unconditionally rebuilds all deps on rust nightly

[matthiaskrgr]

calling cargo build ; cargo build will cause sccache to try to rebuild the entire dep tree + root crate twice, even though nothing changed and it should technically not.

cargo clean ; cargo build ; cargo build
   Compiling serde v1.0.80
   Compiling ryu v0.2.7
   Compiling itoa v0.4.3
   Compiling serde_json v1.0.39 (/home/matthias/vcs/github/serde_json)
    Finished dev [unoptimized + debuginfo] target(s) in 3.75s
   Compiling itoa v0.4.3
   Compiling ryu v0.2.7
   Compiling serde v1.0.80
   Compiling serde_json v1.0.39 (/home/matthias/vcs/github/serde_json)
    Finished dev [unoptimized + debuginfo] target(s) in 1.16s

Cargo with rustc (and no RUSTC_WRAPPER) gets this right and does not rebuild anything on the second "cargo build" run.

This happens since this commit in the cargo repo: rust-lang/cargo@6b28a0c

Related cargo ticket: rust-lang/cargo#6976

[luser]

I tried this out on my simple snippet crate and assuming I set CARGO_INCREMENTAL=0 so that sccache will cache the top-level crate I can reproduce it. I think the issue here is that sccache isn't caching the rmeta files. However, the cause of that seems to be that rustc is not listing the rmeta in its --print=file-names output. I ran cargo +nightly build -v on that snippet crate to get the compiler invocations, and added --print=file-names like sccache does internally and it only shows the rlib:

$ rustc +nightly --crate-name itoa /home/luser/.cargo/registry/src/github.com-1ecc6299db9ec823/itoa-0.3.4/src/lib.rs --color always --crate-type lib --emit=dep-info,metadata,link -C debuginfo=2 -C metadata=9de9e07ff885e8a5 -C extra-filename=-9de9e07ff885e8a5 --out-dir /build/snippet/target/debug/deps -L dependency=/build/snippet/target/debug/deps --cap-lints allow --print=file-names
libitoa-9de9e07ff885e8a5.rlib

I noted that there's an existing workaround in sccache that points to rust-lang/rust#54852 where rustc --emit=metadata --print=file-names prints the rlib name and not the rmeta, perhaps this is related?