Fix SourceEngine cache to refresh buffers after file edits

Author: JoshuaBatty

sway-types/src/source_engine.rs

@@ -64,13 +64,26 @@ impl SourceEngine {
             .contains("<autogenerated>")
     }
 
+    /// Fetch the cached source buffer for `source_id`, replacing it if the caller
+    /// provides newer text so future span lookups observe the latest file contents.
     pub fn get_or_create_source_buffer(
         &self,
         source_id: &SourceId,
         source: span::Source,
     ) -> span::Source {
         let mut map = self.source_to_buffer_map.write();
-        map.entry(*source_id).or_insert(source).clone()
+
+        if let Some(existing) = map.get_mut(source_id) {
+            // Replace the cached source if the contents have changed so that
+            // subsequent spans reflect the latest file text.
+            if existing.text != source.text {
+                *existing = source;
+            }
+            existing.clone()
+        } else {
+            map.insert(*source_id, source.clone());
+            source
+        }
     }
 
     /// This function retrieves an integer-based source ID for a provided path buffer.