Support running without a typical rust installation

[Jake-Shadle]

#54 was closed by #243, but unfortunately that was apparently just the miniboss, as cargo internally calls rustc when gathering metadata

eg, rustc -vV and rustc - --crate-name ___ --print=file-names --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=cfg are called, and while rustc -vV is easy to fake that's...the only one.

The original intention of #54 was to be able to run cargo-deny in minimal container environments, so opening this as a new issue since at least cargo is out of the way, but rustc might be a lot harder.

[luser]

FYI the first invocation comes from Rustc::new which gets called via Config::load_global_rustc. That codepath does use a cache in target/.rustc_info.json, so you could write a dummy file out there to force it to avoid that. It gets called in RustcTargetData::new, which is called from build_resolve_graph , which is called from output_metadata.

The second invocation is in TargetInfo::new, which gets called one or more times for every target specified in RustcTargetData::new. This seems a bit excessive, maybe cargo could be changed to load this information on-demand?

[Jake-Shadle]

Ahh nice, might take a look at that for a workaround!