rust: fix overriding rust flags on musl

If RUSTFLAGS is set in the environment, Cargo will ignore rustflags
settings in its TOML configuration.  So setting RUSTFLAGS=-g (like
separateDebugInfo does) to generate debug info breaks
dynamically-linked Rust packages on musl.  This breakage is visible
for any packages that call into C dynamic libraries.  If the binary is
linked directly to a C dynamic library, it will fail to build, and if
it depends on a Rust library which links a C dynamic library, it will
segfault at runtime when it tries to call a function from the C
library.  I noticed this because pkgsMusl.crosvm is broken for this
reason, since it sets separateDebugInfo = true.

It shouldn't be possible to end up with broken binaries just by using
RUSTFLAGS to do something innocuous like enable debug info, so I think
that, even though we liked the approach of modiyfing .cargo/config
better at the time, it's become clear that it's too brittle, and we
should bite the bullet and patch the compiler instead when targetting
musl.  It does not appear to be necessary to modify the compiler at
all when cross-compiling /from/ dynamically-linked Musl to another
target, so I'm only checking whether the target system is
dynamically-linked Musl when deciding whether to make the modification
to the compiler.

This reverts commit c2eaaae
("cargoSetupHook: pass host config flags"), and implements the
compiler patching approach instead.

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.