backends: add a new "none" backend

It can only be used for projects that don't have any rules at all, i.e.
they are purely using Meson to:

- configure files
- run (script?) tests
- install files that exist by the end of the setup stage

This can be useful e.g. for Meson itself, a pure python project.

6 files changed, 51 insertions, 9 deletions

diff --git a/mesonbuild/backend/backends.py b/mesonbuild/backend/backends.py
index aa9ba3e..fa1629b 100644
--- a/mesonbuild/backend/backends.py
+++ b/mesonbuild/backend/backends.py
@@ -251,6 +251,9 @@ def get_backend_from_name(backend: str, build: T.Optional[build.Build] = None, i
     elif backend == 'xcode':
         from . import xcodebackend
         return xcodebackend.XCodeBackend(build, interpreter)
+    elif backend == 'none':
+        from . import nonebackend
+        return nonebackend.NoneBackend(build, interpreter)
     return None
 
 # This class contains the basic functionality that is needed by all backends.
diff --git a/mesonbuild/backend/nonebackend.py b/mesonbuild/backend/nonebackend.py
new file mode 100644
index 0000000..cf7c3dd
--- /dev/null
+++ b/mesonbuild/backend/nonebackend.py
@@ -0,0 +1,31 @@
+# Copyright 2022 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+from .backends import Backend
+from .. import mlog
+from ..mesonlib import MesonBugException
+
+
+class NoneBackend(Backend):
+
+    name = 'none'
+
+    def generate(self):
+        if self.build.get_targets():
+            raise MesonBugException('None backend cannot generate target rules, but should have failed earlier.')
+        mlog.log('Generating simple install-only backend')
+        self.serialize_tests()
+        self.create_install_data_files()
diff --git a/mesonbuild/coredata.py b/mesonbuild/coredata.py
index 564f54c..c252b22 100644
--- a/mesonbuild/coredata.py
+++ b/mesonbuild/coredata.py
@@ -61,7 +61,7 @@ if T.TYPE_CHECKING:
 # But the corresponding Git tag needs to be '0.1.0rc1'
 version = '1.0.99'
 
-backendlist = ['ninja', 'vs', 'vs2010', 'vs2012', 'vs2013', 'vs2015', 'vs2017', 'vs2019', 'vs2022', 'xcode']
+backendlist = ['ninja', 'vs', 'vs2010', 'vs2012', 'vs2013', 'vs2015', 'vs2017', 'vs2019', 'vs2022', 'xcode', 'none']
 
 DEFAULT_YIELDING = False
 
diff --git a/mesonbuild/interpreter/interpreter.py b/mesonbuild/interpreter/interpreter.py
index b725b99..4c3f723 100644
--- a/mesonbuild/interpreter/interpreter.py
+++ b/mesonbuild/interpreter/interpreter.py
@@ -3090,6 +3090,8 @@ class Interpreter(InterpreterBase, HoldableObject):
                                    "internal use. Please rename.")
 
     def add_target(self, name: str, tobj: build.Target) -> None:
+        if self.backend.name == 'none':
+            raise InterpreterException('Install-only backend cannot generate target rules, try using `--backend=ninja`.')
         if name == '':
             raise InterpreterException('Target name must not be empty.')
         if name.strip() == '':
diff --git a/mesonbuild/minstall.py b/mesonbuild/minstall.py
index 6922c38..e61757b 100644
--- a/mesonbuild/minstall.py
+++ b/mesonbuild/minstall.py
@@ -14,7 +14,6 @@
 from __future__ import annotations
 
 from glob import glob
-from pathlib import Path
 import argparse
 import errno
 import os
@@ -25,8 +24,7 @@ import subprocess
 import sys
 import typing as T
 
-from . import build
-from . import environment
+from . import build, coredata, environment
 from .backend.backends import InstallData
 from .mesonlib import MesonException, Popen_safe, RealPathAction, is_windows, setup_vsenv, pickle_load, is_osx
 from .scripts import depfixer, destdir_join
@@ -755,8 +753,11 @@ class Installer:
                 # file mode needs to be set last, after strip/depfixer editing
                 self.set_mode(outname, install_mode, d.install_umask)
 
-def rebuild_all(wd: str) -> bool:
-    if not (Path(wd) / 'build.ninja').is_file():
+def rebuild_all(wd: str, backend: str) -> bool:
+    if backend == 'none':
+        # nothing to build...
+        return True
+    if backend != 'ninja':
         print('Only ninja backend is supported to rebuild the project before installation.')
         return True
 
@@ -816,7 +817,8 @@ def run(opts: 'ArgumentType') -> int:
     if not opts.no_rebuild:
         b = build.load(opts.wd)
         setup_vsenv(b.need_vsenv)
-        if not rebuild_all(opts.wd):
+        backend = T.cast('str', b.environment.coredata.get_option(coredata.OptionKey('backend')))
+        if not rebuild_all(opts.wd, backend):
             sys.exit(-1)
     os.chdir(opts.wd)
     with open(os.path.join(log_dir, 'install-log.txt'), 'w', encoding='utf-8') as lf:
diff --git a/mesonbuild/mtest.py b/mesonbuild/mtest.py
index e1c6a21..7685023 100644
--- a/mesonbuild/mtest.py
+++ b/mesonbuild/mtest.py
@@ -43,7 +43,7 @@ import xml.etree.ElementTree as et
 from . import build
 from . import environment
 from . import mlog
-from .coredata import major_versions_differ, MesonVersionMismatchException
+from .coredata import MesonVersionMismatchException, OptionKey, major_versions_differ
 from .coredata import version as coredata_version
 from .mesonlib import (MesonException, OrderedSet, RealPathAction,
                        get_wine_shortpath, join_args, split_args, setup_vsenv)
@@ -2099,7 +2099,11 @@ def run(options: argparse.Namespace) -> int:
     setup_vsenv(b.need_vsenv)
 
     if not options.no_rebuild:
-        if not (Path(options.wd) / 'build.ninja').is_file():
+        backend = b.environment.coredata.get_option(OptionKey('backend'))
+        if backend == 'none':
+            # nothing to build...
+            options.no_rebuild = True
+        elif backend != 'ninja':
             print('Only ninja backend is supported to rebuild tests before running them.')
             # Disable, no point in trying to build anything later
             options.no_rebuild = True