assistant: Add MVP for /rustdoc using indexed docs (#12952)

This PR adds an MVP of retrieving docs using the `/rustdoc` command from
an indexed set of docs.

To try this out:

1. Build local docs using `cargo doc`
2. Index the docs for the crate you want to search using `/rustdoc
--index <CRATE_NAME>`
    - Note: This may take a while, depending on the size of the crate
3. Search for docs using `/rustdoc my_crate::path::to::item`
    - You should get completions for the available items

Here are some screenshots of it in action:

<img width="640" alt="Screenshot 2024-06-12 at 6 19 20 PM"
src="https://github.com/zed-industries/zed/assets/1486634/6c49bec9-d084-4dcb-a92c-1b4c557ee9ce">

<img width="636" alt="Screenshot 2024-06-12 at 6 52 56 PM"
src="https://github.com/zed-industries/zed/assets/1486634/636a651c-7d02-48dc-b05c-931f33c49f9c">

Release Notes:

- N/A

crates/assistant/src/slash_command/rustdoc_command.rs

@@ -10,7 +10,8 @@ use gpui::{AppContext, Model, Task, WeakView};
 use http::{AsyncBody, HttpClient, HttpClientWithUrl};
 use language::LspAdapterDelegate;
 use project::{Project, ProjectPath};
-use rustdoc::convert_rustdoc_to_markdown;
+use rustdoc::crawler::LocalProvider;
+use rustdoc::{convert_rustdoc_to_markdown, RustdocStore};
 use ui::{prelude::*, ButtonLike, ElevationIndex};
 use workspace::Workspace;
 
@@ -115,12 +116,19 @@ impl SlashCommand for RustdocSlashCommand {
 
     fn complete_argument(
         &self,
-        _query: String,
+        query: String,
         _cancel: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
-        _cx: &mut AppContext,
+        cx: &mut AppContext,
     ) -> Task<Result<Vec<String>>> {
-        Task::ready(Ok(Vec::new()))
+        let store = RustdocStore::global(cx);
+        cx.background_executor().spawn(async move {
+            let items = store.search(query).await;
+            Ok(items
+                .into_iter()
+                .map(|(crate_name, item)| format!("{crate_name}::{}", item.display()))
+                .collect())
+        })
     }
 
     fn run(
@@ -140,7 +148,67 @@ impl SlashCommand for RustdocSlashCommand {
         let project = workspace.read(cx).project().clone();
         let fs = project.read(cx).fs().clone();
         let http_client = workspace.read(cx).client().http_client();
-        let mut path_components = argument.split("::");
+        let path_to_cargo_toml = Self::path_to_cargo_toml(project, cx);
+
+        let mut item_path = String::new();
+        let mut crate_name_to_index = None;
+
+        let mut args = argument.split(' ').map(|word| word.trim());
+        while let Some(arg) = args.next() {
+            if arg == "--index" {
+                let Some(crate_name) = args.next() else {
+                    return Task::ready(Err(anyhow!("no crate name provided to --index")));
+                };
+                crate_name_to_index = Some(crate_name.to_string());
+                continue;
+            }
+
+            item_path.push_str(arg);
+        }
+
+        if let Some(crate_name_to_index) = crate_name_to_index {
+            let index_task = cx.background_executor().spawn({
+                let rustdoc_store = RustdocStore::global(cx);
+                let fs = fs.clone();
+                let crate_name_to_index = crate_name_to_index.clone();
+                async move {
+                    let cargo_workspace_root = path_to_cargo_toml
+                        .and_then(|path| path.parent().map(|path| path.to_path_buf()))
+                        .ok_or_else(|| anyhow!("no Cargo workspace root found"))?;
+
+                    let provider = Box::new(LocalProvider::new(fs, cargo_workspace_root));
+
+                    rustdoc_store
+                        .index(crate_name_to_index.clone(), provider)
+                        .await?;
+
+                    anyhow::Ok(format!("Indexed {crate_name_to_index}"))
+                }
+            });
+
+            return cx.foreground_executor().spawn(async move {
+                let text = index_task.await?;
+                let range = 0..text.len();
+                Ok(SlashCommandOutput {
+                    text,
+                    sections: vec![SlashCommandOutputSection {
+                        range,
+                        render_placeholder: Arc::new(move |id, unfold, _cx| {
+                            RustdocIndexPlaceholder {
+                                id,
+                                unfold,
+                                source: RustdocSource::Local,
+                                crate_name: SharedString::from(crate_name_to_index.clone()),
+                            }
+                            .into_any_element()
+                        }),
+                    }],
+                    run_commands_in_text: false,
+                })
+            });
+        }
+
+        let mut path_components = item_path.split("::");
         let crate_name = match path_components
             .next()
             .ok_or_else(|| anyhow!("missing crate name"))
@@ -148,29 +216,37 @@ impl SlashCommand for RustdocSlashCommand {
             Ok(crate_name) => crate_name.to_string(),
             Err(err) => return Task::ready(Err(err)),
         };
-        let module_path = path_components.map(ToString::to_string).collect::<Vec<_>>();
-        let path_to_cargo_toml = Self::path_to_cargo_toml(project, cx);
+        let item_path = path_components.map(ToString::to_string).collect::<Vec<_>>();
 
         let text = cx.background_executor().spawn({
+            let rustdoc_store = RustdocStore::global(cx);
             let crate_name = crate_name.clone();
-            let module_path = module_path.clone();
+            let item_path = item_path.clone();
             async move {
-                Self::build_message(
-                    fs,
-                    http_client,
-                    crate_name,
-                    module_path,
-                    path_to_cargo_toml.as_deref(),
-                )
-                .await
+                let item_docs = rustdoc_store
+                    .load(crate_name.clone(), Some(item_path.join("::")))
+                    .await;
+
+                if let Ok(item_docs) = item_docs {
+                    anyhow::Ok((RustdocSource::Local, item_docs))
+                } else {
+                    Self::build_message(
+                        fs,
+                        http_client,
+                        crate_name,
+                        item_path,
+                        path_to_cargo_toml.as_deref(),
+                    )
+                    .await
+                }
             }
         });
 
         let crate_name = SharedString::from(crate_name);
-        let module_path = if module_path.is_empty() {
+        let module_path = if item_path.is_empty() {
             None
         } else {
-            Some(SharedString::from(module_path.join("::")))
+            Some(SharedString::from(item_path.join("::")))
         };
         cx.foreground_executor().spawn(async move {
             let (source, text) = text.await?;
@@ -228,3 +304,31 @@ impl RenderOnce for RustdocPlaceholder {
             .on_click(move |_, cx| unfold(cx))
     }
 }
+
+#[derive(IntoElement)]
+struct RustdocIndexPlaceholder {
+    pub id: ElementId,
+    pub unfold: Arc<dyn Fn(&mut WindowContext)>,
+    pub source: RustdocSource,
+    pub crate_name: SharedString,
+}
+
+impl RenderOnce for RustdocIndexPlaceholder {
+    fn render(self, _cx: &mut WindowContext) -> impl IntoElement {
+        let unfold = self.unfold;
+
+        ButtonLike::new(self.id)
+            .style(ButtonStyle::Filled)
+            .layer(ElevationIndex::ElevatedSurface)
+            .child(Icon::new(IconName::FileRust))
+            .child(Label::new(format!(
+                "rustdoc index ({source}): {crate_name}",
+                crate_name = self.crate_name,
+                source = match self.source {
+                    RustdocSource::Local => "local",
+                    RustdocSource::DocsDotRs => "docs.rs",
+                }
+            )))
+            .on_click(move |_, cx| unfold(cx))
+    }
+}