mlog: move code for printing code with a caret to the mlog module

We need this outside the constructor for the ParseException class, so
let's pull it out. mlog seemed like a good place since it's a text
formatting function, and has no dependencies.

mesonbuild/mlog.py

@@ -495,3 +495,14 @@ def stop_pager() -> None:
             pass
         log_pager.wait()
         log_pager = None
+
+
+def code_line(text: str, line: str, colno: int) -> str:
+    """Print a line with a caret pointing to the colno
+
+    :param text: A message to display before the line
+    :param line: The line of code to be pointed to
+    :param colno: The column number to point at
+    :return: A formatted string of the text, line, and a caret
+    """
+    return f'{text}\n{line}\n{" " * colno}^'

mesonbuild/mparser.py

@@ -47,7 +47,7 @@ def decode_match(match: T.Match[str]) -> str:
 class ParseException(MesonException):
     def __init__(self, text: str, line: str, lineno: int, colno: int) -> None:
         # Format as error message, followed by the line with the error, followed by a caret to show the error column.
-        super().__init__("{}\n{}\n{}".format(text, line, '{}^'.format(' ' * colno)))
+        super().__init__(mlog.code_line(text, line, colno))
         self.lineno = lineno
         self.colno = colno