rustc: add a compiler wrapper

We keep running into situations where we can't get the right
combination of rustc flags through build systems into rustc.
RUSTFLAGS is the only variable supported across build systems, but if
RUSTFLAGS is set, Cargo will ignore all other ways of specifying rustc
flags, including the target-specific ones, which we need to make
dynamic musl builds work.  (This is why pkgsCross.musl64.crosvm is
currently broken — it works if you unset separateDebugInfo, which
causes RUSTFLAGS not to be set.)

So, we need to do the same thing we do for C and C++ compilers, and
add a compiler wrapper so we can inject the flags we need, regardless
of the build system.

Currently the wrapper only supports a single mechanism for injecting
flags — the NIX_RUSTFLAGS environment variable.  As time goes on,
we'll probably want to add additional features, like target-specific
environment variables.

doc/languages-frameworks/rust.section.md

@@ -963,7 +963,7 @@ repository:
      lib.updateManyAttrsByPath [{
        path = [ "packages" "stable" ];
        update = old: old.overrideScope(final: prev: {
-         rustc = prev.rustc.overrideAttrs (_: {
+         rustc-unwrapped = prev.rustc-unwrapped.overrideAttrs (_: {
            src = lib.cleanSource /git/scratch/rust;
            # do *not* put passthru.isReleaseTarball=true here
          });
@@ -1003,4 +1003,3 @@ nix-build $NIXPKGS -A package-broken-by-rust-changes
 The `git submodule update --init` and `cargo vendor` commands above
 require network access, so they can't be performed from within the
 `rustc` derivation, unfortunately.
-

pkgs/build-support/rust/rustc-wrapper/default.nix

@@ -0,0 +1,29 @@
+{ lib, runCommand, rustc-unwrapped }:
+
+runCommand "${rustc-unwrapped.pname}-wrapper-${rustc-unwrapped.version}" {
+  preferLocalBuild = true;
+  strictDeps = true;
+  inherit (rustc-unwrapped) outputs;
+
+  env = {
+    prog = "${rustc-unwrapped}/bin/rustc";
+  };
+
+  passthru = {
+    inherit (rustc-unwrapped) pname version src llvm llvmPackages;
+    unwrapped = rustc-unwrapped;
+  };
+
+  meta = rustc-unwrapped.meta // {
+    description = "${rustc-unwrapped.meta.description} (wrapper script)";
+    priority = 10;
+  };
+} ''
+  mkdir -p $out/bin
+  ln -s ${rustc-unwrapped}/bin/* $out/bin
+  rm $out/bin/rustc
+  substituteAll ${./rustc-wrapper.sh} $out/bin/rustc
+  chmod +x $out/bin/rustc
+  ${lib.concatMapStrings (output: "ln -s ${rustc-unwrapped.${output}} \$${output}\n")
+    (lib.remove "out" rustc-unwrapped.outputs)}
+''

pkgs/build-support/rust/rustc-wrapper/rustc-wrapper.sh

@@ -0,0 +1,11 @@
+#!@shell@
+
+# Optionally print debug info.
+if (( "${NIX_DEBUG:-0}" >= 1 )); then
+    echo "original flags to @prog@:" >&2
+    printf "  %q\n" "$@" >&2
+    echo "extra flags after to @prog@:" >&2
+    printf "  %q\n" $NIX_RUSTFLAGS >&2
+fi
+
+exec @prog@ "$@" $NIX_RUSTFLAGS

pkgs/development/compilers/rust/1_74.nix

@@ -16,6 +16,7 @@
 , CoreFoundation, Security, SystemConfiguration
 , pkgsBuildTarget, pkgsBuildBuild, pkgsBuildHost
 , makeRustPlatform
+, wrapRustc
 , llvmPackages_16, llvm_16
 } @ args:
 

pkgs/development/compilers/rust/binary.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, makeWrapper, bash, curl, darwin, zlib
+{ lib, stdenv, makeWrapper, wrapRustc, bash, curl, darwin, zlib
 , autoPatchelfHook, gcc
 , version
 , src
@@ -19,7 +19,7 @@ let
 in
 
 rec {
-  rustc = stdenv.mkDerivation {
+  rustc-unwrapped = stdenv.mkDerivation {
     pname = "rustc-${versionType}";
 
     inherit version;
@@ -61,6 +61,8 @@ rec {
     setupHooks = ./setup-hook.sh;
   };
 
+  rustc = wrapRustc rustc-unwrapped;
+
   cargo = stdenv.mkDerivation {
     pname = "cargo-${versionType}";
 

pkgs/development/compilers/rust/cargo.nix

@@ -11,7 +11,7 @@ rustPlatform.buildRustPackage.override {
   cargo-auditable = cargo-auditable.bootstrap;
 } ({
   pname = "cargo";
-  inherit (rustc) version src;
+  inherit (rustc.unwrapped) version src;
 
