MSRV: the oldest compiler you promise to support

Level: 201 · working knowledge

One line: rust-version = "1.80" in Cargo.toml is a promise that your crate builds on that compiler — and keeping it means checking it, because a dependency's update can break it without a single line of yours changing.

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

What it has to cover

• What rust-version does today: the error Cargo prints on an older toolchain, and what it does not check (your code's use of newer APIs)
• The MSRV-aware resolver: whether Cargo on the pinned toolchain prefers dependency versions compatible with your rust-version, and which setting controls it — verify in the Cargo reference before writing it down
• Testing it: a CI job on the declared version, cargo hack --rust-version, and cargo +1.80 check
• Minimal versions: -Z minimal-versions / -Z direct-minimal-versions on nightly, and why serde = "1" can be a lie about the oldest serde you actually work with
• Changelogs and unreleased versions: what a changelog promises about MSRV bumps, and treating an MSRV increase as a minor change or a breaking one
• How this library pins its own toolchain instead — pinning the toolchain — and why an application and a library answer this differently

The trap it exists for

Declaring rust-version once and never testing it. The field is checked against the compiler, not against the code, so a crate can claim 1.70 and call a method stabilised in 1.80 until someone on 1.70 tries it.

Where this sits

Pinning the toolchain chooses the compiler you build with. MSRV is the oldest compiler your users may build with. Publishing a crate is where the promise becomes public.

See also

• Pinning the toolchain — one compiler for your own builds
• rustup — cargo +1.80 and installing an older toolchain
• Adding a dependency — the range a version requirement really accepts
• Publishing a crate — where the promise goes public
• Semver hazards — is an MSRV bump a breaking change?

If you are coming from another language

• Python. requires-python = ">=3.10" is the same field with the same weakness: it is metadata, and nothing checks your code against it unless CI runs the oldest interpreter.
• Go. The go 1.22 line in go.mod is stricter since Go 1.21 — the toolchain refuses or downloads a newer one — which is the enforcement rust-version leaves to CI.
• C++. The closest idea is a minimum -std=c++17 plus a compiler version matrix, kept by hand in CI.

Po polsku

MSRV (minimum supported Rust version) to najstarszy kompilator, na którym obiecujesz, że skrzynia się zbuduje; zapisuje się go jako rust-version w Cargo.toml. To pole jest sprawdzane względem kompilatora, a nie kodu, więc obietnica jest coś warta dopiero wtedy, gdy CI faktycznie buduje projekt na tej wersji.

Szukaj po polsku: minimalna wspierana wersja Rusta · rust-version cargo toml · msrv policy rust crate · cargo minimal-versions