MSRV Policy

[mitsuhiko]

I wanted to originally support old Rust versions for as long as I can since insta is a testing tool and as a user you never want to be in a situation where you cannot test your own crate to your own preferred version ranges because your test tool moves you up.

Unfortunately the world is moving up around insta right now:

• Insta currently depends on once_cell, which moved up to 1.56: Minimum Rust Version matklad/once_cell#201: potential workaround is to ditch once_cell
• similar uses bstr which in the 1.x release train moved up to 1.60. This is a bit of a weird situation since this is my own crate but there are users who would like the new version to avoid duplicates: Update bstr to 1.x similar#40
• console uses once_cell which indirectly now moved to 1.56: Drop once_cell / new MSRV Policy console-rs/console#134
• toml is moving up to 1.60 but they are open to making this a semver bump: MSRV policy toml-rs/toml#346

A few other things already moved up, some of which we dropped in the process (see #214)

I have not yet decided what I want to do here, but one way is to drop once_cell for now and not to upgrade in similar to postpone this issue for a while. Potentially the discussion in the community settles by then. It seems like the community is abandoning supporting old rustc versions in general (see rust-lang/libs-team#72).

[mitsuhiko]

For now I'm postponing a decision here. I have restored the intended MSRV policy by dropping once_cell in the interim.

[CosmicHorrorDev]

syn bumped its MSRV up to 1.56 with the v2 release. This alone will bump the MSRV of a lot of projects to at least 1.56 once updates propagate out

I don't think we depend on syn with all features off, but any feature requiring serde will pull it in with a non-semver breaking change