cargo-lock-fetch is a cargo plugin I wrote to solve a build issue at https://www.adbglobal.com/ which would otherwise require manually specifying multiple source files from a rust microservice to be copied into a docker layer for dependency prefetch.

Prefetching dependencies to a docker layer is crucial in reducing build times in docker-based environments. It allows the service to be rebuilt without redownloading all required crates unless Cargo.lock changes without providing cargo with a cache mount, which may not be available.

This can normally be done with cargo fetch. However, cargo fetch requires a large part of the project structure, including, but not limited to, all the Cargo.toml files, which becomes a burden when there is more than one. This makes one end up either "prefetching" after a COPY . . which defeats the purpose, as potentially the same set of crates is downloaded any time any project file changes, or specifying every required file and editing the list on project structure changes.

cargo-lock-fetch solves this very issue by requiring only the Cargo.lock file to prefetch all project's dependencies making prefetching dependencies to a docker layer as simple as:

COPY --from=komar007/cargo-lock-fetch \
    /cargo-lock-fetch /usr/local/cargo/bin

COPY Cargo.lock .
RUN cargo lock-fetch

allowing for later:

COPY . .
RUN cargo build --frozen

As a bonus, cargo-lock-fetch allows for dependencies vendoring, just like cargo vendor, but requiring access only to Cargo.lock.