Significant iOS binary size increase with Rust 1.50

[rfk]

As noted in mozilla-mobile/firefox-ios#8044 (comment), the v73.0.0 release of application-services has significantly increased compiled binary size (by nearly 30%, from 35M to 45M).

Some of that increase is due to the inclusion of Nimbus, but actually the largest contributor seems to be the switch from Rust 1.43 to Rust 1.50. Here's the size of a local build of v73.0.0 done using Rust 1.50, which matches what we see in the release artifact:

[rfk] repos/mozilla/application-services (git)-[tags/v73.0.0] % du -hs Carthage/Build/iOS/MozillaAppServices.framework/MozillaAppServices                                                
 46M    Carthage/Build/iOS/MozillaAppServices.framework/MozillaAppServices

Here's the same when built with Rust 1.43, the toolchain we used for previous releases:

[rfk] repos/mozilla/application-services (git)-[tags/v73.0.0] % du -hs Carthage/Build/iOS/MozillaAppServices.framework/MozillaAppServices
 40M    Carthage/Build/iOS/MozillaAppServices.framework/MozillaAppServices

I confirmed that if I manually remove Nimbus from the build, we get back down to around 35M, which was approximately the size of the previous release.

┆Issue is synchronized with this Jira Task

[rfk]

(Which, +5M still seems like a lot for Nimbus to be honest, but that's a separate question...)

[dmose]

Random thoughts:

• I wonder if running cargo bloat on both versions would give us useful insight
• Longer term, we could catch this sort of thing automagically in CI with something like https://github.com/orf/cargo-bloat-action/
• between 1.43 and 1.50, rustc upgrade from LLVM 9 to LLVM 11
• there was a Build host dependencies with opt-level 0 by default rust-lang/cargo#8500 change in there too, but I'd be pretty surprised if this was the cause
• complete compiler release notes: https://github.com/rust-lang/rust/blob/master/RELEASES.md
• general tips for shrinking things down: https://github.com/johnthagen/min-sized-rust

[dmose]

Other things to try:

• look at the LLVM 10 and 11 release notes, see if anything is mentioned there, particularly stuff controllable from the command-line
• look the LLVM 10 and 11 github issues and PRs for rustc, and see if anyone noticed this along the way

[dmose]

@rfk since you've got both builds around, can you try running cargo bloat to see if it yields any interesting info?

[dmose]

If the problem is LLVM-upgrade-related, interestingly, LLVM12 has landed in nightly rustc, so maybe that'll change things yet again...

[dmose]

LLVM links, including release notes

[dmose]

cargo-bisect-rustc might help us sort this out too...