docgen: implement cross-document links (#20990)

* docgen: implement cross-document links

Fully implements https://github.com/nim-lang/RFCs/issues/125
Follow-up of: https://github.com/nim-lang/Nim/pull/18642 (for internal links)
and https://github.com/nim-lang/Nim/issues/20127.

Overview
--------

Explicit import-like directive is required, called `.. importdoc::`.
(the syntax is % RST, Markdown will use it for a while).

Then one can reference any symbols/headings/anchors, as if they
were in the local file (but they will be prefixed with a module name
or markup document in link text).
It's possible to reference anything from anywhere (any direction
in `.nim`/`.md`/`.rst` files).

See `doc/docgen.md` for full description.

Working is based on `.idx` files, hence one needs to generate
all `.idx` beforehand. A dedicated option `--index:only` is introduced
(and a separate stage for `--index:only` is added to `kochdocs.nim`).

Performance note
----------------

Full run for `./koch docs` now takes 185% of the time before this PR.
(After: 315 s, before: 170 s on my PC).
All the time seems to be spent on `--index:only` run, which takes
almost as much (85%) of normal doc run -- it seems that most time
is spent on file parsing, turning off HTML generation phase has not
helped much.
(One could avoid it by specifying list of files that can be referenced
and pre-processing only them. But it can become error-prone and I assume
that these linke will be **everywhere** in the repository anyway,
especially considering https://github.com/nim-lang/RFCs/issues/478.
So every `.nim`/`.md` file is processed for `.idx` first).

But that's all without significant part of repository converted to
cross-module auto links. To estimate impact I checked the time for
`doc`ing a few files (after all indexes have been generated), and
everywhere difference was **negligible**.
E.g. for `lib/std/private/osfiles.nim` that `importdoc`s large
`os.idx` and hence should have been a case with relatively large
performance impact, but:

* After: 0.59 s.
* Before: 0.59 s.

So Nim compiler works so slow that doc part basically does not matter :-)

Testing
-------

1) added `extlinks` test to `nimdoc/`
2) checked that `theindex.html` is still correct
2) fixed broken auto-links for modules that were derived from `os.nim`
   by adding appropriate ``importdoc``

Implementation note
-------------------

Parsing and formating of `.idx` entries is moved into a dedicated
`rstidx.nim` module from `rstgen.nim`.

`.idx` file format changed:

* fields are not escaped in most cases because we need original
  strings for referencing, not HTML ones
  (the exception is linkTitle for titles and headings).
  Escaping happens later -- on the stage of `rstgen` buildIndex, etc.
* all lines have fixed number of columns 6
* added discriminator tag as a first column,
  it always allows distinguish Nim/markup entries, titles/headings, etc.
  `rstgen` does not rely any more (in most cases) on ad-hoc logic
  to determine what type each entry is.
* there is now always a title entry added at the first line.
* add a line number as 6th column
* linkTitle (4th) column has a different format: before it was like
  `module: funcName()`, now it's `proc funcName()`.
  (This format is also propagated to `theindex.html` and search results,
  I kept it that way since I like it more though it's discussible.)
  This column is what used for Nim symbols resolution.
* also changed details on column format for headings and titles:
  "keyword" is original, "linkTitle" is HTML one

* fix paths on Windows + more clear code

* Update compiler/docgen.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

* Handle .md and .nim paths uniformly in findRefFile

* handle titles better + more comments

* don't allow markup overwrite index title for .nim files

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

1 files changed, 26 insertions, 15 deletions

diff --git a/tools/kochdocs.nim b/tools/kochdocs.nim
index 4fffc7533..f63aea030 100644
--- a/tools/kochdocs.nim
+++ b/tools/kochdocs.nim
@@ -240,27 +240,28 @@ proc buildDocSamples(nimArgs, destPath: string) =
   exec(findNim().quoteShell() & " doc $# -o:$# $#" %
     [nimArgs, destPath / "docgen_sample.html", "doc" / "docgen_sample.nim"])
 
