Compile a crate from its source archive directly

Problem

Today, crates provided by e.g. crates.io are downloaded in a (somewhat loosely defined?) archive format, with a .crate extension. Example: https://static.crates.io/crates/clap/clap-3.0.6.crate

clap-3.0.6.crate: gzip compressed data, was "clap-3.0.6.crate", max compression, original size modulo 2^32 1110528

This gets stored as a local cache and is also extracted in a nearby folder, which then gets fed to cargo / rustc for compilation.

I (and surely others) am developing a $RUSTC_WRAPPER that has to deal with these archives and thus have to replicate the handling of said .crates. Given these open issues, and for optimization reasons, I would like for cargo (or rustc) to be able to take in these archives directly, without prior extraction:

• https://github.com/rust-lang/cargo/issues/2948
• https://github.com/rust-lang/cargo/issues/2526
• https://github.com/rust-lang/cargo/issues/9455

Proposed Solution

I am not familiar with the inner workings of either cargo nor rustc so this is a high level take.

Basically, I'm asking that today's call to rustc be seamlessly interchangeable from:

rustc --crate-name clap ...... $HOME/.cargo/registry/src/index.crates.io-6f17d22bba15001f/clap-3.0.6/src/lib.rs ....

to

rustc --crate-name clap ...... $HOME/.cargo/registry/cache/index.crates.io-6f17d22bba15001f/clap-3.0.6.crate ...

thus moving the crate archive handling code to within the boundary of (I guess) rustc.

This in turn

• encapsulates the archive format
• allows me (and others) to just pass archives down from crates.io straight to rustc

Notes

I realize this issue may need to move to rustc. But hey let's have a small discussion first :)

Start by tracing the current cargo-to-rustc flow described in the issue, including how .crate archives are cached, extracted, and passed to rustc. Compare the existing cargo and rustc entry points, then determine where direct archive handling belongs. Done means rustc or cargo can compile a .crate archive without prior extraction while preserving the current compilation behavior.