mformat: Make --recursive recurse subprojects with --subprojects flag

Currently, `meson format --recursive` only recurse meson.build files
that are reachable via a `subdir` function. Add `--subprojects` flag to
also recurse subprojects.

docs/markdown/snippets/mformat-recursive-subprojects.md

@@ -0,0 +1,4 @@
+## New option `--subprojects` for `meson format`
+
+A new option `--subprojects`, to be specified together with `--recursive`,
+tells `meson format` to also recurse into subprojects.

mesonbuild/mformat.py

@@ -874,11 +874,16 @@ class ComputeLineLengths(FullAstVisitor):
 
 class SubdirFetcher(FullAstVisitor):
 
-    def __init__(self, current_dir: Path):
+    def __init__(self, current_dir: Path, fetch_subprojects: bool):
         self.current_dir = current_dir
+        self.fetch_subprojects = fetch_subprojects
         self.subdirs: T.List[Path] = []
 
     def visit_FunctionNode(self, node: mparser.FunctionNode) -> None:
+        if self.fetch_subprojects and node.func_name.value == 'subproject':
+            if node.args.arguments and isinstance(node.args.arguments[0], mparser.StringNode):
+                subdir = node.args.arguments[0].value
+                self.subdirs.append(self.current_dir / 'subprojects' / subdir)
         if node.func_name.value == 'subdir':
             if node.args.arguments and isinstance(node.args.arguments[0], mparser.StringNode):
                 subdir = node.args.arguments[0].value
@@ -888,8 +893,9 @@ class SubdirFetcher(FullAstVisitor):
 
 class Formatter:
 
-    def __init__(self, configuration_file: T.Optional[Path], use_editor_config: bool, fetch_subdirs: bool):
+    def __init__(self, configuration_file: T.Optional[Path], use_editor_config: bool, fetch_subdirs: bool, fetch_subprojects: bool = False):
         self.fetch_subdirs = fetch_subdirs
+        self.fetch_subprojects = fetch_subprojects
         self.use_editor_config = use_editor_config
         self.config = self.load_configuration(configuration_file)
         self.current_config = self.config
@@ -974,7 +980,7 @@ class Formatter:
 
         ast = mparser.Parser(code, source_file.as_posix()).parse()
         if self.fetch_subdirs:
-            subdir_fetcher = SubdirFetcher(self.current_dir)
+            subdir_fetcher = SubdirFetcher(self.current_dir, self.fetch_subprojects)
             ast.accept(subdir_fetcher)
             self.subdirs = subdir_fetcher.subdirs
 
@@ -1016,6 +1022,11 @@ def add_arguments(parser: argparse.ArgumentParser) -> None:
         action='store_true',
         help='recurse subdirs (requires --check-only, --check-diff or --inplace option)',
     )
+    parser.add_argument(
+        '-s', '--subprojects',
+        action='store_true',
+        help='recurse subprojects (requires --recursive)',
+    )
     parser.add_argument(
         '-c', '--configuration',
         metavar='meson.format',
@@ -1058,6 +1069,8 @@ def run(options: argparse.Namespace) -> int:
         raise MesonException('--output argument implies having exactly one source file')
     if options.recursive and not (options.inplace or options.check_only or options.check_diff):
         raise MesonException('--recursive argument requires one of --inplace, --check-diff or --check-only')
+    if options.subprojects and not options.recursive:
+        raise MesonException('--subprojects argument requires --recursive option')
 
     from_stdin = len(options.sources) == 1 and options.sources[0].name == '-' and options.sources[0].parent == Path()
     if options.recursive and from_stdin:
@@ -1074,7 +1087,7 @@ def run(options: argparse.Namespace) -> int:
     if not options.configuration:
         options.configuration = get_meson_format(sources)
 
-    formatter = Formatter(options.configuration, options.editor_config, options.recursive)
+    formatter = Formatter(options.configuration, options.editor_config, options.recursive, options.subprojects)
     err = 0
 
     while sources:

test cases/format/7 recurse subprojects/compare_format_diff.py

@@ -0,0 +1,44 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: Apache-2.0
+# Copyright © 2026 Arthur Grillo
+
+import argparse
+import subprocess
+import sys
+import difflib
+
+
+def compare(actual: str, expected: str) -> int:
+    if actual == expected:
+        return 0
+
+    diff = difflib.ndiff(expected, actual)
+    for line in diff:
+        print(line, file=sys.stderr, end='')
+    return 1
+
+def main() -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument('meson_cmd_path', nargs='+')
+
+    args = parser.parse_args()
+
+    result = subprocess.run([
+        *args.meson_cmd_path,
+        'format',
+        '--recursive',
+        '--subprojects',
+        '--check-diff',
+    ], capture_output=True, text=True)
+
+    with open('subprojects/find-me/expected.diff', 'r', encoding='utf-8') as f:
+        actual = result.stdout.replace('\\', '/')
+        expected = f.read()
+
+        return compare(
+            actual,
+            expected,
+        )
+
+if __name__ == "__main__":
+    sys.exit(main())

test cases/format/7 recurse subprojects/meson.build

@@ -0,0 +1,14 @@
+project('format')
+
+python = import('python')
+meson_cmd = [python.find_installation(), find_program('meson').full_path()]
+file_compare = find_program(files('compare_format_diff.py'))
+
+test(
+    'recurse subprojects',
+    file_compare,
+    args: meson_cmd,
+    workdir: meson.project_source_root(),
+)
+
+subproject('find-me')

test cases/format/7 recurse subprojects/subprojects/find-me/expected.diff

@@ -0,0 +1,7 @@
+--- subprojects/find-me/meson.build	(original)
++++ subprojects/find-me/meson.build	(reformatted)
+@@ -1,3 +1 @@
+-project(
+-    'a'
+-) # should be on one line
++project('a')  # should be on one line

test cases/format/7 recurse subprojects/subprojects/find-me/meson.build

@@ -0,0 +1,3 @@
+project(
+    'a'
+) # should be on one line