ci: Add nightly Rust toolchain to the CI image (#8182)

Co-authored-by: Bart <bthomee@users.noreply.github.com>

nix/docker/Dockerfile

@@ -10,6 +10,8 @@ RUN mkdir -p ~/.config/nix && \
 COPY nix/ci-env.nix /tmp/build/nix/ci-env.nix
 COPY nix/linux.nix /tmp/build/nix/linux.nix
 COPY nix/packages.nix /tmp/build/nix/packages.nix
+COPY nix/rust-nightly.sh /tmp/build/nix/rust-nightly.sh
+COPY nix/rust.nix /tmp/build/nix/rust.nix
 COPY nix/utils.nix /tmp/build/nix/utils.nix
 COPY flake.nix /tmp/build/
 COPY flake.lock /tmp/build/

nix/packages.nix

@@ -16,23 +16,7 @@ let
     exec ${pkgs.python3}/bin/python3 ${llvmPackages.clang-unwrapped}/bin/run-clang-tidy "$@"
   '';
 
-  # rust-overlay's toolchain propagates the *default* stdenv.cc onto the PATH (so
-  # cargo has a linker). That default may be different from the clang we pin here,
-  # so it shadows our clang and the build can silently use a different compiler
-  # version. Drop that cc from every propagation channel instead of pinning a
-  # replacement: the toolchain then carries no compiler and cargo just uses the
-  # active shell's stdenv cc. Must cover all channels — rust-overlay uses both
-  # propagatedBuildInputs and depsHostHostPropagated.
-  rustToolchainBase = pkgs.rust-bin.fromRustupToolchainFile ../rust-toolchain.toml;
-  rustToolchain =
-    let
-      defaultCc = pkgs.stdenv.cc; # default compiler from nixpkgs stdenv
-      withoutDefaultCc = builtins.filter (dep: (dep.outPath or "") != defaultCc.outPath);
-    in
-    rustToolchainBase.overrideAttrs (old: {
-      propagatedBuildInputs = withoutDefaultCc (old.propagatedBuildInputs or [ ]);
-      depsHostHostPropagated = withoutDefaultCc (old.depsHostHostPropagated or [ ]);
-    });
+  rust = import ./rust.nix { inherit pkgs; };
 
   # Nix wraps its toolchain so that binaries are exposed only under unsuffixed
   # names (gcc, g++, clang-tidy, ...). Several tools probe for a
@@ -108,41 +92,38 @@ in
     mkGcov
     ;
 
-  commonPackages = with pkgs; [
-    clangToolLinks
-    runClangTidyLink
-    ccache
-    clangbuildanalyzer
-    clangTools
-    cmake
-    conan
-    curlMinimal # needed for codecov/codecov-action
-    doxygen
-    file # needed for cpack in Clio
-    gcovr
-    gh
-    git
-    git-cliff
-    git-lfs
-    gnumake
-    gnupg # needed for signing commits & codecov/codecov-action
-    graphviz
-    less # needed for git diff
-    mold
-    nettools # provides netstat, used to debug failures in CI
-    ninja
-    patchelf
-    perl # needed for openssl
-    pkg-config
-    pre-commit
-    python3
-    runClangTidy
-    vim
-    zip
-    # Rust packages
-    cargo-audit
-    cargo-llvm-cov
-    cargo-nextest
-    rustToolchain
-  ];
+  commonPackages =
+    (with pkgs; [
+      clangToolLinks
+      runClangTidyLink
+      ccache
+      clangbuildanalyzer
+      clangTools
+      cmake
+      conan
+      curlMinimal # needed for codecov/codecov-action
+      doxygen
+      file # needed for cpack in Clio
+      gcovr
+      gh
+      git
+      git-cliff
+      git-lfs
+      gnumake
+      gnupg # needed for signing commits & codecov/codecov-action
+      graphviz
+      less # needed for git diff
+      mold
+      nettools # provides netstat, used to debug failures in CI
+      ninja
+      patchelf
+      perl # needed for openssl
+      pkg-config
+      pre-commit
+      python3
+      runClangTidy
+      vim
+      zip
+    ])
+    ++ rust.packages;
 }

