docs: Add a :issue: role and use it in the CHANGELOG

This allows us to write the changelog reasonably simply.

The biggest downside is that pandoc won't be able to handle it anymore
when converting to markdown, but
sphinx-markdown-builder (https://github.com/codejamninja/sphinx-markdown-builder)
should be able to handle it.

doc_src/conf.py

@@ -11,6 +11,7 @@ import os.path
 import pygments
 import subprocess
 from sphinx.errors import SphinxError, SphinxWarning
+from docutils import nodes, utils
 
 # -- Helper functions --------------------------------------------------------
 
@@ -20,8 +21,29 @@ def strip_ext(path):
     return os.path.splitext(path)[0]
 
 
-# -- Load our Pygments lexer -------------------------------------------------
+# A :issue: role to link to github issues.
+# Used like :issue:`2364`
+def issue_role(name, rawtext, text, lineno, inliner, options=None, content=None):
+    options = options or {}
+    config = inliner.document.settings.env.app.config
+    try:
+        issue_num = int(text.strip())
+        if issue_num <= 0:
+            raise ValueError
+    except ValueError:
+        msg = inliner.reporter.error(
+            'Invalid issue number: "%s"' % text, line=lineno)
+        prb = inliner.problematic(rawtext, rawtext, msg)
+        return [prb], [msg]
+    template=issue_url + "/{n}"
+    ref = template.format(n=issue_num)
+    issue_text = "#{issue_no}".format(issue_no=issue_num)
+    link = nodes.reference(text=issue_text, refuri=ref, **options)
+    return [link], []
+
+# -- Load our extensions -------------------------------------------------
 def setup(app):
+    # Our own pygments lexer
     from sphinx.highlighting import lexers
 
     this_dir = os.path.dirname(os.path.realpath(__file__))
@@ -29,12 +51,16 @@ def setup(app):
         os.path.join(this_dir, "fish_indent_lexer.py"), lexername="FishIndentLexer"
     )
     lexers["fish-docs-samples"] = fish_indent_lexer
+
     # add_css_file only appears in Sphinx 1.8.0
     if hasattr(app, "add_css_file"):
         app.add_css_file("custom.css")
     else:
         app.add_stylesheet("custom.css")
 
+    app.add_config_value("issue_url", default=None, rebuild="html")
+    app.add_role("issue", issue_role)
+
 
 # The default language to assume
 highlight_language = "fish-docs-samples"
@@ -55,6 +81,7 @@ highlight_language = "fish-docs-samples"
 project = "fish-shell"
 copyright = "2020, fish-shell developers"
 author = "fish-shell developers"
+issue_url = "https://github.com/fish-shell/fish-shell/issues"
 
 # Parsing FISH-BUILD-VERSION-FILE is possible but hard to ensure that it is in the right place
 # fish_indent is guaranteed to be on PATH for the Pygments highlighter anyway