Store warnings emitted from external resources

lib/mix/lib/mix/compilers/elixir.ex

@@ -744,21 +744,38 @@ defmodule Mix.Compilers.Elixir do
   end
 
   defp apply_warnings(sources, runtime_warnings, compile_warnings) do
+    cwd = File.cwd!()
     runtime_group = Enum.group_by(runtime_warnings, & &1.source)
     compile_group = Enum.group_by(compile_warnings, & &1.source)
 
     for {source_path, source_entry} <- sources, into: %{} do
-      key = Path.absname(source_path)
-
       source(
         runtime_warnings: runtime_warnings,
-        compile_warnings: compile_warnings
+        compile_warnings: compile_warnings,
+        external: external
       ) = source_entry
 
+      keys = [
+        Path.absname(source_path, cwd)
+        | Enum.map(external, &(&1 |> elem(0) |> Path.absname(cwd)))
+      ]
+
+      runtime_warnings =
+        case Enum.flat_map(keys, &Map.get(runtime_group, &1, [])) do
+          [] -> runtime_warnings
+          runtime_warnings -> runtime_warnings
+        end
+
+      compile_warnings =
+        case Enum.flat_map(keys, &Map.get(compile_group, &1, [])) do
+          [] -> compile_warnings
+          compile_warnings -> compile_warnings
+        end
+
       {source_path,
        source(source_entry,
-         runtime_warnings: Map.get(runtime_group, key, runtime_warnings),
-         compile_warnings: Map.get(compile_group, key, compile_warnings)
+         runtime_warnings: runtime_warnings,
+         compile_warnings: compile_warnings
        )}
     end
   end

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

@@ -983,7 +983,6 @@ defmodule Mix.Tasks.Compile.ElixirTest do
         # is not valid but let's ensure we don't crash
         @external_resource "lib"
         @external_resource "lib/a.eex"
-
         @external_resource #{inspect(tmp)}
         def a, do: :ok
       end
@@ -1032,6 +1031,33 @@ defmodule Mix.Tasks.Compile.ElixirTest do
     File.rm(tmp_path("c.eex"))
   end
 
+  test "tracks warnings from external resources" do
+    in_fixture("no_mixfile", fn ->
+      Mix.Project.push(MixTest.Case.Sample)
+      File.touch!("lib/a.eex")
+
+      File.write!("lib/a.ex", """
+      defmodule A do
+        @external_resource "lib/a.eex"
+        IO.warn("oops", file: Path.absname("lib/a.eex"), line: 13)
+      end
+      """)
+
+      # Compiles with missing external resources
+      file = Path.absname("lib/a.eex")
+
+      assert capture_io(:stderr, fn ->
+               assert {:ok, [%Mix.Task.Compiler.Diagnostic{file: ^file, position: 13}]} =
+                        Mix.Tasks.Compile.Elixir.run([])
+
+               assert {:noop, [%Mix.Task.Compiler.Diagnostic{file: ^file, position: 13}]} =
+                        Mix.Tasks.Compile.Elixir.run(["--all-warnings"])
+             end) =~ "oops"
+
+      purge([A])
+    end)
+  end
+
   test "recompiles modules with exports tracking" do
     in_fixture("no_mixfile", fn ->
       Mix.Project.push(MixTest.Case.Sample)