building cargo breaks build-cache for other tools

[jieyouxu]

After #130739 I noticed that whenever I modify run-make tests (on Windows), stage 1 cargo gets unnecessary rebuilt which also triggers rebuilds for other tools like stage 0 rustdoc. I'm pretty sure something in my implementation is wrong w.r.t. to properly caching, but I didn't invest time in figuring out why yet, hence opening this bug to track it.

The build sequence is something like stage 0 std -> stage 1 rustc -> stage 1 std -> stage 1 tool cargo -> stage 0 compiletest / run-make-support / rustdoc.

Example repro steps:

• Run ./x test run-make --stage 1 -> make sure stage 1 cargo is already built.
• Modify a tests/run-make rmake.rs test.
• Observe that stage 1 cargo gets rebuilt even though I didn't modify it.

[jieyouxu]

EDIT: oh, is it because in run-make-support or rustdoc we use prepare_tool_cargo or something that is a different staged cargo compared to the one we built with stage 1 rustc...

[jieyouxu]

Actually unassigning because I don't know how to cleanly fix this just yet.

EDIT: opened thread https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/Modifying.20run-make.20tests.20unnecessarily.20rebuild.20stage.201.20cargo.

[onur-ozkan]

Example repro steps:

Run ./x test run-make --stage 1 -> make sure stage 1 cargo is already built.
Modify a tests/run-make rmake.rs test.
Observe that stage 1 cargo gets rebuilt even though I didn't modify it.

Running ./x test run-make --stage 1 twice is enough to reproduce cargo rebuild issue. Probably some rustflags invalidating the build cache.