rust: fix overriding rust flags on musl

pkgs/build-support/rust/hooks/default.nix

@@ -76,37 +76,14 @@ in {
         # inputs do not cause us to find the wrong `diff`.
         diff = "${lib.getBin buildPackages.diffutils}/bin/diff";
 
-        # We want to specify the correct crt-static flag for both
-        # the build and host platforms. This is important when the wanted
-        # value for crt-static does not match the defaults in the rustc target,
-        # like for pkgsMusl or pkgsCross.musl64; Upstream rustc still assumes
-        # that musl = static[1].
-        #
-        # By default, Cargo doesn't apply RUSTFLAGS when building build.rs
-        # if --target is passed, so the only good way to set crt-static for
-        # build.rs files is to use the unstable -Zhost-config Cargo feature.
-        # This allows us to specify flags that should be passed to rustc
-        # when building for the build platform. We also need to use
-        # -Ztarget-applies-to-host, because using -Zhost-config requires it.
-        #
-        # When doing this, we also have to specify the linker, or cargo
-        # won't pass a -C linker= argument to rustc.  This will make rustc
-        # try to use its default value of "cc", which won't be available
-        # when cross-compiling.
-        #
-        # [1]: https://github.com/rust-lang/compiler-team/issues/422
         cargoConfig = ''
-          [host]
+          [target."${rust.toRustTarget stdenv.buildPlatform}"]
           "linker" = "${ccForBuild}"
-          "rustflags" = [ "-C", "target-feature=${if stdenv.buildPlatform.isStatic then "+" else "-"}crt-static" ]
-
-          [target."${shortTarget}"]
-          "linker" = "${ccForHost}"
+          ${lib.optionalString (stdenv.buildPlatform.config != stdenv.hostPlatform.config) ''
+            [target."${shortTarget}"]
+            "linker" = "${ccForHost}"
+          ''}
           "rustflags" = [ "-C", "target-feature=${if stdenv.hostPlatform.isStatic then "+" else "-"}crt-static" ]
-
-          [unstable]
-          host-config = true
-          target-applies-to-host = true
         '';
       };
     } ./cargo-setup-hook.sh) {};

pkgs/development/compilers/rust/cargo.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, pkgsHostHost
+{ lib, stdenv, pkgsBuildHost, pkgsHostHost
 , file, curl, pkg-config, python3, openssl, cmake, zlib
 , installShellFiles, makeWrapper, rustPlatform, rustc
 , CoreFoundation, Security
@@ -20,6 +20,42 @@ rustPlatform.buildRustPackage {
     inherit (rustc) tests;
   };
 
+  # Upstream rustc still assumes that musl = static[1].  The fix for
+  # this is to disable crt-static by default for non-static musl
+  # targets.
+  #
+  # For every package apart from Cargo, we can fix this by just
+  # patching rustc to not have crt-static by default.  But Cargo is
+  # built with the upstream bootstrap binary for rustc, which we can't
+  # easily patch.  This means we need to find another way to make sure
+  # crt-static is not used during the build of pkgsMusl.cargo.
+  #
+  # By default, Cargo doesn't apply RUSTFLAGS when building build.rs
+  # if --target is passed, so the only good way to set -crt-static for
+  # build.rs files used in the Cargo build is to use the unstable
+  # -Zhost-config Cargo feature.  This allows us to specify flags that
+  # should be passed to rustc when building for the build platform.
+  # We also need to use -Ztarget-applies-to-host, because using
+  # -Zhost-config requires it.
+  #
+  # When doing this, we also have to specify the linker, or cargo
+  # won't pass a -C linker= argument to rustc.  This will make rustc
+  # try to use its default value of "cc", which won't be available
+  # when cross-compiling.
+  #
+  # [1]: https://github.com/rust-lang/compiler-team/issues/422
+  postPatch = lib.optionalString (with stdenv.buildPlatform; isMusl && !isStatic) ''
+    mkdir -p .cargo
+    cat <<EOF >> .cargo/config
+    [host]
+    rustflags = "-C target-feature=-crt-static"
+    linker = "${pkgsBuildHost.stdenv.cc}/bin/${pkgsBuildHost.stdenv.cc.targetPrefix}cc"
+    [unstable]
+    host-config = true
+    target-applies-to-host = true
+    EOF
+  '';
+
   # changes hash of vendor directory otherwise
   dontUpdateAutotoolsGnuConfigScripts = true;
 

pkgs/development/compilers/rust/rustc.nix

@@ -147,6 +147,18 @@ in stdenv.mkDerivation rec {
 
     # Useful debugging parameter
     # export VERBOSE=1
+  '' + lib.optionalString (stdenv.targetPlatform.isMusl && !stdenv.targetPlatform.isStatic) ''
+    # Upstream rustc still assumes that musl = static[1].  The fix for
+    # this is to disable crt-static by default for non-static musl
+    # targets.
+    #
+    # Even though Cargo will build build.rs files for the build platform,
+    # cross-compiling _from_ musl appears to work fine, so we only need
+    # to do this when rustc's target platform is dynamically linked musl.
+    #
+    # [1]: https://github.com/rust-lang/compiler-team/issues/422
+    substituteInPlace compiler/rustc_target/src/spec/linux_musl_base.rs \
+        --replace "base.crt_static_default = true" "base.crt_static_default = false"
   '' + lib.optionalString (stdenv.isDarwin && stdenv.isx86_64) ''
     # See https://github.com/jemalloc/jemalloc/issues/1997
     # Using a value of 48 should work on both emulated and native x86_64-darwin.