Suggest MSRV policy

I don't think there's a strong consensus here yet.

However, there seems to be a weak consensus, and also de-facto
practice for many popular crates (`cfg-if`, `lazy-static`).
Additionally, there's a lot of continuous discussion and re-litigation
across various forums and issue trackers.

So it seems like tentatively documenting current practice might make sense?

Author: matklad

src/checklist.md

@@ -72,6 +72,7 @@
 - **Necessities** *(to whom they matter, they really matter)*
   - [ ] Public dependencies of a stable crate are stable ([C-STABLE])
   - [ ] Crate and its dependencies have a permissive license ([C-PERMISSIVE])
+  - [ ] Crate has MSRV policy ([C-MSRV])
 
 
 [C-CASE]: naming.html#c-case
@@ -139,3 +140,4 @@
 
 [C-STABLE]: necessities.html#c-stable
 [C-PERMISSIVE]: necessities.html#c-permissive
+[C-MSRV]: necessities.html#c-msrv

src/necessities.md

@@ -109,3 +109,38 @@ cannot be used in some situations where most of the Rust runtime stack can.
 The license of a crate's dependencies can affect the restrictions on
 distribution of the crate itself, so a permissively-licensed crate should
 generally only depend on permissively-licensed crates.
+
+<a id="c-msrv"></a>
+## Crate has a MSRV policy (C-MSRV)
+
+A crate should clearly document its Minimal Supported Rust Version:
+
+* Which versions versions of Rust are supported now?
+* Under what conditions is MSRV increased?
+* How MSRV increases are reflected in the semver version of the crate?
+
+MSRV should be tested in CI.
+
+The API guidelines tentatively suggest that, for libraries, MSRV increase should
+*not* be considered a semver-breaking change. Specifically, when increasing
+MSRV:
+
+* for crates past `1.0.0` increment minor version (`1.1.3` -> `1.2.0`),
+* for crates before `1.0.0`, increment patch version (`0.1.3` -> `0.1.4`).
+
+This reduces the amount of ecosystem-wide work for MSRV upgrades and prevents
+incompatibilities. It also is a de-facto practice for many cornerstone crates.
+This policy gives more power to library consumers to manually select working
+combinations of library and compiler versions, at the cost of breaking `cargo
+update` workflow for older compilers.
+
+However, do not increase MSRV without a good reason, and, if possible, batch
+MSRV increases with sevmer-breaking changes.
+
+To reliably test MSRV on CI, use a dedicated Cargo.lock file with dependencies
+pinned to minimal versions:
+
+```bash
+$ cp ci/Cargo.lock.min ./Cargo.lock
+$ cargo +$MSRV test
+```