lsp: Resolve completion items missing documentation on idle (#4406)

Some language servers may not send the `documentation` field if it
is expensive to compute. Clients can request the missing field with
a completionItem/resolve request.

In this change we use the idle-timeout event to ensure that the current
completion item is resolved.

3 files changed, 38 insertions, 4 deletions

diff --git a/helix-term/src/ui/completion.rs b/helix-term/src/ui/completion.rs
index 7348dcf4..a21767f9 100644
--- a/helix-term/src/ui/completion.rs
+++ b/helix-term/src/ui/completion.rs
@@ -295,6 +295,27 @@ impl Completion {
     pub fn is_empty(&self) -> bool {
         self.popup.contents().is_empty()
     }
+
+    pub fn ensure_item_resolved(&mut self, cx: &mut commands::Context) -> bool {
+        // > If computing full completion items is expensive, servers can additionally provide a
+        // > handler for the completion item resolve request. ...
+        // > A typical use case is for example: the `textDocument/completion` request doesn't fill
+        // > in the `documentation` property for returned completion items since it is expensive
+        // > to compute. When the item is selected in the user interface then a
+        // > 'completionItem/resolve' request is sent with the selected completion item as a parameter.
+        // > The returned completion item should have the documentation property filled in.
+        // https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_completion
+        match self.popup.contents_mut().selection_mut() {
+            Some(item) if item.documentation.is_none() => {
+                let doc = doc!(cx.editor);
+                if let Some(resolved_item) = Self::resolve_completion_item(doc, item.clone()) {
+                    *item = resolved_item;
+                }
+                true
+            }
+            _ => false,
+        }
+    }
 }
 
 impl Component for Completion {
diff --git a/helix-term/src/ui/editor.rs b/helix-term/src/ui/editor.rs
index 43b1e7a0..b1bb02c7 100644
--- a/helix-term/src/ui/editor.rs
+++ b/helix-term/src/ui/editor.rs
@@ -1097,10 +1097,15 @@ impl EditorView {
     }
 
     pub fn handle_idle_timeout(&mut self, cx: &mut commands::Context) -> EventResult {
-        if self.completion.is_some()
-            || cx.editor.mode != Mode::Insert
-            || !cx.editor.config().auto_completion
-        {
+        if let Some(completion) = &mut self.completion {
+            return if completion.ensure_item_resolved(cx) {
+                EventResult::Consumed(None)
+            } else {
+                EventResult::Ignored(None)
+            };
+        }
+
+        if cx.editor.mode != Mode::Insert || !cx.editor.config().auto_completion {
             return EventResult::Ignored(None);
         }
 
diff --git a/helix-term/src/ui/menu.rs b/helix-term/src/ui/menu.rs
index 4b1155e3..75769b90 100644
--- a/helix-term/src/ui/menu.rs
+++ b/helix-term/src/ui/menu.rs
@@ -206,6 +206,14 @@ impl<T: Item> Menu<T> {
         })
     }
 
+    pub fn selection_mut(&mut self) -> Option<&mut T> {
+        self.cursor.and_then(|cursor| {
+            self.matches
+                .get(cursor)
+                .map(|(index, _score)| &mut self.options[*index])
+        })
+    }
+
     pub fn is_empty(&self) -> bool {
         self.matches.is_empty()
     }