Tracking Issue for checksum freshness

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Summary

Original issue: #6529
Implementation: https://github.com/rust-lang/cargo/pull/14137
Rustc implementation: https://github.com/rust-lang/rust/pull/126930
Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#checksum-freshness

Add the ability to use content checksums rather than filesystem mtimes to determine if a crate should be recompiled.

Unresolved Issues

• How should we handle checksums for files processed by build scripts?
  • One option is to provide an officially sanctioned API for build scripts. Then we can use that API to compute and output the correct checksum value when the relevant function is invoked in the build script.
• Figure out how this will be stabilized (always on, config)
  • Get benchmarked https://github.com/rust-lang/cargo/issues/14722
• If this is on by default, migrate Cargo's tests to it (hack on and make pass)

Future Extensions

No response

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

### Tasks
- [ ] https://github.com/rust-lang/cargo/pull/14137
- [ ] https://github.com/rust-lang/rust/pull/126930

Start with the checksum freshness tracking issue and review the linked Cargo and Rust implementation pull requests, followed by the unstable Cargo documentation. The remaining work is listed under Unresolved Issues: build-script checksums, stabilization and configuration, benchmarking, and migrating Cargo tests if enabled by default.