To build on this, there are parts of the ecosystem that explicitly outline their Minimum Supported Rust Version. Sometimes it's short (e.g. last two stable rele

To build on this, there are parts of the ecosystem that explicitly outline their Minimum Supported Rust Version. Sometimes it's short (e.g. last two stable releases), and sometimes it's longer (6+ most recent stable releases). What's not well expressed is that is that certain releases prompt a bunch of people to upgrade the newest libraries to require the latest stable release. And these releases are not easily visible or predictable from outside the community. This is driven by Rust releases that stabilize significant features; I'm talking about stuff like async, min const generics, impl trait, proc macro support. So support for older releases tends to wax and wane depending on how recently a big feature was stabilized. There is one other contributing factor - cargo tries to pick the most recent version of library (that's SemVer compliant), not the earliest one, and MSRV changes aren't at this point considered a breaking SemVer change (as far as I know)