hash: fd9c4297ccbee36d39e9a79067edab0b614edb5a
commit-date: 2022-07-01
host: x86_64-unknown-linux-gnu
libgit2: 1.4.2 (sys:0.14.2 vendored)
libcurl: 7.83.1-DEV (sys:0.4.55+curl-7.83.1 vendored ssl:OpenSSL/1.1.1n)
os: Pop!_OS 22.04 (jammy) [64-bit]

[epage]

If people are curious how git does this, https://github.com/gitext-rs/git-config-env/tree/main/src has a rough implementation of parts of it.

[weihanglo]

Semi-related:

• Give more information to third party subcommands and/or a way to query cargo #8842
• cargo subcommands do not pick up [env] #10094

I got an impression that cargo doesn't give external subcommands much info, even cargo-clippy can't inherit a jobserver from cargo today.

[kaspar030]

I'd like to bump this.

Our build needs to pass a custom --config file for setting cargo options. Support for this is spotty for cargo plugins. E.g., cargo bloat added support for passing the config arg down to sub-cargo build, but does not when calling out to cargo metadata.

IMO having a "blessed" way of dealing with this would save plugin authors and users, so, all of us, quite some hassle down the line.

Also, if there is support for this in a plugin, it requires to move the --config after the plugin name. Which is confusing: cargo --config foo build vs cargo clippy --config foo.