add hint for behaviours on UndefinedFunctionErrors (#8517)

This is the first pass on #8513. I would love any feedback you have on this.

Author: dnsbty

lib/elixir/lib/exception.ex

@@ -924,14 +924,38 @@ defmodule UndefinedFunctionError do
 
   defp message(:"function not exported", module, function, arity) do
     formatted_fun = Exception.format_mfa(module, function, arity)
-    {"function #{formatted_fun} is undefined or private", true}
+    fun_message = "function #{formatted_fun} is undefined or private"
+    behaviour_hint = behaviour_hint(module, function, arity)
+    {fun_message <> behaviour_hint, true}
   end
 
   defp message(reason, module, function, arity) do
     formatted_fun = Exception.format_mfa(module, function, arity)
     {"function #{formatted_fun} is undefined (#{reason})", false}
   end
 
+  defp behaviour_hint(module, function, arity) do
+    case behaviours_for(module) do
+      [] ->
+        ""
+
+      behaviours ->
+        case Enum.find(behaviours, &expects_callback?(&1, function, arity)) do
+          nil -> ""
+          behaviour -> ", but the behaviour #{inspect(behaviour)} expects it to be present"
+        end
+    end
+  rescue
+    # In case the module was removed while we are computing this
+    UndefinedFunctionError ->
+      []
+  end
+
+  defp expects_callback?(behaviour, function, arity) do
+    callbacks = behaviour.behaviour_info(:callbacks)
+    Enum.member?(callbacks, {function, arity})
+  end
+
   @impl true
   def blame(exception, stacktrace) do
     %{reason: reason, module: module, function: function, arity: arity} = exception
@@ -997,6 +1021,12 @@ defmodule UndefinedFunctionError do
     ["      * ", Code.Identifier.inspect_as_function(fun), ?/, Integer.to_string(arity), ?\n]
   end
 
+  defp behaviours_for(module) do
+    :attributes
+    |> module.module_info()
+    |> Keyword.get(:behaviour, [])
+  end
+
   defp exports_for(module) do
     if function_exported?(module, :__info__, 1) do
       module.__info__(:macros) ++ module.__info__(:functions)

lib/elixir/test/elixir/exception_test.exs

@@ -729,6 +729,22 @@ defmodule ExceptionTest do
              |> message == "function nil.bar/0 is undefined"
     end
 
+    test "UndefinedFunctionError for a callback" do
+      defmodule Behaviour do
+        @callback callback() :: :ok
+        @optional_callbacks callback: 0
+      end
+
+      defmodule Implementation do
+        @behaviour Behaviour
+      end
+
+      assert %UndefinedFunctionError{module: Implementation, function: :callback, arity: 0}
+             |> message ==
+               "function ExceptionTest.Implementation.callback/0 is undefined or private" <>
+                 ", but the behaviour ExceptionTest.Behaviour expects it to be present"
+    end
+
     test "FunctionClauseError" do
       assert %FunctionClauseError{} |> message == "no function clause matches"
 

lib/elixir/test/elixir/record_test.exs

@@ -6,8 +6,6 @@ defmodule RecordTest do
   require Record
   doctest Record
 
-  import ExUnit.CaptureIO
-
   test "extract/2 extracts information from an Erlang file" do
     assert Record.extract(:file_info, from_lib: "kernel/include/file.hrl") == [
              size: :undefined,