meson: Disable plugins and related outputs if features are disabled

Previously, there was no check performed on features of plugins if these
specify GStreamer plugins. This commit adds that, and ensures that the
plugins and pkg-config targets are skipped if no outputs are to be
generated (this is already done for examples).

Closes #369

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1327>

meson.build

@@ -142,21 +142,6 @@ plugins = {
   'regex': {'library': 'libgstregex'},
   'textwrap': {'library': 'libgsttextwrap'},
 
-  'fallbackswitch': {
-    'library': 'libgstfallbackswitch',
-    'examples': ['gtk-fallbackswitch'],
-    'features': ['gtk', 'gio', 'gst-plugin-gtk4'],
-  },
-  'livesync': {
-    'library': 'libgstlivesync',
-    'examples': ['gtk-livesync'],
-    'features': ['gtk', 'gio', 'gst-plugin-gtk4'],
-  },
-  'togglerecord': {
-    'library': 'libgsttogglerecord',
-    'examples': ['gtk-recording'],
-    'features': ['gtk', 'gio', 'gst-plugin-gtk4'],
-  },
   'tracers': {'library': 'libgstrstracers'},
   'uriplaylistbin': {
     'library': 'libgsturiplaylistbin',
@@ -263,12 +248,29 @@ if get_option('gtk4').allowed()
       gtk4_features += 'x11glx'
     endif
   endif
-  plugins += {'gtk4': {
+  plugins += {
+    'gtk4': {
       'library': 'libgstgtk4',
       'examples': ['gtksink'],
       'extra-deps': {'gtk4': '>=4.6'},
       'features': gtk4_features,
-  }}
+    },
+    'fallbackswitch': {
+      'library': 'libgstfallbackswitch',
+      'examples': ['gtk-fallbackswitch'],
+      'features': ['gtk', 'gio', 'gst-plugin-gtk4'],
+    },
+    'livesync': {
+      'library': 'libgstlivesync',
+      'examples': ['gtk-livesync'],
+      'features': ['gtk', 'gio', 'gst-plugin-gtk4'],
+    },
+    'togglerecord': {
+      'library': 'libgsttogglerecord',
+      'examples': ['gtk-recording'],
+      'features': ['gtk', 'gio', 'gst-plugin-gtk4'],
+    },
+  }
 endif
 
 # Process plugins list
@@ -331,11 +333,22 @@ foreach plugin_name, details: plugins
       deps_cache += {dep_name: dep}
       if not dep.found()
         plugin_deps_found = false
+        break
       endif
     endforeach
+    plugin_features = details.get('features', [])
+    if plugin_deps_found
+      # Validate gst-plugin features
+      foreach feature: features
+        if feature.startswith('gst-plugin') and not packages.contains(feature)
+          plugin_deps_found = false
+          break
+        endif
+      endforeach
+    endif
     if plugin_deps_found
       packages += f'gst-plugin-@plugin_name@'
-      features += details.get('features', [])
+      features += plugin_features
       extra_features = run_command('dependencies.py', meson.current_source_dir(), plugin_name,
         '--feature', '--gst-version', gst_dep.version(), capture: true, check: true).stdout().strip()
       if extra_features != ''
@@ -385,7 +398,9 @@ endif
 # get cmdline for rust
 extra_env += {'RUSTC': ' '.join(rustc.cmd_array())}
 
-rs_plugins = custom_target('gst-plugins-rs',
+plugins = []
+if output.length() > 0
+  rs_plugins = custom_target('gst-plugins-rs',
     build_by_default: true,
     output: output,
     console: true,
@@ -406,8 +421,8 @@ rs_plugins = custom_target('gst-plugins-rs',
       '--depfile', '@DEPFILE@',
       '--lib-suffixes', library_suffixes,
     ] + feature_args + extra_args)
-
-plugins = rs_plugins.to_list()
+  plugins = rs_plugins.to_list()
+endif
 
 # This is used by GStreamer to static link Rust plugins into gst-full
 gst_plugins = []
@@ -476,7 +491,8 @@ subdir('docs')
 # We don't need to pass a command as we depends on the target above
 # but it is currently mandatory ( https://github.com/mesonbuild/meson/issues/8059 )
 # so use python as it's guaranteed to be present on any setup
-custom_target('gst-plugins-rs-pc-files',
+if pc_files.length() > 0
+  custom_target('gst-plugins-rs-pc-files',
     build_by_default: true,
     output: pc_files,
     console: true,
@@ -484,6 +500,7 @@ custom_target('gst-plugins-rs-pc-files',
     install_dir: pkgconfig_install_dir,
     depends: rs_plugins,
     command: [python, '-c', '""'])
+endif
 
 if get_option('webrtc').allowed()
   custom_target('gst-webrtc-signalling-server',