rustc: fix build with unbundled jemalloc and llvm on darwin

jemalloc with stripped prefix would cause segfaults in free:
https://github.com/NixOS/nixpkgs/pull/49557#issuecomment-436734677

Thanks to @danieldk for darwin testing/debugging.

pkgs/development/compilers/rust/rustc.nix

@@ -20,6 +20,8 @@ let
 
   llvmShared = llvm.override { enableSharedLibraries = true; };
 
+  prefixedJemalloc = jemalloc.override { stripPrefix = false; };
+
   target = builtins.replaceStrings [" "] [","] (builtins.toString targets);
 in
 
@@ -43,6 +45,7 @@ stdenv.mkDerivation {
   NIX_LDFLAGS =
        # when linking stage1 libstd: cc: undefined reference to `__cxa_begin_catch'
        optional (stdenv.isLinux && !withBundledLLVM) "--push-state --as-needed -lstdc++ --pop-state"
+    ++ optional (stdenv.isDarwin && !withBundledLLVM) "-lc++"
     ++ optional stdenv.isDarwin "-rpath ${llvmShared}/lib";
 
   # Enable nightly features in stable compiles (used for
@@ -59,7 +62,7 @@ stdenv.mkDerivation {
   configureFlags = configureFlags
                 ++ [ "--enable-local-rust" "--local-rust-root=${rustPlatform.rust.rustc}" "--enable-rpath"
                      "--enable-vendor"
-                     "--jemalloc-root=${jemalloc}/lib"
+                     "--jemalloc-root=${prefixedJemalloc}/lib"
                      "--default-linker=${targetPackages.stdenv.cc}/bin/cc" ]
                 ++ optional (!withBundledLLVM) [ "--enable-llvm-link-shared" "--llvm-root=${llvmShared}" ]
                 ++ optional (targets != []) "--target=${target}";

pkgs/development/libraries/jemalloc/common.nix

@@ -1,6 +1,13 @@
-{ stdenv, fetchurl, version, sha256, ... }@args:
+{ version, sha256 }:
+{ stdenv, fetchurl,
+# By default, jemalloc puts a je_ prefix onto all its symbols on OSX, which
+# then stops downstream builds (mariadb in particular) from detecting it. This
+# option should remove the prefix and give us a working jemalloc.
+# Causes segfaults with some software (ex. rustc), but defaults to true for backward
+# compatibility. Ignored on non OSX.
+stripPrefix ? true }:
 
-stdenv.mkDerivation (rec {
+stdenv.mkDerivation rec {
   name = "jemalloc-${version}";
   inherit version;
 
@@ -9,10 +16,8 @@ stdenv.mkDerivation (rec {
     inherit sha256;
   };
 
-  # By default, jemalloc puts a je_ prefix onto all its symbols on OSX, which
-  # then stops downstream builds (mariadb in particular) from detecting it. This
-  # option should remove the prefix and give us a working jemalloc.
-  configureFlags = stdenv.lib.optional stdenv.isDarwin "--with-jemalloc-prefix=";
+  # see the comment on stripPrefix
+  configureFlags = stdenv.lib.optional (stdenv.isDarwin && stripPrefix) "--with-jemalloc-prefix=";
   doCheck = true;
 
   enableParallelBuilding = true;
@@ -28,4 +33,4 @@ stdenv.mkDerivation (rec {
     platforms = platforms.all;
     maintainers = with maintainers; [ wkennington ];
   };
-} // (builtins.removeAttrs args [ "stdenv" "fetchurl" "version" "sha256" ]))
+}

pkgs/development/libraries/jemalloc/default.nix

@@ -1,6 +1,4 @@
-{ stdenv, fetchurl, fetchpatch }:
 import ./common.nix {
-  inherit stdenv fetchurl;
   version = "5.1.0";
   sha256 = "0s3jpcyhzia8d4k0xyc67is78kg416p9yc3c2f9w6fhhqqffd5jk";
 }

pkgs/development/libraries/jemalloc/jemalloc450.nix

@@ -1,6 +1,4 @@
-{ stdenv, fetchurl }:
 import ./common.nix {
-  inherit stdenv fetchurl;
   version = "4.5.0";
   sha256 = "10373xhpc10pgmai9fkc1z0rs029qlcb3c0qfnvkbwdlcibdh2cl";
-}
+}