How to set a Rust MSRV in Cargo

Rust crate users need to know which compiler release a package still supports before they upgrade dependencies or run continuous integration against older environments. Cargo stores that compatibility boundary in the package manifest so an unsupported compiler fails early with a clear version error.

The manifest key is rust-version in the Cargo.toml package table. Its value is a bare Rust version such as 1.70, not a semver range, toolchain channel, nightly name, or dependency requirement.

Set the value to the oldest Rust release the crate is actually tested with. If the crate uses a newer edition, standard-library API, dependency version, or compiler feature, raise the MSRV or change the code before publishing that compatibility promise.

Steps to set a Rust MSRV in Cargo:

Open a terminal in the package directory that contains Cargo.toml.

For a workspace, set rust-version in each member package that needs an explicit compatibility policy.
Add rust-version to the package table.
Cargo.toml
```
[package]
name = "demo-lib"
version = "0.1.0"
edition = "2021"
rust-version = "1.70"
 
[dependencies]
```
Use a bare version number with at least one component. Values such as >=1.70, 1.70-nightly, or stable are not valid rust-version values.
Install the MSRV toolchain if it is not already installed.
```
$ rustup toolchain install 1.70.0 --profile minimal
```
Use the patch version that matches the MSRV line being tested. The manifest may use 1.70 while rustup installs the release as 1.70.0.
Check the package with the MSRV toolchain.
```
$ cargo +1.70.0 check --all-targets
    Checking demo-lib v0.1.0 (/work/demo-lib)
    Finished dev [unoptimized + debuginfo] target(s) in 0.18s
```
--all-targets includes library, binary, test, bench, and example targets selected by Cargo. Add the same feature flags used by your crate's supported configuration, such as --all-features when every feature must support the MSRV.

Related: How to check Rust code with Cargo
Install one older toolchain when the version boundary needs a failure check.
```
$ rustup toolchain install 1.69.0 --profile minimal
```
Skip this probe when the MSRV check is enough for the change review, or replace 1.69.0 with the immediately older Rust release for your chosen MSRV.
Confirm Cargo rejects the older compiler.
```
$ cargo +1.69.0 check --all-targets
error: package `demo-lib v0.1.0 (/work/demo-lib)` cannot be built because it requires rustc 1.70 or newer, while the currently active rustc version is 1.69.0
```
Do not use --ignore-rust-version in an MSRV check. That option bypasses the manifest guard and is only useful when intentionally testing an unsupported build.

Run the project test suite with the MSRV toolchain before release or CI handoff.

$ cargo +1.70.0 test --all-targets
   Compiling demo-lib v0.1.0 (/work/demo-lib)
    Finished test [unoptimized + debuginfo] target(s) in 0.20s
     Running unittests src/lib.rs (target/debug/deps/demo_lib-e2b4119928f07e2e)

running 1 test
test tests::it_works ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

A cargo check pass proves the manifest boundary and type-checks selected targets. A test run catches code paths that only execute under the crate's tests.
Related: How to run Rust tests with Cargo