Ability to disable `build.build-dir` config

[kornelski]

Problem

Once build.build-dir config is set, it seems that it cannot be unset by usual ways of overriding configuration (I assume the unstable.build-dir = true is going away).

cargo b --config 'build.build-dir=false' # expected a string, but found a boolean
cargo b --config 'build.build-dir=""' # the build directory is set to an empty string
CARGO_BUILD_BUILD_DIR= cargo b # the build directory is set to an empty string

rustc bootstrap is incompatible with build-dir, and the simplest solution would be to turn it off.

Proposed Solution

Perhaps allow false as a valid value, similar to build = false in Cargo.toml. However, true doesn't make sense in this case.

Notes

No response

[epage]

The inability to reset a field to a default value is a more general problem in Cargo's config (#8687).

We could special case this situation. Eventually we plan to change the default, so it won't help to workaround this problem anymore. So the question is how much value do we get out of providing this now verse having this special case stuck inside of cargo.

In terms of considering alternatives, we could offer a {target-dir} variable so you could reset with CARGO_BUILD_BUILD_DIR={target-dir}. However, we also want to phase out target-dir. The burden of being stuck with this is likely less.

In the end, these are workarounds and I suspect it would be best to resolve the root causes than fix the workarounds.