Make sccache2 usable for local developers

[luser]

• Sensible defaults for running on developer machines
• Support using srcdir-relative paths in hashes
• Fetch from S3, store on local disk
• Fetch from S3 and run compilation in parallel, fastest wins

[posix4e]

I'm confused about what needs to be done here

[rillian]

I believe the scope has slipped a bit. I think the expedient things to do would be:

1. cargo publish so cargo install sccache is available.
2. Implement eviction and a settable diskspace limit.
3. Publish and iterate on that.

I understand from Ted the S3 points are blocked on implementing the general fallback-to-another-cache feature. That should also get implemented, but doesn't stop this being usable locally in the meantime. Particularly valuable is that sccache supports the Visual Studio compiler, which traditional ccache does not.

[luser]

I should have filed these as separate issues, but yeah, that's about it. I'm actively working on making the local disk cache support a size limit, and publishing to crates.io is easy. I recently fixed part of the "sensible defaults" thing, in that I made it default to a sensible location for a local disk cache (~/.cache/sccache on Linux, similar on other platforms). Once I make the local disk cache bounded in size I'll publish a release to crates.io.

[bridiver]

Fetch from S3, store on local disk - I was about to file an issue to have the local cache always be active even if you're also using S3 or similar. Is that what you had in mind?

[sylvestre]

it is now much better :)