diff --git a/mesonbuild/msetup.py b/mesonbuild/msetup.py
index 3c98fca7a..a7d28dd43 100644
--- a/mesonbuild/msetup.py
+++ b/mesonbuild/msetup.py
@@ -267,6 +267,13 @@ class MesonApp:
             # by Interpreter() init, but this is most visible at the end
             if env.coredata.options[mesonlib.OptionKey('backend')].value == 'xcode':
                 mlog.warning('xcode backend is currently unmaintained, patches welcome')
+            if env.coredata.options[mesonlib.OptionKey('layout')].value == 'flat':
+                mlog.warning('-Dlayout=flat is unsupported and probably broken. It was a failed experiment at '
+                             'making Windows build artifacts runnable while uninstalled, due to PATH considerations, '
+                             'but was untested by CI and anyways breaks reasonable use of conflicting targets in different subdirs. '
+                             'Please consider using `meson devenv` instead. See https://github.com/mesonbuild/meson/pull/9243 '
+                             'for details.')
+
         except Exception as e:
             mintro.write_meson_info_file(b, [e])
             if 'cdf' in locals():