rustc: propagate libiconv on darwin

Rust binaries are unconditionally linked to libiconv on Darwin (see https://github.com/rust-lang/libc/issues/2870). We already add it as a dependency in `buildRustPackage`, so let's go a step further and propagate it.

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

@@ -127,7 +127,6 @@ stdenv.mkDerivation ((removeAttrs args [ "depsExtraArgs" "cargoUpdateHook" "carg
   ];
 
   buildInputs = buildInputs
-    ++ lib.optionals stdenv.hostPlatform.isDarwin [ libiconv ]
     ++ lib.optionals stdenv.hostPlatform.isMinGW [ windows.pthreads ];
 
   patches = cargoPatches ++ patches;

pkgs/development/compilers/rust/cargo.nix

@@ -1,7 +1,7 @@
 { lib, stdenv, pkgsHostHost
 , file, curl, pkg-config, python3, openssl, cmake, zlib
 , installShellFiles, makeWrapper, cacert, rustPlatform, rustc
-, CoreFoundation, Security
+, libiconv, CoreFoundation, Security
 }:
 
 rustPlatform.buildRustPackage {
@@ -25,7 +25,7 @@ rustPlatform.buildRustPackage {
     (lib.getDev pkgsHostHost.curl)
   ];
   buildInputs = [ cacert file curl python3 openssl zlib ]
-    ++ lib.optionals stdenv.isDarwin [ CoreFoundation Security ];
+    ++ lib.optionals stdenv.isDarwin [ libiconv CoreFoundation Security ];
 
   # cargo uses git-rs which is made for a version of libgit2 from recent master that
   # is not compatible with the current version in nixpkgs.

pkgs/development/compilers/rust/rustc.nix

@@ -148,9 +148,11 @@ in stdenv.mkDerivation rec {
   ];
 
   buildInputs = [ openssl ]
-    ++ optionals stdenv.isDarwin [ libiconv Security ]
+    ++ optionals stdenv.isDarwin [ Security ]
     ++ optional (!withBundledLLVM) llvmShared;
 
+  depsTargetTargetPropagated = optionals stdenv.isDarwin [ libiconv ];
+
   outputs = [ "out" "man" "doc" ];
   setOutputFlags = false;