Improve slash commands (#16195)

This PR:

- Makes slash commands easier to compose by adding a concept,
`CompletionIntent`. When using `tab` on a completion in the assistant
panel, that completion item will be expanded but the associated command
will not be run. Using `enter` will still either run the completion item
or continue command composition as before.
- Fixes a bug where running `/diagnostics` on a project with no
diagnostics will delete the entire command, rather than rendering an
empty header.
- Improves the autocomplete rendering for files, showing when
directories are selected and re-arranging the results to have the file
name or trailing directory show first.

<img width="642" alt="Screenshot 2024-08-13 at 8 12 43 PM"
src="https://github.com/user-attachments/assets/97c96cd2-741f-4f15-ad03-7cf78129a71c">


Release Notes:

- N/A

crates/editor/src/actions.rs

@@ -64,6 +64,12 @@ pub struct ConfirmCompletion {
     pub item_ix: Option<usize>,
 }
 
+#[derive(PartialEq, Clone, Deserialize, Default)]
+pub struct ComposeCompletion {
+    #[serde(default)]
+    pub item_ix: Option<usize>,
+}
+
 #[derive(PartialEq, Clone, Deserialize, Default)]
 pub struct ConfirmCodeAction {
     #[serde(default)]
@@ -140,6 +146,7 @@ impl_actions!(
     [
         ConfirmCodeAction,
         ConfirmCompletion,
+        ComposeCompletion,
         ExpandExcerpts,
         ExpandExcerptsUp,
         ExpandExcerptsDown,

crates/editor/src/editor.rs

@@ -114,7 +114,7 @@ use ordered_float::OrderedFloat;
 use parking_lot::{Mutex, RwLock};
 use project::project_settings::{GitGutterSetting, ProjectSettings};
 use project::{
-    CodeAction, Completion, FormatTrigger, Item, Location, Project, ProjectPath,
+    CodeAction, Completion, CompletionIntent, FormatTrigger, Item, Location, Project, ProjectPath,
     ProjectTransaction, TaskSourceKind, WorktreeId,
 };
 use rand::prelude::*;
@@ -4213,6 +4213,23 @@ impl Editor {
         action: &ConfirmCompletion,
         cx: &mut ViewContext<Self>,
     ) -> Option<Task<Result<()>>> {
+        self.do_completion(action.item_ix, CompletionIntent::Complete, cx)
+    }
+
+    pub fn compose_completion(
+        &mut self,
+        action: &ComposeCompletion,
+        cx: &mut ViewContext<Self>,
+    ) -> Option<Task<Result<()>>> {
+        self.do_completion(action.item_ix, CompletionIntent::Compose, cx)
+    }
+
+    fn do_completion(
+        &mut self,
+        item_ix: Option<usize>,
+        intent: CompletionIntent,
+        cx: &mut ViewContext<Editor>,
+    ) -> Option<Task<std::result::Result<(), anyhow::Error>>> {
         use language::ToOffset as _;
 
         let completions_menu = if let ContextMenu::Completions(menu) = self.hide_context_menu(cx)? {
@@ -4223,7 +4240,7 @@ impl Editor {
 
         let mat = completions_menu
             .matches
-            .get(action.item_ix.unwrap_or(completions_menu.selected_item))?;
+            .get(item_ix.unwrap_or(completions_menu.selected_item))?;
         let buffer_handle = completions_menu.buffer;
         let completions = completions_menu.completions.read();
         let completion = completions.get(mat.candidate_id)?;
@@ -4358,7 +4375,7 @@ impl Editor {
         });
 
         if let Some(confirm) = completion.confirm.as_ref() {
-            (confirm)(cx);
+            (confirm)(intent, cx);
         }
 
         if completion.show_new_completions_on_confirm {

crates/editor/src/element.rs

@@ -372,6 +372,13 @@ impl EditorElement {
                 cx.propagate();
             }
         });
+        register_action(view, cx, |editor, action, cx| {
+            if let Some(task) = editor.compose_completion(action, cx) {
+                task.detach_and_log_err(cx);
+            } else {
+                cx.propagate();
+            }
+        });
         register_action(view, cx, |editor, action, cx| {
             if let Some(task) = editor.confirm_code_action(action, cx) {
                 task.detach_and_log_err(cx);