🚧 Section is work-in-progress.

🌱 Concepts

cargo-msrv requires a list of available Rust releases to determine or verify the MSRV. It can fetch this list of releases from several places (which each vary in completeness, correctness, and size/fetch time).

The following list are all release sources which can be configured (e.g. cargo msrv find --release-source <value>). cargo-msrv can be compiled with or without these release sources (our own release includes all), except for bundled which is always included.

rust-changelog (default)
bundled-unless-outdated
github
rust-dist
bundled

rust-changelog

Fetches the RELEASES.md changelog from the official rust GitHub repository. Does a time check which compares the callers computer date against the latest release date, because the changelog may be slightly ahead when the Rust developers are cutting a release.

This source only provides version numbers and release dates, so toolchains etc. are not included.

This source is the default because it is a single file (unlike say github releases which needs to consider pagination and adhere to the max API limits). It is very fast, consistently up to date and it is reasonably easy to parse.

bundled-unless-outdated

Same as bundled, except that if its configured max age (default is 7 days) has been reached since it was generated, then it falls back to the (configured) fallback release source, which is the same list of release sources (defaults follow the same ordering rules).

Note that the bundled source is complete (it's effectively the rust-dist source packaged in a much smaller format, and pre-packaged as Rust source code for easy embedding), up to its generation day, and includes not just versions but also full toolchain and target information.

If it is outdated and another source is fetched to amend the bundled source, that source may be much less complete which can be a source of confusion if you're combining it with strict filters on e.g. the host target (which the online fetched release source may not have). In general cargo-msrv tries to have best effort defaults. If you find a case where it is not entirely the case, open an issue (be sure to include your use case and suggested best effort!).

github

Uses the GitHub releases list via the GitHub API to determine the available versions. It takes the release date from the API's release date. NB: For versions prior to Rust 1.47.0, this release date is incorrect (for versions <= 1.46.0 releases were created in one big batch on 2020-09-10). cargo-msrv barely uses the release date so this isn't a problem.

rust-dist

The most complete release source. All available Rust release and distribution information can be found here in one place, if you're willing to fetch and parse it all. Fetching it takes a long time because it needs to go through a lot of information, much of which is not relevant if you only care about releases (and not everything, all distributions ever created contain).

Usually version information is enough and then rust-dist's added toolchain data is unnecessary, but if you do stuff like cross-compilation, you may want to actually select toolchains which are really available, iso guessing that they are. This is the place where the nitty gritty gray areas between the differences of "releases" vs "distributions" start to fade, and where a vague general term like "MSRV" (minimum supported Rust version) starts to require a much more guarded definition wrt what "Rust" then means (e.g. is it only the language compatibility, or do the underlying platforms matter?).

The bundled source is generated from rust-dist by the rust-releases' bundled generator

bundled

Same as rust-dist, but generated and embedded as generated Rust code at a certain point in time. Only as up to date as the time where (a) rust-releases-bundled was generated and releases, (b) cargo-msrv updated its lockfile to include the latest rust-releases-bundled and (c) cargo-msrv was released.

In general ever new cargo-msrv release should contain the latest generated set, available on or prior to cargo-msrv's release date. If it didn't, please open an issue.

Resolver

run-toolchain resolver (default): resolver which runs actual toolchains against a crate
rust-version resolver: author defined resolver, used by cargo-msrv list

The cargo-msrv Book 🦀