Tracking Issue for checksum freshness

[Xaeroxe]

Summary

Original issue: #6529
Implementation: #14137
Rustc implementation: rust-lang/rust#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 Tracking the performance of -Zchecksum-freshness #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

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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

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1. initial version of checksum based freshness #14137
   A-build-execution A-dep-info A-rebuild-detection A-unstable S-waiting-on-bors Z-checksum-freshness +6
   
2. Add unstable support for outputting file checksums for use in cargo rust#126930
   A-query-system A-run-make A-testsuite S-waiting-on-bors T-bootstrap T-compiler +6