# V2 Rust Pre-Merge Validation

`V2/ue/Tools/plan-v2-rust-premerge-validation.py` turns a pull request change
list into a fail-fast Rust compile-check plan. It uses
`plan-v2-rust-incremental-build.py` to find impacted crates, then rebuilds the
same affected graph into workspace-aware `cargo check --locked --all-targets`
commands.

The dedicated workflow `.github/workflows/v2-rust-premerge.yml` runs on
`pull_request` and `merge_group`. It gathers changed paths with
`git diff --name-only`, writes `premerge-validation-plan.json`, and executes the
plan with `run-v2-rust-premerge-validation.py`. The runner stops at the first
failed Cargo command and writes `premerge-validation-results.json` for release
evidence.

Changes to Rust build contracts, Rust validation tools, the toolchain, or the
pre-merge workflow force full Rust validation. Normal crate source changes only
compile-check the changed crates plus local reverse dependents. The workflow
uses the same `RUSTC_WRAPPER=sccache` and `CARGO_INCREMENTAL=0` environment as
the distributed Rust build path, so PR checks exercise the same cache and
compiler mode expected in CI.

Horde gates require `rust-premerge-plan-generated`,
`rust-premerge-fail-fast-enabled`, and zero `rust-premerge-compile-check`
failures before a Rust PR can merge.