cargo_metadata requires rustc to be on the $PATH

[illicitonion]

Problem

I tried to run cargo metadata with only a cargo binary on my $PATH, and no rustc.

I got an error that rustc could not be found.

Steps

1. Remove rustc from your $PATH
2. Run cargo metadata in some directory containing a Cargo.toml file

Notes

It appears to be because of this line of code:

cargo/src/cargo/ops/cargo_output_metadata.rs

Line 113 in 58a9613

let target_data = RustcTargetData::new(ws, &requested_kinds)?;

Which calls RustcTargetData::new.

This value doesn't appear to actually be used by any of the following code-paths, it's only used if some conditionals are hit which aren't triggered in the cargo metadata path (or at least, aren't without additional flags).

It would be useful to not need a rustc binary present in order to run cargo metadata.

Possible Solution(s)

I can imagine either making the rustc -vV invocation lazy, or a refactoring to not need to pass that arg around (or accept an Option)?

[ehuss]

Hi! rustc is needed for computing target-specific features. Closing as a duplicate of #8219.