Speeding up Rust builds inside Docker

Currently it is not possible to instruct cargo, the Rust package manager, to build only the dependencies of the software you are compiling! This means you can't easily pre-install build dependencies. Luckily you can workaround this with cargo build -p! I've been using this Python script to parse Cargo.toml:

parse-cargo-toml.py

#!/usr/bin/env python

from __future__ import print_function

import os
import toml

_pwd = os.path.dirname(os.path.abspath(__file__))
cargo = toml.loads(open(os.path.join(_pwd, 'Cargo.toml'), 'r').read())

for section in ['dependencies', 'dev-dependencies']:
    for dep, version in cargo[section].items():
        print('cargo build -p %s' % dep)

and then inside my Dockerfile:

RUN mkdir /bdcs-api-rs/
COPY parse-cargo-toml.py /bdcs-api-rs/

# Manually install cargo dependencies before building
# so we can have a reusable intermediate container.
# This workaround is needed until cargo can do this by itself:
# https://github.com/rust-lang/cargo/issues/2644
# https://github.com/rust-lang/cargo/pull/3567
COPY Cargo.toml /bdcs-api-rs/
WORKDIR /bdcs-api-rs/
RUN python ./parse-cargo-toml.py | while read cmd; do \
        $cmd;                                    \
    done

It doesn't take into account the version constraints specified in Cargo.toml but is still able to produce an intermediate docker layer which I can use to speed-up my tests by caching the dependency compilation part.

As seen in the build log, lines 1173-1182, when doing cargo build it downloads and compiles chrono v0.3.0 and toml v0.3.2. The rest of the dependencies are already available. The logs also show that after Job #285 the build times dropped from 16 minutes down to 3-4 minutes due to Docker caching. This would be even less if the cache is kept locally!

Thanks for reading and happy testing!

Speeding up Rust builds inside Docker

aka cargo build --deps-only

Comments !