mparser: Add partial AST to exceptions

Surprisingly enough we need to do this twice. In some cases
(failing-meson/72 triggers this) we can error out after parsing the
codeblock, but without getting the expected eof.

We need to catch both exceptions as either one can interrupt the built
codeblock object.

Co-authored-by: Xavier Claessens <xavier.claessens@collabora.com>

mesonbuild/mparser.py

@@ -41,6 +41,9 @@ def decode_match(match: T.Match[str]) -> str:
     return codecs.decode(match.group(0).encode(), 'unicode_escape')
 
 class ParseException(MesonException):
+
+    ast: T.Optional[CodeBlockNode] = None
+
     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__(mlog.code_line(text, line, colno))
@@ -548,7 +551,11 @@ class Parser:
 
     def parse(self) -> CodeBlockNode:
         block = self.codeblock()
-        self.expect('eof')
+        try:
+            self.expect('eof')
+        except ParseException as e:
+            e.ast = block
+            raise
         return block
 
     def statement(self) -> BaseNode:
@@ -838,9 +845,13 @@ class Parser:
     def codeblock(self) -> CodeBlockNode:
         block = CodeBlockNode(self.current)
         cond = True
-        while cond:
-            curline = self.line()
-            if not isinstance(curline, EmptyNode):
-                block.lines.append(curline)
-            cond = self.accept('eol')
+        try:
+            while cond:
+                curline = self.line()
+                if not isinstance(curline, EmptyNode):
+                    block.lines.append(curline)
+                cond = self.accept('eol')
+        except ParseException as e:
+            e.ast = block
+            raise
         return block