lyrn msrv

$ lyrn msrv [path]

rust-version in Cargo.toml is a promise to everyone running cargo install, and Cargo believes it. The standard’s msrv job keeps the number from rotting - it builds on exactly that toolchain - but it cannot say whether the number is the true one. lyrn new writes the toolchain in the room, which is the newest that builds the code, not the oldest. lyrn msrv measures the oldest and writes it.

$ lyrn msrv
lyrn msrv: ~/demo-tool, stable 1.99

Cargo.toml
  declared   1.99
  floor      1.85 - assert_cmd 2.2.2 asks for it
  install    1.85 ... done
  check      1.85 ... builds, 12s
  measured   1.85 - rust-version 1.99 → 1.85 in Cargo.toml [package]

removed the toolchains installed to measure: 1.85

How it measures

The search starts at the floor the code cannot go under anyway: its edition - 2024 needs 1.85 - and the highest rust-version any package in the build declares, platform-specific ones included, since the promise is for every platform. The floor is tried first, and it is the answer more often than not. Past it the toolchains up to stable are halved: one that builds the code is taken to mean every newer one does too, so a handful of builds find the oldest.

Each toolchain is asked the question the msrv job asks - cargo check --all-targets - with the lockfile as it is (--locked) and --ignore-rust-version, because otherwise the number being measured would answer it: Cargo refuses a package whose rust-version is above the toolchain before compiling a line.

The builds go into a target directory of their own, which is removed after, so target/ keeps the build the gate made. A toolchain the machine lacks is installed with rustup’s minimal profile and uninstalled when the measuring is done; stable is the one already there. lyrn ci msrv installs the toolchain it needs when it runs.

Where it writes

rust-version goes where the manifest keeps it: the crate’s [package], or the workspace’s [workspace.package] when the crate inherits it or the manifest is a virtual workspace. A desktop app’s shell, src-tauri/Cargo.toml, is measured on its own, unless the top’s workspace already holds it. A number that is already right is left as it is.

lyrn doctor holds the other half: that the number is declared, and that a CI job builds on it.