doc: Write out documented flag list

docs/users_guide/flags.py

@@ -53,6 +53,8 @@ from sphinx.errors import SphinxError
 from distutils.version import LooseVersion
 from utils import build_table_from_list
 
+import os.path
+
 ### Settings
 
 # Categories to titles as well as a canonical list of categories
@@ -590,15 +592,23 @@ class ExtensionPrintDirective(Directive):
 
 ### Additional processing
 
-# Convert every flagprint node into its output format
 def process_print_nodes(app, doctree, fromdocname):
 
+    # Convert every flagprint node into its output format
     for node in doctree.traverse(flagprint):
         node.generate_output(app, fromdocname)
 
     for node in doctree.traverse(extensionprint):
         node.generate_output(app, fromdocname)
 
+    # Write out file listing all documented flags
+    with open(os.path.join(app.outdir, 'ghc-flags.txt'), 'w') as f:
+        flag_names = \
+            {name
+             for flag in app.env.all_flags
+             for name in flag['names']}
+
+        f.write('\n'.join(flag_names))
 
 # To avoid creating duplicates in the serialized environment, clear all
 # flags originating from a file before re-reading it.