This crate can assist you in finding the Minimum Supported Rust Version for a crate.

In this readme you'll find everything to get you started. You can find more detailed explanations in the cargo-msrv book.

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Rustup is required for the cargo msrv (find) and cargo msrv verify commands.

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• pacman -S cargo-msrv

You can use the following commands for building and running via Docker:

• $ docker build -t cargo-msrv .
• $ docker run -t -v "$(pwd)/Cargo.toml":/app/Cargo.toml cargo-msrv

Docker images are also available on Docker Hub.

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• cargo msrv find or cargo msrv find --linear to find the MSRV for a Cargo project in your current working directory.
• cargo msrv --path <dir> find to find the MSRV for a Cargo project in the <dir> directory.
• cargo msrv find -- <command> to use <command> as the compatibility check which decides whether a Rust version is compatible or not. This command should be runnable through rustup as rustup run <toolchain> <command>.
  • Example: cargo msrv find -- cargo check --tests.
  • Example: cargo msrv find -- cargo check --ignore-rust-version. Only available on Rust >= 1.56.
• cargo msrv verify to verify the MSRV as specified by a crate author\
  • A crate author may specify the MSRV using the package.rust-version (Rust >=1.56) or the package.metadata.msrv key in the 'Cargo.toml' manifest. See the book for a more detailed description.
• cargo msrv list to list the MSRV's of your dependencies as specified by their authors
• cargo msrv show to show the currently specified MSRV

Please refer to the commands chapter in the cargo-msrv book for more detailed descriptions of the supported (sub) commands.

Options

Find your Minimum Supported Rust Version!

Usage: cargo msrv [OPTIONS] <COMMAND>

Commands:
  find    Find the MSRV
  list    Display the MSRV's of dependencies
  set     Set the MSRV of the current crate to a given Rust version
  show    Show the MSRV of your crate, as specified in the Cargo manifest
  verify  Verify whether the MSRV is satisfiable
  help    Print this message or the help of the given subcommand(s)

Options:
      --path <Crate Directory>
          Path to cargo project directory

      --manifest-path <Cargo Manifest>
          Path to cargo manifest file

  -h, --help
          Print help (see a summary with '-h')

  -V, --version
          Print version

User output options:
      --output-format <FORMAT>
          Set the format of user output

          [default: human]

          Possible values:
          - human:   Progress bar rendered to stderr
          - json:    Json status updates printed to stdout
          - minimal: Minimal output, usually just the result, such as the MSRV or whether verify succeeded or failed

      --no-user-output
          Disable user output

Debug output options:
      --no-log
          Disable logging

      --log-target <LOG TARGET>
          Specify where the program should output its logs

          [default: file]
          [possible values: file, stdout]

      --log-level <LEVEL>
          Specify the severity of logs which should be

          [default: info]
          [possible values: trace, debug, info, warn, error]


            You may provide a custom compatibility `check` command as the last argument (only
            when this argument is provided via the double dash syntax, e.g. `$ cargo msrv -- custom
            command`.
            This custom check command will then be used to validate whether a Rust version is
            compatible.
            A custom `check` command should be runnable by rustup, as they will be passed on to
            rustup like so: `rustup run <toolchain> <COMMAND...>`. NB: You only need to provide the
            <COMMAND...> part.

            By default, the custom check command is `cargo check`.

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JSON output may be enabled by providing the --output-format json flag: cargo msrv find --output-format json. Events are printed as json lines. The event type is indicated by the type key.

Please see the Output formats and Output format: JSON chapters of the book for documentation of this output format.

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An excerpt

cargo-msrv was originally born out of a desire to find the MSRV for a Rust project (more specifically package). MSRV stands for "minimal supported Rust version" and is the earliest or oldest version supported by a Rust project. For different projects this may mean different things, but for this post I will consider "support" as "does compile with a Rust toolchain of a certain version".

Fast forward a few years, and the MSRV has become somewhat more ubiquitous which can also be seen by its inclusion into Cargo as the rust-version. Over time some additional tools were added to cargo-msrv. One of these was the cargo msrv verify subcommand.

This subcommand can be used to check whether a Rust project supports its defined MSRV (e.g. via this rust-version field in the Cargo manifest). For example, in a CI pipeline you can use this to check whether your project works for the version you promised to your users.

Originally, I kept the cargo msrv top level command aside from the subcommands for backwards compatibility reasons. In hindsight I probably shouldn't have done that, but as is, their coexistence at least provides me with the opportunity to write this blog post 😅.

How cargo msrv works

I described the "support" from "minimal supported Rust version" (MSRV) above as the somewhat simplified "does compile with a Rust toolchain of a certain version".

You may write that as a function like so: fn is_compatible(version) -> bool. If you run this test for some Rust version, when the function produces the value true, then we consider the Rust version to be supported. If instead the function produces the value false, then the Rust version is not supported.

cargo msrv specifically searches for the minimal Rust version which is supported by a given Rust project. While there are some caveats, we build upon Rust's stability promise . In our case that is the idea that Rust versions are backwards compatible.

For a simple example to determine an MSRV, you can linearly walk backwards from the most recent Rust version to the earliest. When your project doesn't compile for a specific Rust version, then the last version that did compile can be considered your MSRV.

Let's make it a bit more concrete with an example. For this example, we assume that Rust the following Rust versions exist: 1.0.0 up to and including 1.5.0.

Consider a project which uses the Duration API which was stabilised by Rust 1.3.0 (and nothing more recent 😉).

Then, if you would compile this project with Rust version from most recent to least recent, you would expect the following to happen:

• is_compatible(Rust 1.5.0) returns true ✅
• is_compatible(Rust 1.4.0) returns true ✅
• is_compatible(Rust 1.3.0) returns true ✅
• is_compatible(Rust 1.2.0) returns false ❌ ("Duration is not stable")
• is_compatible(Rust 1.1.0) returns false ❌
• is_compatible(Rust 1.0.0) returns false ❌

Since we only care about the minimal Rust version, you could have stopped searching after compiling Rust 1.2.0; Rust 1.3.0 was the earliest released Rust version which worked.

In reality doing a linear search is quite slow (at the time of writing, there are 79 minor versions), so we primarily use a binary search instead to incrementally reduce the search space.

cargo msrv verify works quite similar to "finding the MSRV", but instead of running a search which produces as primary output the MSRV, in this case the MSRV is already known in advance. So given a MSRV of 1.3.0 we just run the is_compatible(Rust 1.3.0) function once. If it returns true we can say that the 1.3.0 is an acceptable MSRV ( although not necessarily strictly so). More importantly, if it returns false, then the specified version is actually not supported, and thus can not be an MSRV).

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Licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.