Store filename in node location

Warnings have a location node object (with subdir and lineno
attributes), which is passed as a location: kwarg to mlog.warning() and
formatted in _log_error().

Re-purpose the subdir attribute (path relative to the source root dir,
with an implied filename of 'meson.build'), which is stored into the
node by parser(), to contain a pathname.

(Properly I should rename 'subdir' -> 'file' everywhere, but that's a
lot of churn just to see if this works)

Notes:
The warning location node may also have a colno attribute, which is
currently ignored by _log_error().

We can't currently issue warnings with locations in meson_options.txt
because the filename isn't part of the location (as it's assumed to be
'meson.build).

mesonbuild/ast/interpreter.py

@@ -154,7 +154,7 @@ class AstInterpreter(interpreterbase.InterpreterBase):
             code = f.read()
         assert(isinstance(code, str))
         try:
-            codeblock = mparser.Parser(code, subdir).parse()
+            codeblock = mparser.Parser(code, absname).parse()
         except mesonlib.MesonException as me:
             me.file = absname
             raise me

mesonbuild/interpreter.py

@@ -3748,7 +3748,7 @@ This will become a hard error in the future.''' % kwargs['input'], location=self
             code = f.read()
         assert(isinstance(code, str))
         try:
-            codeblock = mparser.Parser(code, self.subdir).parse()
+            codeblock = mparser.Parser(code, absname).parse()
         except mesonlib.MesonException as me:
             me.file = absname
             raise me

mesonbuild/interpreterbase.py

@@ -381,7 +381,7 @@ class InterpreterBase:
             raise InvalidCode('Builder file is empty.')
         assert(isinstance(code, str))
         try:
-            self.ast = mparser.Parser(code, self.subdir).parse()
+            self.ast = mparser.Parser(code, mesonfile).parse()
         except mesonlib.MesonException as me:
             me.file = mesonfile
             raise me

mesonbuild/mlog.py

@@ -251,7 +251,7 @@ def _log_error(severity: str, *rargs: T.Union[str, AnsiDecorator],
 
     location = kwargs.pop('location', None)
     if location is not None:
-        location_file = os.path.join(location.subdir, build_filename)
+        location_file = location.subdir
         location_str = get_error_location_string(location_file, location.lineno)
         # Unions are frankly awful, and we have to T.cast here to get mypy
         # to understand that the list concatenation is safe

mesonbuild/optinterpreter.py

@@ -140,7 +140,7 @@ class OptionInterpreter:
     def process(self, option_file):
         try:
             with open(option_file, 'r', encoding='utf8') as f:
-                ast = mparser.Parser(f.read(), '').parse()
+                ast = mparser.Parser(f.read(), option_file).parse()
         except mesonlib.MesonException as me:
             me.file = option_file
             raise me