language: Add closure expressions

tests/sample_errors/expr_closure_not_cast.blp

@@ -0,0 +1,5 @@
+using Gtk 4.0;
+
+Label {
+  label: bind $closure();
+}

tests/sample_errors/expr_closure_not_cast.err

@@ -0,0 +1 @@
+4,15,10,Closure expression must be cast to the closure's return type

tests/samples/expr_closure.blp

@@ -0,0 +1,5 @@
+using Gtk 4.0;
+
+Label my-label {
+  label: bind ($my-closure(my-label.margin-bottom)) as (string);
+}

tests/samples/expr_closure.ui

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <requires lib="gtk" version="4.0"/>
+  <object class="GtkLabel" id="my-label">
+    <binding name="label">
+      <closure function="my-closure" type="gchararray">
+        <lookup name="margin-bottom" type="GtkLabel">
+          <constant>my-label</constant>
+        </lookup>
+      </closure>
+    </binding>
+  </object>
+</interface>

tests/test_samples.py

@@ -151,6 +151,7 @@ class TestSamples(unittest.TestCase):
         self.assert_sample("combo_box_text")
         self.assert_sample("comments")
         self.assert_sample("enum")
+        self.assert_sample("expr_closure", skip_run=True)  # The closure doesn't exist
         self.assert_sample("expr_lookup")
         self.assert_sample("file_filter")
         self.assert_sample("flags")
@@ -208,6 +209,7 @@ class TestSamples(unittest.TestCase):
         self.assert_sample_error("empty")
         self.assert_sample_error("enum_member_dne")
         self.assert_sample_error("expr_cast_conversion")
+        self.assert_sample_error("expr_closure_not_cast")
         self.assert_sample_error("expr_lookup_dne")
         self.assert_sample_error("expr_lookup_no_properties")
         self.assert_sample_error("filters_in_non_file_filter")