build: Tweak make validate-rust

This took me some thinking and experimenting. Basically we want:

- Hard deny some warnings (this is covered by the Cargo.toml
  workspace.lints.rust)
- Gate merging to main in CI on an exact set of warnings we
  want to forbid, but *without* also using a blanket
  -Dwarning deny policy because that could break our build
  when the compiler revs.
- A corollary to the previous: allow developing locally
  without killing the build just because
  you have an unused import or some dead code (for example).
  So we don't want to add `dead_code = deny` into the Cargo.toml.
- Be able to easily reproduce locally what CI is gating on
  in an efficient way.

We already had `make validate-rust` which was intending to navigate
this, but what was missing was the "deny extended set of warnings"
so we got code committed to git main which hit `unused_imports`.

Clippy upstream docs recommend the `RUSTFLAGS = -Dwarnings`
approach in e.g.
https://doc.rust-lang.org/clippy/continuous_integration/github_actions.html
but again I think this is a problem because it can break with
updated Rust/clippy versions (unless you pin on those, but that
becomes a pain in and of itself).

The problem also with doing `RUSTFLAGS = -Dwarnings` *locally*
is it blows out the cargo cache.

So here's the solution I came to: We run `cargo clippy -A clippy:all`,
and then deny some specific clippy lints *and* the core Rust
warnings we want (`unused_imports` type things) at this stage.
The advantage is this doesn't blow out the main Cargo cache,
and I can easily reproduce locally exactly what CI would gate on.

Also while we're here, add `make fix-rust` which is a handy
way to use the existing `clippy --fix` to locally fix things
like unused imports as well as other machine-applicable
things that are in e.g. `clippy::suspicious`.

Signed-off-by: Colin Walters <[email protected]>

Author: cgwalters

Makefile

@@ -59,15 +59,22 @@ test-bin-archive: all
 test-tmt:
 	cargo xtask test-tmt
 
-# Checks extra rust things (formatting, a few extra rust warnings, and select clippy lints)
+# This gates CI by default. Note that for clippy, we gate on
+# only the clippy correctness and suspicious lints, plus a select
+# set of default rustc warnings.
+# We intentionally don't gate on this for local builds in cargo.toml
+# because it impedes iteration speed.
+CLIPPY_CONFIG = -A clippy::all -D clippy::correctness -D clippy::suspicious -Dunused_imports -Ddead_code
 validate-rust:
 	cargo fmt -- --check -l
-	cargo check
-	(cd lib && cargo check --no-default-features)
 	cargo test --no-run
-	cargo clippy -- -D clippy::correctness -D clippy::suspicious
+	(cd lib && cargo check --no-default-features)
+	cargo clippy -- $(CLIPPY_CONFIG)
 	env RUSTDOCFLAGS='-D warnings' cargo doc --lib
 .PHONY: validate-rust
+fix-rust:
+	cargo clippy --fix --allow-dirty -- $(CLIPPY_CONFIG)
+.PHONY: fix-rust
 
 validate: validate-rust
 	ruff check