rustc: Expose llvmPackages set used to build rustc

Reusing the same llvmPackages version used for building rustc is
required for LTO in Firefox.

pkgs/development/compilers/rust/1_55.nix

@@ -32,6 +32,9 @@ import ./default.nix {
   # For use at runtime
   llvmShared = llvm_12.override { enableSharedLibraries = true; };
 
+  # Expose llvmPackages used for rustc from rustc via passthru for LTO in Firefox
+  llvmPackagesForBuild = pkgsBuildBuild.llvmPackages_12;
+
   # Note: the version MUST be one version prior to the version we're
   # building
   bootstrapVersion = "1.54.0";

pkgs/development/compilers/rust/default.nix

@@ -10,6 +10,7 @@
 , llvmSharedForBuild
 , llvmSharedForHost
 , llvmSharedForTarget
+, llvmPackagesForBuild # Exposed through rustc for LTO in Firefox
 }:
 { stdenv, lib
 , buildPackages
@@ -85,7 +86,7 @@
         version = rustcVersion;
         sha256 = rustcSha256;
         inherit enableRustcDev;
-        inherit llvmShared llvmSharedForBuild llvmSharedForHost llvmSharedForTarget;
+        inherit llvmShared llvmSharedForBuild llvmSharedForHost llvmSharedForTarget llvmPackagesForBuild;
 
         patches = rustcPatches;
 

pkgs/development/compilers/rust/rustc.nix

@@ -1,5 +1,5 @@
 { lib, stdenv, removeReferencesTo, pkgsBuildBuild, pkgsBuildHost, pkgsBuildTarget
-, llvmShared, llvmSharedForBuild, llvmSharedForHost, llvmSharedForTarget
+, llvmShared, llvmSharedForBuild, llvmSharedForHost, llvmSharedForTarget, llvmPackagesForBuild
 , fetchurl, file, python3
 , darwin, cmake, rust, rustPlatform
 , pkg-config, openssl
@@ -174,7 +174,10 @@ in stdenv.mkDerivation rec {
 
   requiredSystemFeatures = [ "big-parallel" ];
 
-  passthru.llvm = llvmShared;
+  passthru = {
+    llvm = llvmShared;
+    llvmPackages = llvmPackagesForBuild;
+  };
 
   meta = with lib; {
     homepage = "https://www.rust-lang.org/";