Codify linting toolchain (#1251)

* Codify linting toolchain in a toolchain file

Moves the nightly toolchain we use for linting out of a GitHub action
and into a dedicated linting `rust-toolchain.toml` file.

rustup doesn't currently support specifying a toolchain file, though
[it will soon](rust-lang/rustup#2686), so we
hack around it by keeping the toolchain files in separate directories.

* Add a Nix devShell for the linting toolchain

.github/workflows/rust.yml

@@ -103,11 +103,11 @@ jobs:
       run: >
         find linera-* -name '*.rs' -a -not -wholename '*/target/*' -print0
         | xargs -0 -L1 ./scripts/target/release/check_copyright_header
+    - name: Put lint toolchain file in place
+      run: |
+        ln -sf rust-toolchain.lint.toml rust-toolchain.toml
     - uses: Twey/setup-rust-toolchain@v1
-      with:
-        toolchain: nightly-2023-10-22
-        target: wasm32-unknown-unknown
-        components: clippy rustfmt
+      working-directory: toolchains/lint
     - name: Install cargo-machete
       run: |
         cargo install cargo-machete --locked

flake.nix

@@ -26,12 +26,10 @@
           pkg-config
           nodejs
         ];
-        rustToolchain = (pkgs.rust-bin.fromRustupToolchainFile
-          ./rust-toolchain.toml);
-        rustPlatform = pkgs.makeRustPlatform {
-          rustc = rustToolchain;
-          cargo = rustToolchain;
-        };
+        rustBuildToolchain = (pkgs.rust-bin.fromRustupToolchainFile
+          ./toolchains/build/rust-toolchain.toml);
+        rustLintToolchain = (pkgs.rust-bin.fromRustupToolchainFile
+          ./toolchains/lint/rust-toolchain.toml);
       in {
         _module.args.pkgs = import inputs.nixpkgs {
           inherit system;
@@ -45,17 +43,21 @@
           ];
           shellHook = ''
             # For rust-analyzer 'hover' tooltips to work.
-            export RUST_SRC_PATH=${rustToolchain.availableComponents.rust-src}
+            export RUST_SRC_PATH=${rustBuildToolchain.availableComponents.rust-src}
             export LIBCLANG_PATH=${pkgs.libclang.lib}/lib
             export PATH=$PWD/target/release:~/.cargo/bin:$PATH
           '';
           buildInputs = nonRustDeps;
           nativeBuildInputs = with pkgs; [
-            rustToolchain
+            rustBuildToolchain
             rust-analyzer
           ];
         };
 
+        devShells.lint = pkgs.mkShell {
+          nativeBuildInputs = [ rustLintToolchain ];
+        };
+
         # Add your auto-formatters here.
         # cf. https://numtide.github.io/treefmt/
         treefmt.config = {

rust-toolchain.toml

@@ -0,0 +1 @@
+toolchains/build/rust-toolchain.toml

toolchains/build/rust-toolchain.toml

@@ -0,0 +1,5 @@
+[toolchain]
+channel = "1.73.0"
+components = [ "clippy", "rustfmt", "rust-src" ]
+targets = [ "wasm32-unknown-unknown" ]
+profile = "minimal"

toolchains/lint/rust-toolchain.toml

@@ -0,0 +1,4 @@
+[toolchain]
+channel = "nightly-2023-10-23"
+components = [ "clippy", "rustfmt" ]
+profile = "minimal"