nix/rust-nightly.sh

@@ -0,0 +1,23 @@
+#!@runtimeShell@
+# Reaches the nightly Rust toolchain, which is deliberately kept off PATH.
+# Packaged by nix/rust.nix, which explains why.
+
+set -euo pipefail
+
+usage() {
+    echo "usage: rust-nightly (path | run <command>...)" >&2
+    exit 2
+}
+
+case "${1-}" in
+    path) printf '%s\n' "@rustNightlyBin@" ;;
+    run)
+        shift
+        if [[ $# -eq 0 ]]; then
+            usage
+        fi
+        export PATH="@rustNightlyBin@:${PATH}"
+        exec "$@"
+        ;;
+    *) usage ;;
+esac

nix/rust.nix

@@ -0,0 +1,84 @@
+# The Rust half of the tool set shared by the CI environment and the dev shell:
+# the stable toolchain pinned by rust-toolchain.toml, the nightly the Rust
+# coverage job needs, and the cargo plugins. Consumed by packages.nix.
+{ pkgs }:
+let
+  # rust-overlay's toolchain propagates the *default* stdenv.cc onto the PATH (so
+  # cargo has a linker). That default may be different from the clang we pin
+  # elsewhere, so it shadows our clang and the build can silently use a different
+  # compiler version. Drop that cc from every propagation channel instead of
+  # pinning a replacement: the toolchain then carries no compiler and cargo just
+  # uses the active shell's stdenv cc.
+  #
+  # The channel list is every list mkDerivation propagates to a dependent's
+  # environment (including the two legacy aliases). rust-overlay currently only
+  # uses propagatedBuildInputs and depsHostHostPropagated, but covering all of
+  # them means an upstream switch to another channel cannot quietly put the
+  # compiler back on PATH.
+  dropDefaultCc =
+    toolchain:
+    let
+      defaultCc = pkgs.stdenv.cc; # default compiler from nixpkgs stdenv
+      withoutDefaultCc = builtins.filter (dep: (dep.outPath or "") != defaultCc.outPath);
+    in
+    toolchain.overrideAttrs (
+      old:
+      pkgs.lib.genAttrs [
+        "depsBuildBuildPropagated"
+        "propagatedNativeBuildInputs" # alias of depsBuildHostPropagated
+        "depsBuildTargetPropagated"
+        "depsHostHostPropagated"
+        "propagatedBuildInputs" # alias of depsHostTargetPropagated
+        "depsTargetTargetPropagated"
+      ] (channel: withoutDefaultCc (old.${channel} or [ ]))
+    );
+
+  rustToolchain = dropDefaultCc (pkgs.rust-bin.fromRustupToolchainFile ../rust-toolchain.toml);
+
+  # cargo-llvm-cov honours the #[coverage(off)] that keeps unit tests out of the
+  # coverage report only under a nightly rustc, and looks for llvm-profdata and
+  # llvm-cov in that same toolchain's sysroot — hence llvm-tools-preview.
+  #
+  # Not every nightly ships every component, so `nightly.latest` breaks on the
+  # days llvm-tools-preview is absent; selectLatestNightlyWith walks back to the
+  # newest one that has it. The result is the newest such nightly *known to the
+  # locked rust-overlay*, which means updating flake.lock moves the compiler that
+  # produces the coverage numbers — and with it the rustc version recorded in
+  # nix/check-tools/*.txt, so those snapshots need regenerating alongside.
+  rustNightly = dropDefaultCc (
+    pkgs.rust-bin.selectLatestNightlyWith (
+      toolchain: toolchain.minimal.override { extensions = [ "llvm-tools-preview" ]; }
+    )
+  );
+
+  # A second toolchain cannot go on PATH: its cargo and rustc would collide with
+  # the pinned stable's in the ci-env buildEnv, which resolves collisions by
+  # picking one silently. Reaching the nightly only through this wrapper keeps it
+  # in the image closure (the Docker build copies the whole closure, not just
+  # what is linked into /bin) while leaving it inactive everywhere that does not
+  # ask for it.
+  #
+  # The script's `path` subcommand exists for scopes wider than one command — a
+  # CI job appending to $GITHUB_PATH, so that the cargo cache action's own
+  # `rustc -vV` probe, which runs in a step of its own, agrees with the toolchain
+  # the build will use.
+  rustNightlyScript = pkgs.replaceVarsWith {
+    name = "rust-nightly";
+    src = ./rust-nightly.sh;
+    dir = "bin";
+    isExecutable = true;
+    replacements = {
+      inherit (pkgs) runtimeShell;
+      rustNightlyBin = "${rustNightly}/bin";
+    };
+  };
+in
+{
+  packages = [
+    pkgs.cargo-audit
+    pkgs.cargo-llvm-cov
+    pkgs.cargo-nextest
+    rustNightlyScript
+    rustToolchain
+  ];
+}