feat(ui): deprecated completions (#5932)

* feat(ui): deprecated completions

Mark deprecated completions using strike-through
(CROSSED_OUT modifier). The deprection information
is taken either from the `deprecated` field of the
completion item or from the completion tags.

The field seems to be the older way of passing
the deprecated information and it was already
marked as deprecated for Symbol. In completion
item the field is still valid but it seems that
the LSP is moving in the general direction of using
tags for this kind of information and as such
relying on tags as well seems reasonable and
future-proof.

helix-lsp/src/client.rs

@@ -329,6 +329,10 @@ impl Client {
                                 ],
                             }),
                             insert_replace_support: Some(true),
+                            deprecated_support: Some(true),
+                            tag_support: Some(lsp::TagSupport {
+                                value_set: vec![lsp::CompletionItemTag::DEPRECATED],
+                            }),
                             ..Default::default()
                         }),
                         completion_item_kind: Some(lsp::CompletionItemKindCapability {

helix-term/src/ui/completion.rs

@@ -1,6 +1,10 @@
 use crate::compositor::{Component, Context, Event, EventResult};
-use helix_view::{editor::CompleteAction, ViewId};
+use helix_view::{
-use tui::buffer::Buffer as Surface;
+    editor::CompleteAction,
+    theme::{Modifier, Style},
+    ViewId,
+};
+use tui::{buffer::Buffer as Surface, text::Span};
 
 use std::borrow::Cow;
 
@@ -33,8 +37,19 @@ impl menu::Item for CompletionItem {
     }
 
     fn format(&self, _data: &Self::Data) -> menu::Row {
+        let deprecated = self.deprecated.unwrap_or_default()
+            || self.tags.as_ref().map_or(false, |tags| {
+                tags.contains(&lsp::CompletionItemTag::DEPRECATED)
+            });
         menu::Row::new(vec![
-            menu::Cell::from(self.label.as_str()),
+            menu::Cell::from(Span::styled(
+                self.label.as_str(),
+                if deprecated {
+                    Style::default().add_modifier(Modifier::CROSSED_OUT)
+                } else {
+                    Style::default()
+                },
+            )),
             menu::Cell::from(match self.kind {
                 Some(lsp::CompletionItemKind::TEXT) => "text",
                 Some(lsp::CompletionItemKind::METHOD) => "method",