assistant: Add /docs slash command (#13794)

This PR adds a new `/docs` slash command to the Assistant. This slash
command replaces `/rustdoc`.

The `/docs` slash command works with different providers. There is
currently a built-in provider for rustdoc, but new providers can be
defined within extensions. The Gleam extension contains an example of
this.

When you first type `/docs` a completion menu will be shown with the
list of available providers:


https://github.com/zed-industries/zed/assets/1486634/32287000-5855-44d9-a2eb-569596f5abd9

After completing the provider you want to use then you can type the
package name and/or item path to search for the relevant docs:


https://github.com/zed-industries/zed/assets/1486634/6fc55a63-7fcd-42ea-80ce-08c670bf03fc

There are still some rough edges around completions that I would like to
get cleaned up in a future PR. Both of these seem to stem from the fact
that we're using an intermediate completion in the slash command:

1. Accepting a provider completion will show an error until you press
<kbd>Space</kbd> to continue typing.
- We need a way of not submitting a slash command when a completion is
accepted.
2. We currently need to show the provider name in the documentation item
completion list.
- Without it, the provider name gets wiped out when accepting a
completion, causing the slash command to become invalid.

Release Notes:

- N/A

crates/indexed_docs/src/providers/rustdoc.rs

@@ -16,16 +16,6 @@ use http::{AsyncBody, HttpClient, HttpClientWithUrl};
 
 use crate::{IndexedDocsDatabase, IndexedDocsProvider, PackageName, ProviderId};
 
-#[derive(Debug, Clone, Copy)]
-pub enum RustdocSource {
-    /// The docs were sourced from Zed's rustdoc index.
-    Index,
-    /// The docs were sourced from local `cargo doc` output.
-    Local,
-    /// The docs were sourced from `docs.rs`.
-    DocsDotRs,
-}
-
 #[derive(Debug)]
 struct RustdocItemWithHistory {
     pub item: RustdocItem,

crates/indexed_docs/src/registry.rs

@@ -34,6 +34,14 @@ impl IndexedDocsRegistry {
         }
     }
 
+    pub fn list_providers(&self) -> Vec<ProviderId> {
+        self.stores_by_provider
+            .read()
+            .keys()
+            .cloned()
+            .collect::<Vec<_>>()
+    }
+
     pub fn register_provider(
         &self,
         provider: Box<dyn IndexedDocsProvider + Send + Sync + 'static>,

crates/indexed_docs/src/store.rs

@@ -94,22 +94,12 @@ impl IndexedDocsStore {
         self.indexing_tasks_by_package.read().contains_key(package)
     }
 
-    pub async fn load(
-        &self,
-        package: PackageName,
-        item_path: Option<String>,
-    ) -> Result<MarkdownDocs> {
-        let item_path = if let Some(item_path) = item_path {
-            format!("{package}::{item_path}")
-        } else {
-            package.to_string()
-        };
-
+    pub async fn load(&self, key: String) -> Result<MarkdownDocs> {
         self.database_future
             .clone()
             .await
             .map_err(|err| anyhow!(err))?
-            .load(item_path)
+            .load(key)
             .await
     }
 
@@ -160,10 +150,6 @@ impl IndexedDocsStore {
         let executor = self.executor.clone();
         let database_future = self.database_future.clone();
         self.executor.spawn(async move {
-            if query.is_empty() {
-                return Vec::new();
-            }
-
             let Some(database) = database_future.await.map_err(|err| anyhow!(err)).log_err() else {
                 return Vec::new();
             };