nixos-render-docs: parse GFM-alerts as admonitions

Mostly useful for out-of-tree users of nixos-render-docs, but could also
be useful if NixOS/Nixpkgs decides to transition to GFM's flavour of
admonition syntax in the future.

Both GFM-style and pandoc-fenced-div will now be tokenised as
"admonitions".

pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/md.py

@@ -579,6 +579,58 @@ def _block_titles(block: str) -> Callable[[markdown_it.MarkdownIt], None]:
 
     return do_add
 
+
+def _gfm_alerts(md: markdown_it.MarkdownIt) -> None:
+
+    _ALERT_PATTERN = re.compile(r"^\[\!(TIP|NOTE|IMPORTANT|WARNING|CAUTION)\][ \t]*(?:\n|$)", re.IGNORECASE)
+
+    @dataclasses.dataclass
+    class Entry:
+        open: Token
+        content: Token | None = None
+
+    """
+    Find blockquote tokens and convert GFM-alert-style blockquotes to admonition tokens.
+    """
+    def gfm_alert(state: markdown_it.rules_core.StateCore) -> None:
+        stack: list[Entry] = []
+        size = len(state.tokens)
+
+        for i, token in enumerate(state.tokens):
+            match token.type:
+                case "blockquote_open":
+                    entry = Entry(token)
+                    # Get the first inline token of the blockquote's first paragraph
+                    if i + 2 < size:
+                        para = state.tokens[i + 1]
+                        inline = state.tokens[i + 2]
+                        if para and para.type == "paragraph_open" and inline and inline.type == "inline":
+                            entry.content = inline
+                    stack.append(entry)
+
+                case "blockquote_close":
+                    entry = stack.pop()
+
+                    if entry.content is None:
+                        continue
+
+                    m = _ALERT_PATTERN.match(entry.content.content)
+                    if m is None:
+                        continue
+
+                    # Remove the alert marker from the rendered text.
+                    entry.content.content = entry.content.content[m.end() :]
+
+                    # Rewrite the enclosing blockquote as an admonition.
+                    entry.open.type = "admonition_open"
+                    entry.open.tag = "div"
+                    entry.open.meta["kind"] = m.group(1).lower()
+                    token.type = "admonition_close"
+                    token.tag = "div"
+
+    md.core.ruler.after("block", "github-alerts", gfm_alert)
+
+
 TR = TypeVar('TR', bound='Renderer')
 
 class Converter(ABC, Generic[TR]):
@@ -626,6 +678,7 @@ class Converter(ABC, Generic[TR]):
         self._md.use(_block_attr)
         self._md.use(_block_titles("example"))
         self._md.use(_block_titles("figure"))
+        self._md.use(_gfm_alerts)
         self._md.enable(["smartquotes", "replacements"])
 
     def _parse(self, src: str) -> list[Token]:

pkgs/by-name/ni/nixos-render-docs/src/tests/sample_md.py

@@ -1,4 +1,10 @@
 sample1 = """\
+> [!NOTE]
+> This is a *GFM* note.
+>
+> > [!caution]
+> > This is a **nested** GFM alert.
+
 :::: {.warning}
 foo
 ::: {.note}

pkgs/by-name/ni/nixos-render-docs/src/tests/test_asciidoc.py

@@ -45,6 +45,18 @@ f
 def test_full() -> None:
     c = Converter({ 'man(1)': 'http://example.org' })
     assert c._render(sample1) == """\
+[NOTE]
+====
+This is a __GFM__ note{zwsp}.
+
+[CAUTION]
+=====
+This is a **nested** GFM alert{zwsp}.
+=====
+
+====
+
+
 [WARNING]
 ====
 foo

pkgs/by-name/ni/nixos-render-docs/src/tests/test_commonmark.py

@@ -27,6 +27,10 @@ def test_indented_fence() -> None:
 def test_full() -> None:
     c = Converter({ 'man(1)': 'http://example.org' })
     assert c._render(sample1) == """\
+**Note:** This is a *GFM* note\\.
+
+**Caution:** This is a **nested** GFM alert\\.
+
 **Warning:** foo
 
 **Note:** nested

pkgs/by-name/ni/nixos-render-docs/src/tests/test_html.py

@@ -149,6 +149,14 @@ def test_footnotes() -> None:
 def test_full() -> None:
     c = Converter({ 'man(1)': 'http://example.org' }, {})
     assert c._render(sample1) == unpretty("""
+        <div class="note">
+          <h3 class="title">Note</h3>
+          <p>This is a <span class="emphasis"><em>GFM</em></span> note.</p>
+          <div class="caution">
+            <h3 class="title">Caution</h3>
+            <p>This is a <span class="strong"><strong>nested</strong></span> GFM alert.</p>
+          </div>
+        </div>
         <div class="warning">
          <h3 class="title">Warning</h3>
          <p>foo</p>

pkgs/by-name/ni/nixos-render-docs/src/tests/test_manpage.py

@@ -41,6 +41,18 @@ def test_full() -> None:
     assert c._render(sample1) == """\
 .sp
 .RS 4
+\\fBNote\\fP
+.br
+This is a \\fIGFM\\fR note\\&.
+.sp
+.RS 4
+\\fBCaution\\fP
+.br
+This is a \\fBnested\\fR GFM alert\\&.
+.RE
+.RE
+.sp
+.RS 4
 \\fBWarning\\fP
 .br
 foo

pkgs/by-name/ni/nixos-render-docs/src/tests/test_plugins.py

@@ -429,6 +429,90 @@ def test_admonitions() -> None:
               children=None, content='', markup=':::', info='', meta={}, block=True, hidden=False)
     ]
 
