Document rust-lang/rust pr-subset-of-merge CI invariant

src/infra/docs/rustc-ci.md

@@ -5,4 +5,23 @@ is always in a valid state.
 
 The CI infrastructure is described in detail in the [rustc-dev-guide][rustc-dev-guide].
 
+## `rust-lang/rust` CI invariant
+
+Changes to the default and `beta`/`stable` release branches of the [rust-lang/rust] repository go
+through two sets of CI jobs:
+
+1. **PR CI**. This is a fast and restricted set of CI jobs that run against PRs to provide faster
+   feedback.
+2. **Merge CI**: This is the full set of CI jobs that all changes to [rust-lang/rust] must pass.
+   This includes, but are not limited to, e.g. testing jobs for all Tier 1 targets.
+
+We enforce an [rust-lang/rust] CI invariant where **PR CI jobs are a subset of Merge CI jobs modulo
+carve outs**. The carve out differences between PR CI and Merge CI jobs are tracked in
+<https://github.com/rust-lang/rust/issues/144259>. The reason for this subset invariant is because
+if you have PR-only CI jobs, a PR can fail the PR-only jobs but pass the Merge CI. This can cause
+all subsequent PR CI to fail, in _completely unrelated PRs_, which is very frustrating for other
+contributors.
+
+
+[rust-lang/rust]: https://github.com/rust-lang/rust
 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/tests/ci.html