Support word-based completions (#26410)

Closes https://github.com/zed-industries/zed/issues/4957


https://github.com/user-attachments/assets/ff491378-376d-48ec-b552-6cc80f74200b

Adds `"completions"` language settings section, to configure LSP and
word completions per language.
Word-based completions may be turned on never, always (returned along
with the LSP ones), and as a fallback if no LSP completion items were
returned.

Future work:

* words are matched with the same fuzzy matching code that the rest of
the completions are

This might worsen the completion menu's usability even more, and will
require work on better completion sorting.

* completion entries currently have no icons or other ways to indicate
those are coming from LSP or from word search, or from something else

* we may work with language scopes more intelligently, group words by
them and distinguish during completions

Release Notes:

- Supported word-based completions

---------

Co-authored-by: Max Brunsfeld <max@zed.dev>

crates/editor/src/editor.rs

@@ -101,6 +101,7 @@ use itertools::Itertools;
 use language::{
     language_settings::{
         self, all_language_settings, language_settings, InlayHintSettings, RewrapBehavior,
+        WordsCompletionMode,
     },
     point_from_lsp, text_diff_with_options, AutoindentMode, BracketMatch, BracketPair, Buffer,
     Capability, CharKind, CodeLabel, CursorShape, Diagnostic, DiffOptions, EditPredictionsMode,
@@ -4012,9 +4013,8 @@ impl Editor {
             } else {
                 return;
             };
-        let show_completion_documentation = buffer
-            .read(cx)
-            .snapshot()
+        let buffer_snapshot = buffer.read(cx).snapshot();
+        let show_completion_documentation = buffer_snapshot
             .settings_at(buffer_position, cx)
             .show_completion_documentation;
 
@@ -4038,6 +4038,51 @@ impl Editor {
         };
         let completions =
             provider.completions(&buffer, buffer_position, completion_context, window, cx);
+        let (old_range, word_kind) = buffer_snapshot.surrounding_word(buffer_position);
+        let (old_range, word_to_exclude) = if word_kind == Some(CharKind::Word) {
+            let word_to_exclude = buffer_snapshot
+                .text_for_range(old_range.clone())
+                .collect::<String>();
+            (
+                buffer_snapshot.anchor_before(old_range.start)
+                    ..buffer_snapshot.anchor_after(old_range.end),
+                Some(word_to_exclude),
+            )
+        } else {
+            (buffer_position..buffer_position, None)
+        };
+
+        let completion_settings = language_settings(
+            buffer_snapshot
+                .language_at(buffer_position)
+                .map(|language| language.name()),
+            buffer_snapshot.file(),
+            cx,
+        )
+        .completions;
+
+        // The document can be large, so stay in reasonable bounds when searching for words,
+        // otherwise completion pop-up might be slow to appear.
+        const WORD_LOOKUP_ROWS: u32 = 5_000;
+        let buffer_row = text::ToPoint::to_point(&buffer_position, &buffer_snapshot).row;
+        let min_word_search = buffer_snapshot.clip_point(
+            Point::new(buffer_row.saturating_sub(WORD_LOOKUP_ROWS), 0),
+            Bias::Left,
+        );
+        let max_word_search = buffer_snapshot.clip_point(
+            Point::new(buffer_row + WORD_LOOKUP_ROWS, 0).min(buffer_snapshot.max_point()),
+            Bias::Right,
+        );
+        let word_search_range = buffer_snapshot.point_to_offset(min_word_search)
+            ..buffer_snapshot.point_to_offset(max_word_search);
+        let words = match completion_settings.words {
+            WordsCompletionMode::Disabled => Task::ready(HashMap::default()),
+            WordsCompletionMode::Enabled | WordsCompletionMode::Fallback => {
+                cx.background_spawn(async move {
+                    buffer_snapshot.words_in_range(None, word_search_range)
+                })
+            }
+        };
         let sort_completions = provider.sort_completions();
 
         let id = post_inc(&mut self.next_completion_id);
@@ -4046,8 +4091,55 @@ impl Editor {
                 editor.update(&mut cx, |this, _| {
                     this.completion_tasks.retain(|(task_id, _)| *task_id >= id);
                 })?;
-                let completions = completions.await.log_err();
-                let menu = if let Some(completions) = completions {
+                let mut completions = completions.await.log_err().unwrap_or_default();
+
+                match completion_settings.words {
+                    WordsCompletionMode::Enabled => {
+                        completions.extend(
+                            words
+                                .await
+                                .into_iter()
+                                .filter(|(word, _)| word_to_exclude.as_ref() != Some(word))
+                                .map(|(word, word_range)| Completion {
+                                    old_range: old_range.clone(),
+                                    new_text: word.clone(),
+                                    label: CodeLabel::plain(word, None),
+                                    documentation: None,
+                                    source: CompletionSource::BufferWord {
+                                        word_range,
+                                        resolved: false,
+                                    },
+                                    confirm: None,
+                                }),
+                        );
+                    }
+                    WordsCompletionMode::Fallback => {
+                        if completions.is_empty() {
+                            completions.extend(
+                                words
+                                    .await
+                                    .into_iter()
+                                    .filter(|(word, _)| word_to_exclude.as_ref() != Some(word))
+                                    .map(|(word, word_range)| Completion {
+                                        old_range: old_range.clone(),
+                                        new_text: word.clone(),
+                                        label: CodeLabel::plain(word, None),
+                                        documentation: None,
+                                        source: CompletionSource::BufferWord {
+                                            word_range,
+                                            resolved: false,
+                                        },
+                                        confirm: None,
+                                    }),
+                            );
+                        }
+                    }
+                    WordsCompletionMode::Disabled => {}
+                }
+
+                let menu = if completions.is_empty() {
+                    None
+                } else {
                     let mut menu = CompletionsMenu::new(
                         id,
                         sort_completions,
@@ -4061,8 +4153,6 @@ impl Editor {
                         .await;
 
                     menu.visible().then_some(menu)
-                } else {
-                    None
                 };
 
                 editor.update_in(&mut cx, |editor, window, cx| {