+@pytest.mark.parametrize(
+    ("alert", "kind"),
+    [
+        ("NOTE", "note"),
+        ("TIP", "tip"),
+        ("IMPORTANT", "important"),
+        ("WARNING", "warning"),
+        ("CAUTION", "caution"),
+    ],
+)
+def test_gfm_alert_kinds(alert: str, kind: str) -> None:
+    c = Converter({})
+    assert c._parse(f"> [!{alert}]\n> body") == [
+        Token(type='admonition_open', tag='div', markup='>', meta={'kind': kind}, nesting=1, map=[0, 2], block=True),
+        Token(type='paragraph_open', tag='p', nesting=1, map=[0, 2], level=1, block=True),
+        Token(type='inline', tag='', nesting=0, map=[0, 2], level=2, block=True, content='body', children=[
+            Token(type='text', tag='', nesting=0, content='body'),
+        ]),
+        Token(type='paragraph_close', tag='p', nesting=-1, level=1, block=True),
+        Token(type='admonition_close', tag='div', nesting=-1, markup='>', block=True)
+    ]
+
+def test_gfm_alert_basic_structure_preserved() -> None:
+    c = Converter({})
+    tokens = c._parse("> [!NOTE]\n> body text")
+    assert any(t.type == "inline" for t in tokens)
+
+@pytest.mark.parametrize(
+    "md",
+    [
+        "> # [!NOTE]",
+        "> - [!NOTE]",
+        "> - [!WARNING] body",
+        "> ### [!TIP]",
+        "> ```\n> [!NOTE]\n> ```",
+        "> This is not [!NOTE] an alert",
+        "> [!NOTE] with trailing text is ignored",
+    ],
+)
+def test_gfm_alert_rejected(md: str) -> None:
+    c = Converter({})
+    tokens = c._parse(md)
+    assert all(t.type != "admonition_open" for t in tokens)
+    assert tokens[0].type == "blockquote_open"
+    assert tokens[-1].type == "blockquote_close"
+
+def test_gfm_alert_multi_paragraph() -> None:
+    c = Converter({})
+    assert c._parse(
+        "> [!NOTE]\n"
+        "> line 1\n"
+        ">\n"
+        "> line 2"
+    ) == [
+        Token(type='admonition_open', tag='div', markup='>', meta={'kind': 'note'}, nesting=1, map=[0, 4], block=True),
+        Token(type='paragraph_open', tag='p', nesting=1, map=[0, 2], level=1, block=True),
+        Token(type='inline', tag='', nesting=0, map=[0, 2], level=2, block=True, content='line 1', children=[
+            Token(type='text', tag='', nesting=0, content='line 1'),
+        ]),
+        Token(type='paragraph_close', tag='p', nesting=-1, level=1, block=True),
+        Token(type='paragraph_open', tag='p', nesting=1, map=[3, 4], level=1, block=True),
+        Token(type='inline', tag='', nesting=0, map=[3, 4], level=2, block=True, content='line 2', children=[
+            Token(type='text', tag='', nesting=0, content='line 2')
+        ]),
+        Token(type='paragraph_close', tag='p', nesting=-1, level=1, block=True),
+        Token(type='admonition_close', tag='div', markup='>', block=True, nesting=-1),
+    ]
+
+def test_gfm_alert_whitespace_tolerant() -> None:
+    c = Converter({})
+    tokens = c._parse("> [!NOTE]   \n> body")
+    assert tokens[0].type == "admonition_open"
+    assert tokens[0].meta["kind"] == "note"
+
+def test_gfm_alert_empty_body_allowed() -> None:
+    c = Converter({})
+    assert c._parse("> [!NOTE]\n>") == [
+        Token(type='admonition_open', tag='div', markup='>', meta={'kind': 'note'}, nesting=1, map=[0, 2], block=True),
+        Token(type='paragraph_open', tag='p', nesting=1, map=[0, 1], level=1, block=True),
+        Token(type='inline', tag='', nesting=0, map=[0, 1], level=2, block=True, children=[]),
+        Token(type='paragraph_close', tag='p', nesting=-1, level=1, block=True),
+        Token(type='admonition_close', tag='div', markup='>', nesting=-1, block=True),
+    ]
+
 def test_example() -> None:
     c = Converter({})
     assert c._parse("::: {.example}\n# foo") == [