Fix some issues with branch buffers (#18945)

* `Open Excerpts` command always opens the locations in the base buffer
* LSP features like document-highlights, go-to-def, and inlay hints work
correctly in branch buffers
* Other LSP features like completions, code actions, and rename are
disabled in branch buffers

Release Notes:

- N/A

crates/editor/src/proposed_changes_editor.rs

@@ -1,4 +1,4 @@
-use crate::{Editor, EditorEvent};
+use crate::{Editor, EditorEvent, SemanticsProvider};
 use collections::HashSet;
 use futures::{channel::mpsc, future::join_all};
 use gpui::{AppContext, EventEmitter, FocusableView, Model, Render, Subscription, Task, View};
@@ -6,7 +6,7 @@ use language::{Buffer, BufferEvent, Capability};
 use multi_buffer::{ExcerptRange, MultiBuffer};
 use project::Project;
 use smol::stream::StreamExt;
-use std::{any::TypeId, ops::Range, time::Duration};
+use std::{any::TypeId, ops::Range, rc::Rc, time::Duration};
 use text::ToOffset;
 use ui::prelude::*;
 use workspace::{
@@ -35,6 +35,12 @@ struct RecalculateDiff {
     debounce: bool,
 }
 
+/// A provider of code semantics for branch buffers.
+///
+/// Requests in edited regions will return nothing, but requests in unchanged
+/// regions will be translated into the base buffer's coordinates.
+struct BranchBufferSemanticsProvider(Rc<dyn SemanticsProvider>);
+
 impl ProposedChangesEditor {
     pub fn new<T: ToOffset>(
         buffers: Vec<ProposedChangesBuffer<T>>,
@@ -66,6 +72,13 @@ impl ProposedChangesEditor {
             editor: cx.new_view(|cx| {
                 let mut editor = Editor::for_multibuffer(multibuffer.clone(), project, true, cx);
                 editor.set_expand_all_diff_hunks();
+                editor.set_completion_provider(None);
+                editor.clear_code_action_providers();
+                editor.set_semantics_provider(
+                    editor
+                        .semantics_provider()
+                        .map(|provider| Rc::new(BranchBufferSemanticsProvider(provider)) as _),
+                );
                 editor
             }),
             recalculate_diffs_tx,
@@ -76,7 +89,7 @@ impl ProposedChangesEditor {
 
                     while recalculate_diff.debounce {
                         cx.background_executor()
-                            .timer(Duration::from_millis(250))
+                            .timer(Duration::from_millis(50))
                             .await;
                         let mut had_further_changes = false;
                         while let Ok(next_recalculate_diff) = recalculate_diffs_rx.try_next() {
@@ -245,3 +258,103 @@ impl ToolbarItemView for ProposedChangesEditorToolbar {
         self.get_toolbar_item_location()
     }
 }
+
+impl BranchBufferSemanticsProvider {
+    fn to_base(
+        &self,
+        buffer: &Model<Buffer>,
+        positions: &[text::Anchor],
+        cx: &AppContext,
+    ) -> Option<Model<Buffer>> {
+        let base_buffer = buffer.read(cx).diff_base_buffer()?;
+        let version = base_buffer.read(cx).version();
+        if positions
+            .iter()
+            .any(|position| !version.observed(position.timestamp))
+        {
+            return None;
+        }
+        Some(base_buffer)
+    }
+}
+
+impl SemanticsProvider for BranchBufferSemanticsProvider {
+    fn hover(
+        &self,
+        buffer: &Model<Buffer>,
+        position: text::Anchor,
+        cx: &mut AppContext,
+    ) -> Option<Task<Vec<project::Hover>>> {
+        let buffer = self.to_base(buffer, &[position], cx)?;
+        self.0.hover(&buffer, position, cx)
+    }
+
+    fn inlay_hints(
+        &self,
+        buffer: Model<Buffer>,
+        range: Range<text::Anchor>,
+        cx: &mut AppContext,
+    ) -> Option<Task<anyhow::Result<Vec<project::InlayHint>>>> {
+        let buffer = self.to_base(&buffer, &[range.start, range.end], cx)?;
+        self.0.inlay_hints(buffer, range, cx)
+    }
+
+    fn resolve_inlay_hint(
+        &self,
+        hint: project::InlayHint,
+        buffer: Model<Buffer>,
+        server_id: lsp::LanguageServerId,
+        cx: &mut AppContext,
+    ) -> Option<Task<anyhow::Result<project::InlayHint>>> {
+        let buffer = self.to_base(&buffer, &[], cx)?;
+        self.0.resolve_inlay_hint(hint, buffer, server_id, cx)
+    }
+
+    fn supports_inlay_hints(&self, buffer: &Model<Buffer>, cx: &AppContext) -> bool {
+        if let Some(buffer) = self.to_base(&buffer, &[], cx) {
+            self.0.supports_inlay_hints(&buffer, cx)
+        } else {
+            false
+        }
+    }
+
+    fn document_highlights(
+        &self,
+        buffer: &Model<Buffer>,
+        position: text::Anchor,
+        cx: &mut AppContext,
+    ) -> Option<Task<gpui::Result<Vec<project::DocumentHighlight>>>> {
+        let buffer = self.to_base(&buffer, &[position], cx)?;
+        self.0.document_highlights(&buffer, position, cx)
+    }
+
+    fn definitions(
+        &self,
+        buffer: &Model<Buffer>,
+        position: text::Anchor,
+        kind: crate::GotoDefinitionKind,
+        cx: &mut AppContext,
+    ) -> Option<Task<gpui::Result<Vec<project::LocationLink>>>> {
+        let buffer = self.to_base(&buffer, &[position], cx)?;
+        self.0.definitions(&buffer, position, kind, cx)
+    }
+
+    fn range_for_rename(
+        &self,
+        _: &Model<Buffer>,
+        _: text::Anchor,
+        _: &mut AppContext,
+    ) -> Option<Task<gpui::Result<Option<Range<text::Anchor>>>>> {
+        None
+    }
+
+    fn perform_rename(
+        &self,
+        _: &Model<Buffer>,
+        _: text::Anchor,
+        _: String,
+        _: &mut AppContext,
+    ) -> Option<Task<gpui::Result<project::ProjectTransaction>>> {
+        None
+    }
+}