Make nested-trans-block exceptions nicer

Refs python-babel/babel#1049

src/jinja2/ext.py

@@ -495,16 +495,26 @@ def _parse_block(
                 parser.stream.expect("variable_end")
             elif parser.stream.current.type == "block_begin":
                 next(parser.stream)
-                if parser.stream.current.test("name:endtrans"):
+                block_name = (
+                    parser.stream.current.value if
+                    parser.stream.current.type == "name"
+                    else None
+                )
+                if block_name == "endtrans":
                     break
-                elif parser.stream.current.test("name:pluralize"):
+                elif block_name == "pluralize":
                     if allow_pluralize:
                         break
                     parser.fail(
                         "a translatable section can have only one pluralize section"
                     )
+                elif block_name == "trans":
+                    parser.fail(
+                        f"trans blocks can't be nested; did you mean `endtrans`?"
+                    )
                 parser.fail(
-                    "control structures in translatable sections are not allowed"
+                    f"control structures in translatable sections are not allowed; "
+                    f"saw `{block_name}`"
                 )
             elif parser.stream.eos:
                 parser.fail("unclosed translation block")

tests/test_ext.py

@@ -5,6 +5,7 @@
 
 from jinja2 import DictLoader
 from jinja2 import Environment
+from jinja2 import TemplateSyntaxError
 from jinja2 import nodes
 from jinja2 import pass_context
 from jinja2.exceptions import TemplateAssertionError
@@ -468,6 +469,18 @@ def test_extract_context(self):
             (3, "npgettext", ("babel", "%(users)s user", "%(users)s users", None), []),
         ]
 
+    def test_nested_trans_error(self):
+        s = '{% trans %}foo{% trans %}{% endtrans %}'
+        with pytest.raises(TemplateSyntaxError) as excinfo:
+            i18n_env.from_string(s)
+        assert "trans blocks can't be nested" in str(excinfo.value)
+
+    def test_trans_block_error(self):
+        s = '{% trans %}foo{% wibble bar %}{% endwibble %}{% endtrans %}'
+        with pytest.raises(TemplateSyntaxError) as excinfo:
+            i18n_env.from_string(s)
+        assert "saw `wibble`" in str(excinfo.value)
+
 
 class TestScope:
     def test_basic_scope_behavior(self):