rust: restore libLLVM symlink

Fixes Homebrew#205571 (comment)

This reverts commit c255cc3.

Author: carlocab

Formula/r/rust.rb

@@ -122,6 +122,10 @@ class Rust < Formula
     end
   end
 
+  def llvm
+    Formula["llvm"]
+  end
+
   def install
     # Ensure that the `openssl` crate picks up the intended library.
     # https://docs.rs/openssl/latest/openssl/#manual
@@ -169,7 +173,7 @@ def install
       --prefix=#{prefix}
       --sysconfdir=#{etc}
       --tools=#{tools.join(",")}
-      --llvm-root=#{Formula["llvm"].opt_prefix}
+      --llvm-root=#{llvm.opt_prefix}
       --enable-llvm-link-shared
       --enable-profiler
       --enable-vendor
@@ -207,6 +211,14 @@ def post_install
       MachO.codesign!(dylib) if Hardware::CPU.arm?
       chmod 0444, dylib
     end
+    return unless OS.mac?
+
+    # Symlink our LLVM here to make sure the adjacent bin/rust-lld can find it.
+    # Needs to be done in `postinstall` to avoid having `change_dylib_id` done on it.
+    lib.glob("rustlib/*/lib") do |dir|
+      # Use `ln_sf` instead of `install_symlink` to avoid resolving this into a Cellar path.
+      ln_sf llvm.opt_lib/shared_library("libLLVM"), dir
+    end
   end
 
   test do