rustdoc_to_markdown: Handle "stabs" in item name entries (#12494)

This PR extends `rustdoc_to_markdown` with support for rustdoc's
"stabs".

These are used in item name lists to indicate that the construct is
behind a feature flag:

<img width="641" alt="Screenshot 2024-05-30 at 1 34 53 PM"
src="https://github.com/zed-industries/zed/assets/1486634/0216f325-dc4e-4302-b6db-149ace31deea">

We now treat these specially in the Markdown output:

<img width="813" alt="Screenshot 2024-05-30 at 1 35 27 PM"
src="https://github.com/zed-industries/zed/assets/1486634/96396305-123d-40b2-af49-7eed71b62971">

Release Notes:

- N/A

crates/rustdoc_to_markdown/src/markdown_writer.rs

@@ -224,10 +224,13 @@ impl MarkdownWriter {
                     return StartTagOutcome::Skip;
                 }
 
-                if tag.attrs.borrow().iter().any(|attr| {
-                    attr.name.local.to_string() == "class" && attr.value.to_string() == "item-name"
-                }) {
-                    self.push_str("`");
+                if self.is_inside_item_name() {
+                    if tag.attrs.borrow().iter().any(|attr| {
+                        attr.name.local.to_string() == "class"
+                            && attr.value.split(' ').any(|class| class.trim() == "stab")
+                    }) {
+                        self.push_str(" [");
+                    }
                 }
             }
             _ => {}
@@ -265,11 +268,20 @@ impl MarkdownWriter {
             "table" => {
                 self.current_table_columns = 0;
             }
-            "div" => {
+            "div" | "span" => {
                 if tag.attrs.borrow().iter().any(|attr| {
                     attr.name.local.to_string() == "class" && attr.value.to_string() == "item-name"
                 }) {
-                    self.push_str("`: ");
+                    self.push_str(": ");
+                }
+
+                if self.is_inside_item_name() {
+                    if tag.attrs.borrow().iter().any(|attr| {
+                        attr.name.local.to_string() == "class"
+                            && attr.value.split(' ').any(|class| class.trim() == "stab")
+                    }) {
+                        self.push_str("]");
+                    }
                 }
             }
             _ => {}
@@ -283,8 +295,24 @@ impl MarkdownWriter {
         }
 
         let trimmed_text = text.trim_matches(|char| char == '\n' || char == '\r' || char == '§');
+
+        if self.is_inside_item_name() && !self.is_inside("span") && !self.is_inside("code") {
+            self.push_str(&format!("`{trimmed_text}`"));
+            return Ok(());
+        }
+
         self.push_str(trimmed_text);
 
         Ok(())
     }
+
+    /// Returns whether we're currently inside of an `.item-name` element, which
+    /// rustdoc uses to display Rust items in a list.
+    fn is_inside_item_name(&self) -> bool {
+        self.current_element_stack.iter().any(|element| {
+            element.attrs.borrow().iter().any(|attr| {
+                attr.name.local.to_string() == "class" && attr.value.to_string() == "item-name"
+            })
+        })
+    }
 }

crates/rustdoc_to_markdown/src/rustdoc_to_markdown.rs

@@ -139,6 +139,42 @@ mod tests {
         )
     }
 
+    #[test]
+    fn test_item_table() {
+        let html = indoc! {r##"
+            <h2 id="structs" class="section-header">Structs<a href="#structs" class="anchor">§</a></h2>
+            <ul class="item-table">
+            <li><div class="item-name"><a class="struct" href="struct.Error.html" title="struct axum::Error">Error</a></div><div class="desc docblock-short">Errors that can happen when using axum.</div></li>
+            <li><div class="item-name"><a class="struct" href="struct.Extension.html" title="struct axum::Extension">Extension</a></div><div class="desc docblock-short">Extractor and response for extensions.</div></li>
+            <li><div class="item-name"><a class="struct" href="struct.Form.html" title="struct axum::Form">Form</a><span class="stab portability" title="Available on crate feature `form` only"><code>form</code></span></div><div class="desc docblock-short">URL encoded extractor and response.</div></li>
+            <li><div class="item-name"><a class="struct" href="struct.Json.html" title="struct axum::Json">Json</a><span class="stab portability" title="Available on crate feature `json` only"><code>json</code></span></div><div class="desc docblock-short">JSON Extractor / Response.</div></li>
+            <li><div class="item-name"><a class="struct" href="struct.Router.html" title="struct axum::Router">Router</a></div><div class="desc docblock-short">The router type for composing handlers and services.</div></li></ul>
+            <h2 id="functions" class="section-header">Functions<a href="#functions" class="anchor">§</a></h2>
+            <ul class="item-table">
+            <li><div class="item-name"><a class="fn" href="fn.serve.html" title="fn axum::serve">serve</a><span class="stab portability" title="Available on crate feature `tokio` and (crate features `http1` or `http2`) only"><code>tokio</code> and (<code>http1</code> or <code>http2</code>)</span></div><div class="desc docblock-short">Serve the service with the supplied listener.</div></li>
+            </ul>
+        "##};
+        let expected = indoc! {r#"
+            ## Structs
+
+            - `Error`: Errors that can happen when using axum.
+            - `Extension`: Extractor and response for extensions.
+            - `Form` [`form`]: URL encoded extractor and response.
+            - `Json` [`json`]: JSON Extractor / Response.
+            - `Router`: The router type for composing handlers and services.
+
+            ## Functions
+
+            - `serve` [`tokio` and (`http1` or `http2`)]: Serve the service with the supplied listener.
+        "#}
+        .trim();
+
+        assert_eq!(
+            convert_rustdoc_to_markdown(html.as_bytes()).unwrap(),
+            expected
+        )
+    }
+
     #[test]
     fn test_table() {
         let html = indoc! {r##"