Comment by epage

That work is being tracked in https://github.com/rust-lang/cargo/issues/5931

Someone has taken up the work on this though there are some foundational steps first.

1. We need to delineate intermediate and final build artifacts so people have a clearer understanding in `target/` what has stability guarantees (implemented, awaiting stabilization).

2. We then need to re-organize the target directory from being organized by file type to being organized by crate instance.

3. We need to re-do the file locking for `target/` so when we share things, one cargo process won't lock out your entire system

4. We can then start exploring moving intermediate artifacts into a central location.

There are some caveats to this initial implementation

- To avoid cache poisoning, this will only items with immutable source that and an idempotent build, leaving out your local source and stuff that depends on build scripts and proc-macros. There is work to reduce the reliance on build scripts and proc-macros. We may also need a "trust me, this is idempotent" flag for some remaining cases.

- A new instance of a crate will be created in the cache if any dependency changes versions, reducing reuse. This becomes worse when foundation crates release frequently and when adding or updating a specific dependency, Cargo prefers to keep all existing versions, creating a very unpredictable dependency tree. Support for remote caches, especially if you can use your project's CI as a cache source, would help a lot with this.