rustc --emit=dep-info does too much work

[luser]

I'm working on caching Rust compilation in sccache, and I got it working but it's not as fast as I had hoped. As part of the design that @alexcrichton, @glandium and I hashed out, I'm running rustc --emit=dep-info to get a list of source files as input to the hash used as the cache key. It turns out that running that takes almost as long as actually compiling the crate for some crates. For example, on my machine compiling the rand crate takes ~1.9s, and running just rustc --emit=dep-info for the same crate takes ~1.1s.

@eddyb looked at the -Z time-passes output and determined that rustc is doing too much work in this case. The fix looks simple, thankfully, so I'm going to try to fix it.

[luser]

That's significantly better!

$ rustc +stable --version
rustc 1.15.1 (021bd294c 2017-02-08)

$ time rustc --crate-name rand /home/luser/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.14/src/lib.rs --crate-type lib -g -C metadata=a266d0434ee969c4 -C extra-filename=-a266d0434ee969c4 -L dependency=/build/read-process-memory/target/debug/deps --extern libc=/build/read-process-memory/target/debug/deps/liblibc-8c7226756bef4c8b.rlib --cap-lints allow --emit dep-info -o /tmp/deps.d

real	0m1.396s
user	0m1.268s
sys	0m0.024s

$ rustc +nightly --version
rustc 1.17.0-nightly (fd182c401 2017-03-13)

$ time rustc +nightly --crate-name rand /home/luser/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.14/src/lib.rs --crate-type lib -C debuginfo=2 -C metadata=7436655aefac27bf -C extra-filename=-7436655aefac27bf -L dependency=/build/read-process-memory/target/debug/deps --extern libc=/build/read-process-memory/target/debug/deps/liblibc-89a24418d48d484a.rlib --cap-lints allow --emit dep-info -o /tmp/deps.d

real	0m0.111s
user	0m0.100s
sys	0m0.004s

[mimoo]

I'm running into the same type of issue, I want to run RUSTFLAGS="--emit=dep-info" cargo check to obtain .d files and understand what files will be used for the compilation. But I do not want to compile anything. This does not seem to work unfortunately :(

[eddyb]

@mimoo --emit doesn't override previous --emits AFAIK, it's additive. So your RUSTFLAGS does nothing because cargo check already passes --emit=dep-info,metadata.

You need to use Cargo as a library, or (as I've seen recommended by other people) use cargo_metadata, in order to find all the crates, and then you can attempt to run rustc yourself with --emit=dep-info, but that's not guaranteed to work.

Looking at the PR that closed this issue, --emit=dep-info stops compilation after expansion, which means it needs to be able to load macros from dependency crates (and e.g. include!(...) can add to the .d output AFAIK).

So you probably need the full cargo check anyway, unless you want some approximation (that --emit=dep-info won't give you).