fixes #12998 nim doc regression (#13117)

compiler/docgen.nim

@@ -17,7 +17,7 @@ import
   packages/docutils/rst, packages/docutils/rstgen,
   json, xmltree, cgi, trees, types,
   typesrenderer, astalgo, lineinfos, intsets,
-  pathutils, trees
+  pathutils, trees, nimconf
 
 const
   exportSection = skField
@@ -43,6 +43,7 @@ type
     destFile*: AbsoluteFile
     thisDir*: AbsoluteDir
     examples: string
+    wroteCss*: bool
 
   PDoc* = ref TDocumentor ## Alias to type less.
 
@@ -1069,10 +1070,16 @@ proc writeOutput*(d: PDoc, useWarning = false) =
     template outfile: untyped = d.destFile
     #let outfile = getOutFile2(d.conf, shortenDir(d.conf, filename), outExt, "htmldocs")
     createDir(outfile.splitFile.dir)
+    d.conf.outFile = outfile.extractFilename.RelativeFile
     if not writeRope(content, outfile):
       rawMessage(d.conf, if useWarning: warnCannotOpenFile else: errCannotOpenFile,
         outfile.string)
-    d.conf.outFile = outfile.extractFilename.RelativeFile
+    elif not d.wroteCss:
+      let cssSource = $d.conf.getPrefixDir() / "doc" / "nimdoc.css"
+      let cssDest = $d.conf.outDir / "nimdoc.out.css"
+        # renamed to make it easier to use with gitignore in user's repos
+      copyFile(cssSource, cssDest)
+      d.wroteCss = true
 
 proc writeOutputJson*(d: PDoc, useWarning = false) =
   runAllExamples(d)

compiler/installer.ini

@@ -139,4 +139,4 @@ licenses: "bin/nim,MIT;lib/*,MIT;"
 
 [nimble]
 pkgName: "compiler"
-pkgFiles: "compiler/*;doc/basicopt.txt;doc/advopt.txt"
+pkgFiles: "compiler/*;doc/basicopt.txt;doc/advopt.txt;doc/nimdoc.css"

compiler/main.nim

@@ -305,6 +305,7 @@ proc mainCommand*(graph: ModuleGraph) =
 
       var dumpdata = %[
         (key: "version", val: %VersionAsString),
+        (key: "prefixdir", val: %conf.getPrefixDir().string),
         (key: "project_path", val: %conf.projectFull.string),
         (key: "defined_symbols", val: definedSymbols),
         (key: "lib_paths", val: %libpaths),

compiler/options.nim

@@ -509,6 +509,9 @@ proc getPrefixDir*(conf: ConfigRef): AbsoluteDir =
   ##
   ## This is overridden by some tools (namely nimsuggest) via the ``conf.prefixDir``
   ## field.
+  ## This should resolve to root of nim sources, whether running nim from a local
+  ##  clone or using installed nim, so that these exist: `result/doc/advopt.txt`
+  ## and `result/lib/system.nim`
   if not conf.prefixDir.isEmpty: result = conf.prefixDir
   else: result = AbsoluteDir splitPath(getAppDir()).head