Warn about deprecated syntax features in imported modules.

This fixes #1072.

pkg/eval/builtin_fn_misc.go

@@ -283,7 +283,7 @@ func source(fm *Frame, fname string) error {
 		return err
 	}
 	src := parse.Source{Name: fname, Code: code, IsFile: true}
-	tree, err := parse.Parse(src)
+	tree, err := parse.ParseWithDeprecation(src, fm.ports[2].File)
 	if err != nil {
 		return err
 	}

pkg/eval/builtin_special.go

@@ -299,7 +299,7 @@ func loadModuleFile(fm *Frame, r diag.Ranger, spec, path string) (Ns, error) {
 }
 
 func evalModule(fm *Frame, r diag.Ranger, key string, src parse.Source) (Ns, error) {
-	tree, err := parse.Parse(src)
+	tree, err := parse.ParseWithDeprecation(src, fm.ports[2].File)
 	if err != nil {
 		return nil, err
 	}

pkg/eval/builtin_special_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"github.com/elves/elvish/pkg/eval/errs"
+	"github.com/elves/elvish/pkg/prog"
 	"github.com/elves/elvish/pkg/util"
 )
 
@@ -160,3 +161,17 @@ func TestUse(t *testing.T) {
 		That("use a b c").DoesNotCompile(),
 	)
 }
+
+// Regression test for #1072
+func TestUse_WarnsAboutDeprecatedSyntax(t *testing.T) {
+	restore := prog.SetShowDeprecations(true)
+	defer restore()
+	libdir, cleanup := util.InTestDir()
+	defer cleanup()
+	mustWriteFile("dep.elv", []byte("a = \\\n1"), 0600)
+
+	TestWithSetup(t, func(ev *Evaler) { ev.SetLibDir(libdir) },
+		// Importing module triggers check for deprecated syntax features
+		That("use dep").PrintsStderrWith("is deprecated"),
+	)
+}