fixes #9462; jsondoc --index can generate a theindex.json (#20205)

compiler/docgen.nim

@@ -1124,6 +1124,8 @@ proc genJsonItem(d: PDoc, n, nameNode: PNode, k: TSymKind): JsonItem =
         for kind in genericParam.sym.typ.sons:
           param["types"].add %($kind)
         result.json["signature"]["genericParams"].add param
+  if optGenIndex in d.conf.globalOptions:
+    genItem(d, n, nameNode, k, kForceExport)
 
 proc setDoctype(d: PDoc, n: PNode) =
   ## Processes `{.doctype.}` pragma changing Markdown/RST parsing options.
@@ -1782,3 +1784,17 @@ proc commandBuildIndex*(conf: ConfigRef, dir: string, outFile = RelativeFile"")
     writeFile(filename, code)
   except:
     rawMessage(conf, errCannotOpenFile, filename.string)
+
+proc commandBuildIndexJson*(conf: ConfigRef, dir: string, outFile = RelativeFile"") =
+  var (modules, symbols, docs) = readIndexDir(dir)
+  let documents = toSeq(keys(Table[IndexEntry, seq[IndexEntry]](docs)))
+  let body = %*({"documents": documents, "modules": modules, "symbols": symbols})
+
+  var outFile = outFile
+  if outFile.isEmpty: outFile = theindexFname.RelativeFile.changeFileExt("")
+  let filename = getOutFile(conf, outFile, JsonExt)
+
+  try:
+    writeFile(filename, $body)
+  except:
+    rawMessage(conf, errCannotOpenFile, filename.string)

compiler/main.nim

@@ -325,7 +325,11 @@ proc mainCommand*(graph: ModuleGraph) =
       else:
         docLikeCmd commandDoc2(graph, TexExt)
   of cmdJsondoc0: docLikeCmd commandJson(cache, conf)
-  of cmdJsondoc: docLikeCmd commandDoc2(graph, JsonExt)
+  of cmdJsondoc:
+    docLikeCmd():
+      commandDoc2(graph, JsonExt)
+      if optGenIndex in conf.globalOptions and optWholeProject in conf.globalOptions:
+        commandBuildIndexJson(conf, $conf.outDir)
   of cmdCtags: docLikeCmd commandTags(cache, conf)
   of cmdBuildindex: docLikeCmd commandBuildIndex(conf, $conf.projectFull, conf.outFile)
   of cmdGendepend: commandGenDepend(graph)