Improve error message for invalid module attribute usage, closes #12656

Author: josevalim

lib/elixir/lib/kernel.ex

@@ -3555,10 +3555,12 @@ defmodule Kernel do
       not bootstrapped?(Macro) ->
         nil
 
-      not function? and __CALLER__.context == :match ->
+      not function? and (__CALLER__.context == :match or __CALLER__.context == :guard) ->
         raise ArgumentError,
               """
-              invalid write attribute syntax. If you want to define an attribute, don't do this:
+              invalid usage of module attributes. Module attributes cannot be used inside \
+              pattern matching (and guards) outside of a function. If you are trying to \
+              define an attribute, do not do this:
 
                   @foo = :value
 

lib/elixir/test/elixir/kernel_test.exs

@@ -831,11 +831,25 @@ defmodule KernelTest do
     end
 
     test "matching attribute" do
-      assert_raise ArgumentError, ~r"invalid write attribute syntax", fn ->
+      assert_raise ArgumentError, ~r"invalid usage of module attributes", fn ->
         defmodule MatchAttributeInModule do
           @foo = 42
         end
       end
+
+      assert_raise ArgumentError, ~r"invalid usage of module attributes", fn ->
+        defmodule MatchAttributeInModule do
+          @foo 16
+          <<_::@foo>> = "ab"
+        end
+      end
+
+      assert_raise ArgumentError, ~r"invalid usage of module attributes", fn ->
+        defmodule MatchAttributeInModule do
+          @foo 16
+          <<_::size(@foo)>> = "ab"
+        end
+      end
     end
   end