msubprojects: Speedup subproject_dir extraction

The interpreter takes significant amount of time to initialize
everything in project() function. We only need to extract a string from
AST, just like we do in handle_meson_version_from_ast().

mesonbuild/ast/introspection.py

@@ -362,3 +362,22 @@ class IntrospectionInterpreter(AstInterpreter):
         self.sanity_check_ast()
         self.parse_project()
         self.run()
+
+    def extract_subproject_dir(self) -> T.Optional[str]:
+        '''Fast path to extract subproject_dir kwarg.
+           This is faster than self.parse_project() which also initialize options
+           and also calls parse_project() on every subproject.
+        '''
+        if not self.ast.lines:
+            return
+        project = self.ast.lines[0]
+        # first line is always project()
+        if not isinstance(project, FunctionNode):
+            return
+        for kw, val in project.args.kwargs.items():
+            assert isinstance(kw, IdNode), 'for mypy'
+            if kw.value == 'subproject_dir':
+                # mypy does not understand "and isinstance"
+                if isinstance(val, StringNode):
+                    return val.value
+        return None

mesonbuild/msubprojects.py

@@ -14,7 +14,7 @@ import tarfile
 import zipfile
 
 from . import mlog
-from .ast import IntrospectionInterpreter, AstIDGenerator
+from .ast import IntrospectionInterpreter
 from .mesonlib import quiet_git, GitException, Popen_safe, MesonException, windows_proof_rmtree
 from .wrap.wrap import (Resolver, WrapException, ALL_TYPES, PackageDefinition,
                         parse_patch_url, update_wrap_file, get_releases)
@@ -691,11 +691,9 @@ def run(options: 'Arguments') -> int:
         mlog.error('Directory', mlog.bold(source_dir), 'does not seem to be a Meson source directory.')
         return 1
     with mlog.no_logging():
-        intr = IntrospectionInterpreter(source_dir, '', 'none', visitors = [AstIDGenerator()])
+        intr = IntrospectionInterpreter(source_dir, '', 'none')
         intr.load_root_meson_file()
-        intr.sanity_check_ast()
-        intr.parse_project()
-    subproject_dir = intr.subproject_dir
+        subproject_dir = intr.extract_subproject_dir() or 'subprojects'
     if not os.path.isdir(os.path.join(source_dir, subproject_dir)):
         mlog.log('Directory', mlog.bold(source_dir), 'does not seem to have subprojects.')
         return 0