buildRustCrate: fix target config environment variables on 32-bit ARM

pkgs/build-support/rust/build-rust-crate/configure-crate.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, echo_colored, noisily, mkRustcDepArgs, mkRustcFeatureArgs }:
+{ lib, stdenv, rust, echo_colored, noisily, mkRustcDepArgs, mkRustcFeatureArgs }:
 {
   build
 , buildDependencies
@@ -17,7 +17,6 @@
 , libName
 , libPath
 , release
-, target_os
 , verbose
 , workspace_member }:
 let version_ = lib.splitString "-" crateVersion;
@@ -124,8 +123,8 @@ in ''
   export CARGO_PKG_AUTHORS="${authors}"
   export CARGO_PKG_DESCRIPTION="${crateDescription}"
 
-  export CARGO_CFG_TARGET_ARCH=${stdenv.hostPlatform.parsed.cpu.name}
+  export CARGO_CFG_TARGET_ARCH=${rust.toTargetArch stdenv.hostPlatform}
-  export CARGO_CFG_TARGET_OS=${target_os}
+  export CARGO_CFG_TARGET_OS=${rust.toTargetOs stdenv.hostPlatform}
   export CARGO_CFG_TARGET_FAMILY="unix"
   export CARGO_CFG_UNIX=1
   export CARGO_CFG_TARGET_ENV="gnu"
@@ -136,8 +135,8 @@ in ''
   export CARGO_MANIFEST_DIR=$(pwd)
   export DEBUG="${toString (!release)}"
   export OPT_LEVEL="${toString optLevel}"
-  export TARGET="${stdenv.hostPlatform.config}"
+  export TARGET="${rust.toRustTarget stdenv.hostPlatform}"
-  export HOST="${stdenv.hostPlatform.config}"
+  export HOST="${rust.toRustTarget stdenv.buildPlatform}"
   export PROFILE=${if release then "release" else "debug"}
   export OUT_DIR=$(pwd)/target/build/${crateName}.out
   export CARGO_PKG_VERSION_MAJOR=${lib.elemAt version 0}

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

@@ -8,12 +8,6 @@
 , cargo, jq }:
 
 let
-    # This doesn't appear to be officially documented anywhere yet.
-    # See https://github.com/rust-lang-nursery/rust-forge/issues/101.
-    target_os = if stdenv.hostPlatform.isDarwin
-      then "macos"
-      else stdenv.hostPlatform.parsed.kernel.name;
-
     # Create rustc arguments to link against the given list of dependencies
     # and renames.
     #
@@ -52,7 +46,7 @@ let
    inherit (import ./log.nix { inherit lib; }) noisily echo_colored;
 
    configureCrate = import ./configure-crate.nix {
-     inherit lib stdenv echo_colored noisily mkRustcDepArgs mkRustcFeatureArgs;
+     inherit lib stdenv rust echo_colored noisily mkRustcDepArgs mkRustcFeatureArgs;
    };
 
    buildCrate = import ./build-crate.nix {
@@ -284,7 +278,7 @@ stdenv.mkDerivation (rec {
       inherit crateName buildDependencies completeDeps completeBuildDeps crateDescription
               crateFeatures crateRenames libName build workspace_member release libPath crateVersion
               extraLinkFlags extraRustcOpts
-              crateAuthors crateHomepage verbose colors target_os;
+              crateAuthors crateHomepage verbose colors;
     };
     buildPhase = buildCrate {
       inherit crateName dependencies

pkgs/development/compilers/rust/default.nix

@@ -13,12 +13,23 @@
 , llvmPackages_5
 , pkgsBuildTarget, pkgsBuildBuild
 }: rec {
+  # https://doc.rust-lang.org/reference/conditional-compilation.html#target_arch
+  toTargetArch = platform:
+    if platform.isAarch32 then "arm"
+    else platform.parsed.cpu.name;
+
+  # https://doc.rust-lang.org/reference/conditional-compilation.html#target_os
+  toTargetOs = platform:
+    if platform.isDarwin then "macos"
+    else platform.parsed.kernel.name;
+
+  # Target triple. Rust has slightly different naming conventions than we use.
   toRustTarget = platform: with platform.parsed; let
-    cpu_ = {
+    cpu_ = platform.rustc.arch or {
       "armv7a" = "armv7";
       "armv7l" = "armv7";
       "armv6l" = "arm";
-    }.${cpu.name} or platform.rustc.arch or cpu.name;
+    }.${cpu.name} or cpu.name;
   in platform.rustc.config
     or "${cpu_}-${vendor.name}-${kernel.name}${lib.optionalString (abi.name != "unknown") "-${abi.name}"}";