check-templates: Extract/improve report_problem.

We extract the function for modularity and to
allow early-return.

We also add checks for "else" and improve a few
error messages.

tools/lib/template_parser.py

@@ -308,17 +308,22 @@ def validate(fn: Optional[str] = None, text: Optional[str] = None) -> None:
             else:
                 max_lines = 1
 
-            problem = None
+            def report_problem() -> Optional[str]:
                 if (start_tag == "code") and (end_line == start_line + 1):
-                problem = "Code tag is split across two lines."
+                    return "Code tag is split across two lines."
-            if is_else_tag:
-                pass
-            elif start_tag != end_tag:
-                problem = "Mismatched tag."
 
-            if not problem and (end_line > start_line + max_lines):
+                if is_else_tag:
+                    # We are not completely rigorous about having a sensible
+                    # order of if/elif/elif/else, but we catch obviously
+                    # mismatching else tags.
+                    if start_tag not in ("if", "else", "unless"):
+                        return f"Unexpected else/elif tag encountered after {start_tag} tag."
+                elif start_tag != end_tag:
+                    return f"Mismatched tags: ({start_tag} != {end_tag})"
+
+                if end_line > start_line + max_lines:
                     if end_col != start_col:
-                    problem = "Bad indentation."
+                        return "Indentation for start/end tags does not match."
 
                 if end_line >= start_line + 2:
                     # We have 3+ lines in the tag's block.
@@ -326,8 +331,11 @@ def validate(fn: Optional[str] = None, text: Optional[str] = None) -> None:
                     start_indent = indent_level(start_row_text)
                     if start_indent != start_col - 1 and start_row_text[start_indent] not in "<{":
                         junk = start_row_text[start_indent : start_col - 1]
-                    problem = f"There is junk before the start tag: {junk}"
+                        return f"There is junk before the start tag: {junk}"
 
+                return None
+
+            problem = report_problem()
             if problem:
                 raise TemplateParserException(
                     f"""

tools/tests/test_template_parser.py

@@ -84,7 +84,7 @@ class ParserTest(unittest.TestCase):
         my_html = """
             <b>foo</i>
         """
-        self._assert_validate_error("Mismatched tag.", text=my_html)
+        self._assert_validate_error(r"Mismatched tags: \(b != i\)", text=my_html)
 
     def test_validate_bad_indentation(self) -> None:
         my_html = """
@@ -92,7 +92,7 @@ class ParserTest(unittest.TestCase):
                 foo
                 </p>
         """
-        self._assert_validate_error("Bad indentation.", text=my_html)
+        self._assert_validate_error("Indentation for start/end tags does not match.", text=my_html)
 
     def test_validate_state_depth(self) -> None:
         my_html = """