I find myself writing a lot of Rust nowadays. I’m by no means an expert, but I need to make do with my limited knowledge. A constant problem I’m facing is that the binaries outputted by Rust are huge. This is especially a problem on mobile, where each megabyte counts.

The documentation is a bit confusing, but here is the configuration I ended up using to get somewhat OK sizes. In the cargo.toml file:

[profile.release] # When compiling in release mode
debug = false # Exclude debug symbols
strip = "symbols" # Exclude the rest of the symbols
# opt-level = "z" # Did not use this, but it equals C++'s optimize for size (O3?)
lto = true # Link time optimization, not sure what this does but it helps reduce the size
codegen-units = 1 # Cargo specifies 16 parallel codegen units for release builds. This improves compile times, but prevents some optimizations.
panic = "abort" # when Rust code encounters a situation when it must call panic!(), it unwinds the stack and produces a helpful backtrace. The unwinding code, however, does require extra binary size. rustc can be instructed to abort immediately rather than unwind, which removes the need for this extra unwinding code.

Cargo Bloat

I haven’t fully explored what this tool can do, but it does point to large sections of the code. Install it with cargo install cargo-bloat and then run:

cargo bloat --release --target=<your-target>

Building both a static and dylib

In my experience static binaries on iOS are OK, but on Android they can be huge. Ideally you would specify crate-type = ['staticlib', 'dylib'] and just be on your merry way, however, it seems there is a bug in the rust compiles and it bloats the static lib massively. In order to get a static binary for iOS and a dynamic one for Android you can set crate-type = ['dylib'] and change the compilation command for iOS to cargo rustc --crate-type=staticlib ...

OpenSSL

The OpenSSL adds a lot of weight to a crate unless you need it, you can use ring which reduced further 4 MB to 6 MB of my crate.

Further reading

Here is a great repo with a lot more flags and tweaks, mostly under experimental flags or nightly Rust versions so use at your own risk.

Results

Using all the compile optimizations, I was able to reduce the output of one of my binaries from 66 MB to 24.7 MB. It’s still large but better.