@PRIVATE_DIR@ in run_target() trigger an unhandled exception

[DevilishSpirits]

Describe the bug
Using @PRIVATE_DIR@ in a run_target() command array trigger an error.

$ meson setup builddir
The Meson build system
Version: 1.6.1
Source dir: /tmp/run_target_private_dir
Build dir: /tmp/run_target_private_dir/builddir
Build type: native build
Project name: run_target_private_dir
Project version: undefined
Host machine cpu family: x86_64
Host machine cpu: x86_64
Build targets in project: 1

Found ninja-1.12.1 at /usr/bin/ninja
Traceback (most recent call last):                                                                                                                                                                                  
  File "/usr/lib/python3.13/site-packages/mesonbuild/mesonmain.py", line 193, in run
    return options.run_func(options)
           ~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/mesonbuild/msetup.py", line 365, in run
    app.generate()
    ~~~~~~~~~~~~^^
  File "/usr/lib/python3.13/site-packages/mesonbuild/msetup.py", line 188, in generate
    return self._generate(env, capture, vslite_ctx)
           ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/mesonbuild/msetup.py", line 253, in _generate
    captured_compile_args = intr.backend.generate(capture, vslite_ctx)
  File "/usr/lib/python3.13/site-packages/mesonbuild/backend/ninjabackend.py", line 656, in generate
    self.generate_target(t)
    ~~~~~~~~~~~~~~~~~~~~^^^
  File "/usr/lib/python3.13/site-packages/mesonbuild/backend/ninjabackend.py", line 883, in generate_target
    self.generate_run_target(target)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/usr/lib/python3.13/site-packages/mesonbuild/backend/ninjabackend.py", line 1239, in generate_run_target
    _, _, cmd = self.eval_custom_target_command(target)
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/usr/lib/python3.13/site-packages/mesonbuild/backend/backends.py", line 1586, in eval_custom_target_command
    pdir = self.get_target_private_dir_abs(target)
  File "/usr/lib/python3.13/site-packages/mesonbuild/backend/backends.py", line 395, in get_target_private_dir_abs
    return os.path.join(self.environment.get_build_dir(), self.get_target_private_dir(target))
                                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/usr/lib/python3.13/site-packages/mesonbuild/backend/backends.py", line 392, in get_target_private_dir
    return os.path.join(self.get_target_filename(target, warn_multi_output=False) + '.p')
                        ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/mesonbuild/backend/backends.py", line 304, in get_target_filename
    assert isinstance(t, build.BuildTarget), t
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
AssertionError: <RunTarget private_dir_failure@run: /usr/bin/mkdir>

ERROR: Unhandled python exception

    This is a Meson bug and should be reported!

To Reproduce

Use this meson.build file :

project('run_target_private_dir')

run_target('private_dir_failure', command: [
	'mkdir','@PRIVATE_DIR@'
])

Expected behavior
I initially expected Meson to adopt the same behavior than in custom_target() by allocating a directory path for private data.
Though after reading the documentation this substitution not listed among the supported ones.

In case this is not supported I expect Meson to catch to produce a clear error such as @PRIVATE_DIR@ substitutions are not supported in run_target() command.

I did more testing and found that @BUILD_ROOT@ and @OUTPUT@ substitution works, the later expanding to the target name in Ninja.build.

system parameters

• Is this a cross build or just a plain native build (for the same computer)? native
• what operating system : ArchLinux
• what Python version : Python 3.13.1
• what meson --version : 1.6.1
• what ninja --version : 1.12.1