Add dangling changes from branches ghc-8.6 and ghc-8.8 (#1243)

* Fix multiple typos and inconsistencies in doc/markup.rst

Note: I noticed some overlap with #1112 from @wygulmage and #1081 from
@parsonsmatt after creating these proposed changes - mea culpa for not
looking at the open PRs sooner.

* Fix #1113 If no Signatures, no section of index.html

* Change the formatting of missing link destinations

The current formatting of the missing link destination does not really
help user to understand the reasons of the missing link.

To address this, I've changed the formatting in two ways:

- the missing link symbol name is now fully qualified. This way you
immediately know which haskell module cannot be linked. It is then easier
to understand why this module does not have documentation (hidden module
or broken documentation).
- one line per missing link, that's more readable now that symbol name
can be longer due to qualification.

For example, before haddock was listing missing symbol such as:

```
could not find link destinations for:
  Word8 Word16 mapMaybe
```

Now it is listed as:

```
could not find link destinations for:
  - Data.Word.Word8
  - Data.Word.Word16
  - Data.Maybe.mapMaybe
```

* Add `--ignore-link-symbol` command line argument

This argument can be used multiples time. A missing link to a symbol
listed by `--ignore-link-symbol` won't trigger "missing link" warning.

* Forbid spaces in anchors (#1148)

* Improve error messages with context information (#1060)

Co-authored-by: Matt Audesse <matt@mattaudesse.com>
Co-authored-by: Mike Pilgrem <mpilgrem@users.noreply.github.com>
Co-authored-by: Guillaume Bouchard <guillaume.bouchard@tweag.io>
Co-authored-by: Pepe Iborra <pepeiborra@gmail.com>

3 files changed, 9 insertions, 4 deletions

diff --git a/haddock-library/src/Documentation/Haddock/Parser.hs b/haddock-library/src/Documentation/Haddock/Parser.hs
index bd01f354..a3bba38a 100644
--- a/haddock-library/src/Documentation/Haddock/Parser.hs
+++ b/haddock-library/src/Documentation/Haddock/Parser.hs
@@ -227,7 +227,7 @@ takeWhile1_ = mfilter (not . T.null) . takeWhile_
 -- DocAName "Hello world"
 anchor :: Parser (DocH mod a)
 anchor = DocAName . T.unpack <$>
-         disallowNewline ("#" *> takeWhile1_ (/= '#') <* "#")
+         ("#" *> takeWhile1_ (\x -> x /= '#' && not (isSpace x)) <* "#")
 
 -- | Monospaced strings.
 --
diff --git a/haddock-library/src/Documentation/Haddock/Types.hs b/haddock-library/src/Documentation/Haddock/Types.hs
index d8c7a9fa..12ccd28d 100644
--- a/haddock-library/src/Documentation/Haddock/Types.hs
+++ b/haddock-library/src/Documentation/Haddock/Types.hs
@@ -126,7 +126,7 @@ data DocH mod id
   | DocMathInline String
   | DocMathDisplay String
   | DocAName String
-  -- ^ A (HTML) anchor.
+  -- ^ A (HTML) anchor. It must not contain any spaces.
   | DocProperty String
   | DocExamples [Example]
   | DocHeader (Header (DocH mod id))
diff --git a/haddock-library/test/Documentation/Haddock/ParserSpec.hs b/haddock-library/test/Documentation/Haddock/ParserSpec.hs
index 9bf9b6ea..f264dbba 100644
--- a/haddock-library/test/Documentation/Haddock/ParserSpec.hs
+++ b/haddock-library/test/Documentation/Haddock/ParserSpec.hs
@@ -289,8 +289,10 @@ spec = do
       it "parses a single word anchor" $ do
         "#foo#" `shouldParseTo` DocAName "foo"
 
-      it "parses a multi word anchor" $ do
-        "#foo bar#" `shouldParseTo` DocAName "foo bar"
+      -- Spaces are not allowed:
+      -- https://www.w3.org/TR/html51/dom.html#the-id-attribute
+      it "doesn't parse a multi word anchor" $ do
+        "#foo bar#" `shouldParseTo` "#foo bar#"
 
       it "parses a unicode anchor" $ do
         "#灼眼のシャナ#" `shouldParseTo` DocAName "灼眼のシャナ"
@@ -305,6 +307,9 @@ spec = do
       it "does not accept empty anchors" $ do
         "##" `shouldParseTo` "##"
 
+      it "does not accept anchors containing spaces" $ do
+        "{-# LANGUAGE GADTs #-}" `shouldParseTo` "{-# LANGUAGE GADTs #-}"
+
     context "when parsing emphasised text" $ do
       it "emphasises a word on its own" $ do
         "/foo/" `shouldParseTo` DocEmphasis "foo"