language: Do not fetch diagnostics when iterating over text without language awareness (#16824)

This PR fixes a regression from
https://github.com/zed-industries/zed/pull/15646 where we've started
fetching diagnostic spans unconditionally (whereas previously that
wasn't done when iterating over raw text).

Closes #16764

Release Notes:

- Fixed performance regression in handling buffers with large quantities
of diagnostics.

crates/language/src/buffer.rs

@@ -452,7 +452,7 @@ pub struct BufferChunks<'a> {
     buffer_snapshot: Option<&'a BufferSnapshot>,
     range: Range<usize>,
     chunks: text::Chunks<'a>,
-    diagnostic_endpoints: Peekable<vec::IntoIter<DiagnosticEndpoint>>,
+    diagnostic_endpoints: Option<Peekable<vec::IntoIter<DiagnosticEndpoint>>>,
     error_depth: usize,
     warning_depth: usize,
     information_depth: usize,
@@ -2578,8 +2578,9 @@ impl BufferSnapshot {
         if language_aware {
             syntax = Some(self.get_highlights(range.clone()));
         }
-
-        BufferChunks::new(self.text.as_rope(), range, syntax, Some(self))
+        // We want to look at diagnostic spans only when iterating over language-annotated chunks.
+        let diagnostics = language_aware;
+        BufferChunks::new(self.text.as_rope(), range, syntax, diagnostics, Some(self))
     }
 
     /// Invokes the given callback for each line of text in the given range of the buffer.
@@ -3798,6 +3799,7 @@ impl<'a> BufferChunks<'a> {
         text: &'a Rope,
         range: Range<usize>,
         syntax: Option<(SyntaxMapCaptures<'a>, Vec<HighlightMap>)>,
+        diagnostics: bool,
         buffer_snapshot: Option<&'a BufferSnapshot>,
     ) -> Self {
         let mut highlights = None;
@@ -3810,7 +3812,7 @@ impl<'a> BufferChunks<'a> {
             })
         }
 
-        let diagnostic_endpoints = Vec::new().into_iter().peekable();
+        let diagnostic_endpoints = diagnostics.then(|| Vec::new().into_iter().peekable());
         let chunks = text.chunks_in_range(range.clone());
 
         let mut this = BufferChunks {
@@ -3871,25 +3873,27 @@ impl<'a> BufferChunks<'a> {
     }
 
     fn initialize_diagnostic_endpoints(&mut self) {
-        if let Some(buffer) = self.buffer_snapshot {
-            let mut diagnostic_endpoints = Vec::new();
-            for entry in buffer.diagnostics_in_range::<_, usize>(self.range.clone(), false) {
-                diagnostic_endpoints.push(DiagnosticEndpoint {
-                    offset: entry.range.start,
-                    is_start: true,
-                    severity: entry.diagnostic.severity,
-                    is_unnecessary: entry.diagnostic.is_unnecessary,
-                });
-                diagnostic_endpoints.push(DiagnosticEndpoint {
-                    offset: entry.range.end,
-                    is_start: false,
-                    severity: entry.diagnostic.severity,
-                    is_unnecessary: entry.diagnostic.is_unnecessary,
-                });
+        if let Some(diagnostics) = self.diagnostic_endpoints.as_mut() {
+            if let Some(buffer) = self.buffer_snapshot {
+                let mut diagnostic_endpoints = Vec::new();
+                for entry in buffer.diagnostics_in_range::<_, usize>(self.range.clone(), false) {
+                    diagnostic_endpoints.push(DiagnosticEndpoint {
+                        offset: entry.range.start,
+                        is_start: true,
+                        severity: entry.diagnostic.severity,
+                        is_unnecessary: entry.diagnostic.is_unnecessary,
+                    });
+                    diagnostic_endpoints.push(DiagnosticEndpoint {
+                        offset: entry.range.end,
+                        is_start: false,
+                        severity: entry.diagnostic.severity,
+                        is_unnecessary: entry.diagnostic.is_unnecessary,
+                    });
+                }
+                diagnostic_endpoints
+                    .sort_unstable_by_key(|endpoint| (endpoint.offset, !endpoint.is_start));
+                *diagnostics = diagnostic_endpoints.into_iter().peekable();
             }
-            diagnostic_endpoints
-                .sort_unstable_by_key(|endpoint| (endpoint.offset, !endpoint.is_start));
-            self.diagnostic_endpoints = diagnostic_endpoints.into_iter().peekable();
         }
     }
 
@@ -3975,15 +3979,19 @@ impl<'a> Iterator for BufferChunks<'a> {
             }
         }
 
-        while let Some(endpoint) = self.diagnostic_endpoints.peek().copied() {
-            if endpoint.offset <= self.range.start {
-                self.update_diagnostic_depths(endpoint);
-                self.diagnostic_endpoints.next();
-            } else {
-                next_diagnostic_endpoint = endpoint.offset;
-                break;
+        let mut diagnostic_endpoints = std::mem::take(&mut self.diagnostic_endpoints);
+        if let Some(diagnostic_endpoints) = diagnostic_endpoints.as_mut() {
+            while let Some(endpoint) = diagnostic_endpoints.peek().copied() {
+                if endpoint.offset <= self.range.start {
+                    self.update_diagnostic_depths(endpoint);
+                    diagnostic_endpoints.next();
+                } else {
+                    next_diagnostic_endpoint = endpoint.offset;
+                    break;
+                }
             }
         }
+        self.diagnostic_endpoints = diagnostic_endpoints;
 
         if let Some(chunk) = self.chunks.peek() {
             let chunk_start = self.range.start;

crates/language/src/language.rs

@@ -1358,7 +1358,9 @@ impl Language {
                 });
             let highlight_maps = vec![grammar.highlight_map()];
             let mut offset = 0;
-            for chunk in BufferChunks::new(text, range, Some((captures, highlight_maps)), None) {
+            for chunk in
+                BufferChunks::new(text, range, Some((captures, highlight_maps)), false, None)
+            {
                 let end_offset = offset + chunk.text.len();
                 if let Some(highlight_id) = chunk.syntax_highlight_id {
                     if !highlight_id.is_default() {