Warn on mixing ESM and commonJS (#3807)

As part of testing for #3265 it was noticed this isn't uncommon.

As there is no way for this to be supported, and it will be against how
the specification works we prefer to warn users on this at least for a
little while.

Co-authored-by: Théo Crevon <oleiade@users.noreply.github.com>
Co-authored-by: Ivan <2103732+codebien@users.noreply.github.com>

js/compiler/compiler.go

@@ -251,7 +251,17 @@ func (c *Compiler) compileImpl(
 			return nil, code, err
 		}
 		// the compatibility mode "decreases" here as we shouldn't transform twice
-		return c.compileImpl(code, filename, wrap, lib.CompatibilityModeBase, state.srcMap)
+		var prg *sobek.Program
+		prg, code, err = c.compileImpl(code, filename, wrap, lib.CompatibilityModeBase, state.srcMap)
+		if err == nil && strings.Contains(src, "module.exports") {
+			c.logger.Warningf(
+				"During the compilation of %q, it has been detected that the file combines ECMAScript modules (ESM) "+
+					"import/export syntax with commonJS module.exports. "+
+					"Mixing these two module systems is non-standard and will not be supported anymore in future releases. "+
+					"Please ensure to use solely one or the other syntax.",
+				filename)
+		}
+		return prg, code, err
 	}
 
 	if compatibilityMode == lib.CompatibilityModeExperimentalEnhanced {

js/compiler/compiler_test.go

@@ -241,3 +241,26 @@ func TestMinimalSourceMap(t *testing.T) {
 	require.NoError(t, err)
 	require.Empty(t, hook.Drain())
 }
+
+func TestMixingImportExport(t *testing.T) {
+	t.Parallel()
+	logger := logrus.New()
+	logger.SetLevel(logrus.DebugLevel)
+	logger.Out = io.Discard
+	hook := testutils.NewLogHook(logrus.InfoLevel, logrus.WarnLevel)
+	logger.AddHook(hook)
+
+	compiler := New(logger)
+	compiler.Options = Options{
+		CompatibilityMode: lib.CompatibilityModeExtended,
+		Strict:            true,
+	}
+	_, _, err := compiler.Compile("export let s = 5;\nmodule.exports = 'something';", "somefile", false)
+	require.NoError(t, err)
+	entries := hook.Drain()
+	require.Len(t, entries, 1)
+	msg, err := entries[0].String() // we need this in order to get the field error
+	require.NoError(t, err)
+
+	require.Contains(t, msg, `it has been detected that the file combines`)
+}