Adopt the phase 2 Rust baseline (toolchain, rustfmt, lints)

.cargo/config.toml

@@ -1,9 +1,3 @@
-[unstable]
-codegen-backend = true
-
-[profile.dev]
-codegen-backend = "cranelift"
-
 [target.x86_64-unknown-linux-gnu]
 linker = "clang"
 rustflags = ["-C", "link-arg=-fuse-ld=mold"]

.github/workflows/release.yml

@@ -56,8 +56,8 @@ jobs:
           key: cross-${{ env.CROSS_REVISION }}
       - name: Install cross
         env:
-          # Clear repository build flags from .cargo/config.toml (mold linker
-          # and Cranelift codegen-backend) before installing cross from CROSS_REVISION.
+          # Clear the repository's mold linker rustflags from
+          # .cargo/config.toml before installing cross from CROSS_REVISION.
           RUSTFLAGS: ""
         run: |
           if [ -x "$HOME/.cargo/bin/cross" ]; then
@@ -74,8 +74,9 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-cargo-${{ matrix.target }}-
       - name: Build release binary
-        # Use +stable to override rust-toolchain.toml (which specifies nightly
-        # with Cranelift for development) and ensure release builds use stable.
+        # Use +stable to override rust-toolchain.toml's nightly pin (needed
+        # for the repository's lint and dev-fast tooling) so release builds
+        # use stable regardless.
         env:
           # Build release artifacts without repository-local linker flags,
           # including mold rustflags from .cargo/config.toml.

.rustfmt.toml

@@ -1,3 +1,10 @@
+# Canonical rustfmt configuration for the estate. In a consuming
+# repository this file lives at the root as `.rustfmt.toml`; keep it
+# there, aligned with canon (the rust-rustfmt-baseline rule FMT-001
+# compares the parsed keys).
+#
+# `unstable_features = true` means rustfmt must run on the nightly channel;
+# the rust-rustfmt-baseline rule (FMT-002) checks for that evidence.
 unstable_features = true
 comment_width = 100
 format_code_in_doc_comments = true

AGENTS.md

@@ -372,3 +372,26 @@ The following tooling is available in this environment:
 
 These practices help maintain a high-quality codebase and facilitate
 collaboration.
+
+## Fast development builds
+
+`make dev-build` and `make dev-test` compile with the opt-in Cranelift
+backend and the mold linker configured in `tools/dev-fast/config.toml`.
+They require a nightly toolchain and, on Linux, a `mold` binary on the
+`PATH`. The fragment is passed explicitly with `--config`, so release,
+coverage, and verification builds are unaffected; never copy its contents
+into `.cargo/config.toml`, which Cargo applies to every build.
+
+## Standard development path
+
+The dev-fast profile is the standard development path, not a side path:
+`make build`, `make test`, `make lint`, and `make typecheck` all pass
+`--config tools/dev-fast/config.toml` to Cargo. An agent or human calling
+`cargo build`, `cargo test`, `cargo clippy`, `cargo check`, or `cargo doc`
+directly for a development build, test, lint, or typecheck run must pass
+that same `--config` flag, or the run will use a different codegen
+backend and linker than `make` does. The fragment must never be applied
+to coverage, release, or verification builds. Mixing direct-cargo and
+`make` invocations without the flag thrashes the incremental build
+cache, because Cargo fingerprints the codegen backend and the two runs
+produce different fingerprints for what looks like the same build.