Use LZ4 instead of Deflate in metadata

[emberian]

No description provided.

[emberian]

Closes #6902

[brson]

@cmr what impact does this have on rustc size and running time?

[emberian]

@brson Cuts down compilation time of an empty file from 0.260s to 0.220s on my computer. It does cause the binaries to be larger:

deflate

3.5M    libextra-4782a756585a81-0.7-pre.so
4.0K    libmorestack.a
40K librust-2569481ffcb56381-0.7-pre.so
14M librustc-e786add3fc752ef5-0.7-pre.so
1.9M    librustdoc-94c552b7af5a89e1-0.7-pre.so
472K    librusti-ff2e0bf4a1967bd-0.7-pre.so
788K    librustpkg-e811e18a789111-0.7-pre.so
316K    librustrt.so
4.4M    libstd-c3ca5d77d81b46c1-0.7-pre.so
4.3M    libsyntax-db358e1e4f18873e-0.7-pre.so

lz4

4.0M    libextra-4782a756585a81-0.7-pre.so
4.0K    libmorestack.a
52K librust-2569481ffcb56381-0.7-pre.so
16M librustc-e786add3fc752ef5-0.7-pre.so
2.4M    librustdoc-94c552b7af5a89e1-0.7-pre.so
568K    librusti-ff2e0bf4a1967bd-0.7-pre.so
1.1M    librustpkg-e811e18a789111-0.7-pre.so
1.4M    librustrt.so
5.4M    libstd-c3ca5d77d81b46c1-0.7-pre.so
5.0M    libsyntax-db358e1e4f18873e-0.7-pre.so

Probably not worth it for a 16% improvement?

[emberian]

"Empty file" meaning

fn main() {}

When linking to extra it's 0.286s vs 0.331, still 16% speedup.

[brson]

@cmr how does the time to compile rustc change?

[emberian]

@brson Up to 16:58.86 from 16:50.72, but that could be noise.

[huonw]

What about the time to run the test suite?

[pcwalton]

16% speedup for small files seems definitely worth it to me. Small crates are the "first run experience" on Rust.

[emberian]

@pcwalton the problem is the 22% increase in binary size and a 50% increase
in compression time. I'll be experimenting with other compression
algorithms.