libexpr: Introduce Deprecated features

They are like experimental features, but opt-in instead of opt-out. They
will allow us to gracefully remove language features. See #437

Change-Id: I9ca04cc48e6926750c4d622c2b229b25cc142c42

doc/manual/src/command-ref/meson.build

@@ -7,10 +7,10 @@ xp_features_json = custom_target(
 experimental_features_shortlist_md = custom_target(
   command : nix_eval_for_docs + [
     '--expr',
-    'import @INPUT0@ (builtins.fromJSON (builtins.readFile @INPUT1@))',
+    'import @INPUT0@ "experimental" "xp" (builtins.fromJSON (builtins.readFile @INPUT1@))',
   ],
   input : [
-    '../../generate-xp-features-shortlist.nix',
+    '../../generate-features-shortlist.nix',
     xp_features_json,
   ],
   capture : true,
@@ -18,6 +18,26 @@ experimental_features_shortlist_md = custom_target(
   env : nix_env_for_docs,
 )
 
+dp_features_json = custom_target(
+  command : [nix, '__dump-dp-features'],
+  capture : true,
+  output : 'dp-features.json',
+)
+
+deprecated_features_shortlist_md = custom_target(
+  command : nix_eval_for_docs + [
+    '--expr',
+    'import @INPUT0@ "deprecated" "dp" (builtins.fromJSON (builtins.readFile @INPUT1@))',
+  ],
+  input : [
+    '../../generate-features-shortlist.nix',
+    dp_features_json,
+  ],
+  capture : true,
+  output : 'deprecated-features-shortlist.md',
+  env : nix_env_for_docs,
+)
+
 # Intermediate step for manpage generation.
 # This splorks the output of generate-manpage.nix as JSON,
 # which gets written as a directory tree below.
@@ -60,6 +80,7 @@ conf_file_md = custom_target(
     '../../utils.nix',
     conf_file_json,
     experimental_features_shortlist_md,
+    deprecated_features_shortlist_md,
   ],
   output : 'conf-file.md',
   env : nix_env_for_docs,