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