[rust-lang/rust] rustdoc-json: Move `#[doc` attribute out of `Attribute::Other` to own well-typed variant

### ROOT CAUSE The `#[doc(html_root_url = "...")]` attribute is currently stored in `Attribute::Other` in the rustdoc JSON backend. This is not ideal because `Attribute::Other` is intended for attributes that are not predefined and have a simple string representation. The `html_root_url` attribute, however, has a specific meaning and requires proper handling when generating links. By keeping it in `Attribute::Other`, we lose the ability to specifically process it in the JSON output and downstream tools. ### CODE FIX 1. In the rustdoc JSON backend, modify the `Attribute` enum to include a new variant `DocHtmlRootUrl` (or `HtmlRootUrl`) that holds the URL string. 2. Update the attribute parsing code to recognize the `#[doc(html_root_url = "...")]` attribute and convert it to the new variant. 3. Update the JSON emission code to output the new variant appropriately. Here is an example of the code changes: ```rust // In the rustdoc_json crate, modify the Attribute enum: #[derive(Serialize, Deserialize, Clone, PartialEq, JsonSpec)] #[json_schema::schemas(schema = "rustdoc")] pub enum Attribute { // ... other variants ... DocHtmlRootUrl(String), Other(String), } // Then, in the attribute parsing code (e.g., in the function that processes attributes): fn process_attribute(attr: &Attribute) -> Result { match attr { // ... other cases ... Attr::HtmlRootUrl(url) => { Ok(Attribute::DocHtmlRootUrl(url.clone())) } _ => { Ok(Attribute::Other(attr.to_string())) } } } // Finally, update the JSON emission code to handle the new variant: fn emit_attribute(attr: &Attribute) -> String { match attr { // ... other cases ... Attribute::DocHtmlRootUrl(url) => { format!("\"html_root_url\": \"{}\"", url) } Attribute::Other(s) => { format!("\"other_attribute\": \"{}\"", s) } } } ``` This change ensures that the `html_root_url` attribute is properly typed and can be easily identified and processed by downstream tools.