Fix protocols diff calculation between compilations (#6339)

lib/mix/lib/mix/tasks/compile.protocols.ex

@@ -193,14 +193,20 @@ defmodule Mix.Tasks.Compile.Protocols do
           Map.put(protocols, protocol, true)
       end)
 
+    removed_metadata =
+      old_metadata -- new_metadata
+
+    removed_protocols =
+      for {protocol, :protocol, _beam} <- removed_metadata,
+          remove_consolidated(protocol, output),
+          do: {protocol, true},
+          into: %{}
+
     protocols =
-      Enum.reduce(old_metadata -- new_metadata, protocols, fn
-        {_, {:impl, protocol}, _beam}, protocols ->
-          Map.put(protocols, protocol, true)
-        {protocol, :protocol, _beam}, protocols ->
-          remove_consolidated(protocol, output)
-          protocols
-      end)
+      for {_, {:impl, protocol}, _beam} <- removed_metadata,
+          not Map.has_key?(removed_protocols, protocol),
+          do: {protocol, true},
+          into: protocols
 
     Map.keys(protocols)
   end

lib/mix/test/mix/tasks/compile.protocols_test.exs

@@ -42,6 +42,41 @@ defmodule Mix.Tasks.Compile.ProtocolsTest do
     end
   end
 
+  test "compiles after converting a protocol into a standard module", context do
+    Mix.Project.push MixTest.Case.Sample
+
+    in_tmp context.test, fn ->
+      File.mkdir_p!("lib")
+      assert Mix.Task.run("compile")
+
+      # Define a local protocol
+      File.write!("lib/protocol.ex", """
+      defprotocol Compile.Protocol do
+        def foo(a)
+      end
+
+      defimpl Compile.Protocol, for: Integer do
+        def foo(a), do: a
+      end
+      """)
+      assert compile_elixir_and_protocols() == :ok
+      mark_as_old!("_build/dev/lib/sample/consolidated/Elixir.Compile.Protocol.beam")
+      File.rm!("lib/protocol.ex")
+
+      # Define a standard module
+      File.write!("lib/protocol.ex", """
+      defmodule Compile.Protocol do
+      end
+      """)
+      assert compile_elixir_and_protocols() == :noop
+
+      # Delete a local protocol
+      File.rm!("lib/protocol.ex")
+      assert compile_elixir_and_protocols() == :noop
+      refute File.regular?("_build/dev/lib/sample/consolidated/Elixir.Compile.Protocol.beam")
+    end
+  end
+
   test "compiles and consolidates deps protocols", context do
     Mix.Project.push MixTest.Case.Sample