   # the rust source tarball already has all the dependencies vendored, no need to fetch them again
   cargoVendorDir = "vendor";

pkgs/development/compilers/rust/default.nix

@@ -18,6 +18,7 @@
 , CoreFoundation, Security, SystemConfiguration
 , pkgsBuildBuild
 , makeRustPlatform
+, wrapRustc
 }:
 
 let
@@ -64,7 +65,7 @@ in
       buildRustPackages = (selectRustPackage buildPackages).packages.stable;
       # Analogous to stdenv
       rustPlatform = makeRustPlatform self.buildRustPackages;
-      rustc = self.callPackage ./rustc.nix ({
+      rustc-unwrapped = self.callPackage ./rustc.nix ({
         version = rustcVersion;
         sha256 = rustcSha256;
         inherit enableRustcDev;
@@ -75,6 +76,7 @@ in
         # Use boot package set to break cycle
         inherit (bootstrapRustPackages) cargo rustc rustfmt;
       });
+      rustc = wrapRustc self.rustc-unwrapped;
       rustfmt = self.callPackage ./rustfmt.nix {
         inherit Security;
         inherit (self.buildRustPackages) rustc;

pkgs/development/compilers/rust/rustc.nix

@@ -154,9 +154,9 @@ in stdenv.mkDerivation (finalAttrs: {
     runHook preBuild
 
     mkdir -p build/${stdenv.hostPlatform.rust.rustcTargetSpec}/stage0-{std,rustc}/${stdenv.hostPlatform.rust.rustcTargetSpec}/release/
-    ln -s ${rustc}/lib/rustlib/${stdenv.hostPlatform.rust.rustcTargetSpec}/libstd-*.so build/${stdenv.hostPlatform.rust.rustcTargetSpec}/stage0-std/${stdenv.hostPlatform.rust.rustcTargetSpec}/release/libstd.so
+    ln -s ${rustc.unwrapped}/lib/rustlib/${stdenv.hostPlatform.rust.rustcTargetSpec}/libstd-*.so build/${stdenv.hostPlatform.rust.rustcTargetSpec}/stage0-std/${stdenv.hostPlatform.rust.rustcTargetSpec}/release/libstd.so
-    ln -s ${rustc}/lib/rustlib/${stdenv.hostPlatform.rust.rustcTargetSpec}/librustc_driver-*.so build/${stdenv.hostPlatform.rust.rustcTargetSpec}/stage0-rustc/${stdenv.hostPlatform.rust.rustcTargetSpec}/release/librustc.so
+    ln -s ${rustc.unwrapped}/lib/rustlib/${stdenv.hostPlatform.rust.rustcTargetSpec}/librustc_driver-*.so build/${stdenv.hostPlatform.rust.rustcTargetSpec}/stage0-rustc/${stdenv.hostPlatform.rust.rustcTargetSpec}/release/librustc.so
-    ln -s ${rustc}/bin/rustc build/${stdenv.hostPlatform.rust.rustcTargetSpec}/stage0-rustc/${stdenv.hostPlatform.rust.rustcTargetSpec}/release/rustc-main
+    ln -s ${rustc.unwrapped}/bin/rustc build/${stdenv.hostPlatform.rust.rustcTargetSpec}/stage0-rustc/${stdenv.hostPlatform.rust.rustcTargetSpec}/release/rustc-main
     touch build/${stdenv.hostPlatform.rust.rustcTargetSpec}/stage0-std/${stdenv.hostPlatform.rust.rustcTargetSpec}/release/.libstd.stamp
     touch build/${stdenv.hostPlatform.rust.rustcTargetSpec}/stage0-rustc/${stdenv.hostPlatform.rust.rustcTargetSpec}/release/.librustc.stamp
     python ./x.py --keep-stage=0 --stage=1 build library/std
@@ -169,9 +169,9 @@ in stdenv.mkDerivation (finalAttrs: {
 
     python ./x.py --keep-stage=0 --stage=1 install library/std
     mkdir -v $out/bin $doc $man
-    makeWrapper ${rustc}/bin/rustc $out/bin/rustc --add-flags "--sysroot $out"
+    makeWrapper ${rustc.unwrapped}/bin/rustc $out/bin/rustc --add-flags "--sysroot $out"
-    makeWrapper ${rustc}/bin/rustdoc $out/bin/rustdoc --add-flags "--sysroot $out"
+    makeWrapper ${rustc.unwrapped}/bin/rustdoc $out/bin/rustdoc --add-flags "--sysroot $out"
-    ln -s ${rustc}/lib/rustlib/{manifest-rust-std-,}${stdenv.hostPlatform.rust.rustcTargetSpec} $out/lib/rustlib/
+    ln -s ${rustc.unwrapped}/lib/rustlib/{manifest-rust-std-,}${stdenv.hostPlatform.rust.rustcTargetSpec} $out/lib/rustlib/
     echo rust-std-${stdenv.hostPlatform.rust.rustcTargetSpec} >> $out/lib/rustlib/components
     lndir ${rustc.doc} $doc
     lndir ${rustc.man} $man

pkgs/top-level/all-packages.nix

@@ -16897,6 +16897,9 @@ with pkgs;
     inherit (darwin) apple_sdk;
   };
 
+  wrapRustcWith = { rustc-unwrapped, ... } @ args: callPackage ../build-support/rust/rustc-wrapper args;
+  wrapRustc = rustc-unwrapped: wrapRustcWith { inherit rustc-unwrapped; };
+
   rust_1_74 = callPackage ../development/compilers/rust/1_74.nix {
     inherit (darwin.apple_sdk.frameworks) CoreFoundation Security SystemConfiguration;
     llvm_16 = llvmPackages_16.libllvm;
@@ -16915,7 +16918,7 @@ with pkgs;
   inherit (rustPackages) cargo cargo-auditable cargo-auditable-cargo-wrapper clippy rustc rustPlatform;
 
   # https://github.com/NixOS/nixpkgs/issues/89426
-  rustc-wasm32 = (rustc.override {
+  rustc-wasm32 = wrapRustc ((rustc.unwrapped.override {
     stdenv = stdenv.override {
       targetPlatform = lib.systems.elaborate {
         # lib.systems.elaborate won't recognize "unknown" as the last component.
@@ -16925,7 +16928,7 @@ with pkgs;
     };
   }).overrideAttrs (old: {
     configureFlags = old.configureFlags ++ ["--set=build.docs=false"];
-  });
+  }));
 
   makeRustPlatform = callPackage ../development/compilers/rust/make-rust-platform.nix { };