-proc buildDocPackages(nimArgs, destPath: string) =
+proc buildDocPackages(nimArgs, destPath: string, indexOnly: bool) =
   # compiler docs; later, other packages (perhaps tools, testament etc)
   let nim = findNim().quoteShell()
     # to avoid broken links to manual from compiler dir, but a multi-package
     # structure could be supported later
 
   proc docProject(outdir, options, mainproj: string) =
-    exec("$nim doc --project --outdir:$outdir $nimArgs --git.url:$gitUrl $options $mainproj" % [
+    exec("$nim doc --project --outdir:$outdir $nimArgs --git.url:$gitUrl $index $options $mainproj" % [
       "nim", nim,
       "outdir", outdir,
       "nimArgs", nimArgs,
       "gitUrl", gitUrl,
       "options", options,
       "mainproj", mainproj,
+      "index", if indexOnly: "--index:only" else: ""
       ])
   let extra = "-u:boot"
   # xxx keep in sync with what's in $nim_prs_D/config/nimdoc.cfg, or, rather,
   # start using nims instead of nimdoc.cfg
   docProject(destPath/"compiler", extra, "compiler/index.nim")
 
-proc buildDoc(nimArgs, destPath: string) =
+proc buildDoc(nimArgs, destPath: string, indexOnly: bool) =
   # call nim for the documentation:
   let rst2html = getMd2html()
   var
@@ -268,17 +269,19 @@ proc buildDoc(nimArgs, destPath: string) =
               officialPackagesList.len + officialPackagesListWithoutIndex.len)
     i = 0
   let nim = findNim().quoteShell()
+
+  let index = if indexOnly: "--index:only" else: ""
   for d in items(rst2html):
-    commands[i] = nim & " md2html $# --git.url:$# -o:$# --index:on $#" %
+    commands[i] = nim & " md2html $# --git.url:$# -o:$# $# $#" %
       [nimArgs, gitUrl,
-      destPath / changeFileExt(splitFile(d).name, "html"), d]
+      destPath / changeFileExt(splitFile(d).name, "html"), index, d]
     i.inc
   for d in items(doc):
     let extra = if isJsOnly(d): "--backend:js" else: ""
     var nimArgs2 = nimArgs
     if d.isRelativeTo("compiler"): doAssert false
-    commands[i] = nim & " doc $# $# --git.url:$# --outdir:$# --index:on $#" %
-      [extra, nimArgs2, gitUrl, destPath, d]
+    commands[i] = nim & " doc $# $# --git.url:$# --outdir:$# $# $#" %
+      [extra, nimArgs2, gitUrl, destPath, index, d]
     i.inc
   for d in items(withoutIndex):
     commands[i] = nim & " doc $# --git.url:$# -o:$# $#" %
@@ -300,12 +303,6 @@ proc buildDoc(nimArgs, destPath: string) =
     i.inc
 
   mexec(commands)
-  exec(nim & " buildIndex -o:$1/theindex.html $1" % [destPath])
-    # caveat: this works so long it's called before `buildDocPackages` which
-    # populates `compiler/` with unrelated idx files that shouldn't be in index,
-    # so should work in CI but you may need to remove your generated html files
-    # locally after calling `./koch docs`. The clean fix would be for `idx` files
-    # to be transient with `--project` (eg all in memory).
 
 proc nim2pdf(src: string, dst: string, nimArgs: string) =
   # xxx expose as a `nim` command or in some other reusable way.
@@ -351,9 +348,23 @@ proc buildDocsDir*(args: string, dir: string) =
   gitClonePackages(@["asyncftpclient", "punycode", "smtp", "db_connector"])
   createDir(dir)
   buildDocSamples(args, dir)
-  buildDoc(args, dir) # bottleneck
+
+  # generate `.idx` files and top-level `theindex.html`:
+  buildDoc(args, dir, indexOnly=true) # bottleneck
+  let nim = findNim().quoteShell()
+  exec(nim & " buildIndex -o:$1/theindex.html $1" % [dir])
+    # caveat: this works so long it's called before `buildDocPackages` which
+    # populates `compiler/` with unrelated idx files that shouldn't be in index,
+    # so should work in CI but you may need to remove your generated html files
+    # locally after calling `./koch docs`. The clean fix would be for `idx` files
+    # to be transient with `--project` (eg all in memory).
+  buildDocPackages(args, dir, indexOnly=true)
+
+  # generate HTML and package-level `theindex.html`:
+  buildDoc(args, dir, indexOnly=false) # bottleneck
+  buildDocPackages(args, dir, indexOnly=false)
+
   copyFile(dir / "overview.html", dir / "index.html")
-  buildDocPackages(args, dir)
   copyFile(docHackJsSource, dir / docHackJsSource.lastPathPart)
 
 proc buildDocs*(args: string, localOnly = false, localOutDir = "") =