diff --git a/lib/ASTGen/Sources/ASTGen/LexicalLookup.swift b/lib/ASTGen/Sources/ASTGen/LexicalLookup.swift
index 7315ce6c1b4c8..405651f60362b 100644
--- a/lib/ASTGen/Sources/ASTGen/LexicalLookup.swift
+++ b/lib/ASTGen/Sources/ASTGen/LexicalLookup.swift
@@ -52,6 +52,15 @@ public func unqualifiedLookup(
   }
   let sourceLocationConverter = sourceFile.pointee.sourceLocationConverter
   let configuredRegions = sourceFile.pointee.configuredRegions(astContext: astContext)
+  
+  let lookupCache: LookupCache
+  
+  if let sourceFileCache = sourceFile.pointee.lookupCache {
+    lookupCache = sourceFileCache
+  } else {
+    lookupCache = LookupCache(drop: 4)
+    sourceFile.pointee.lookupCache = lookupCache
+  }
 
   guard let lookupPosition = sourceFile.pointee.position(of: lookupAt),
     let lookupToken = sourceFileSyntax.token(at: lookupPosition)
@@ -70,7 +79,8 @@ public func unqualifiedLookup(
   let sllResults = sllConsumedResults(
     lookupToken: lookupToken,
     finishInSequentialScope: finishInSequentialScope,
-    configuredRegions: configuredRegions
+    configuredRegions: configuredRegions,
+    cache: lookupCache
   )
 
   // Add header to the output
@@ -196,11 +206,13 @@ private func astConsumedResults(
 private func sllConsumedResults(
   lookupToken: TokenSyntax,
   finishInSequentialScope: Bool,
-  configuredRegions: ConfiguredRegions
+  configuredRegions: ConfiguredRegions,
+  cache: LookupCache
 ) -> [ConsumedLookupResult] {
   let resultsWithoutMacroReordering = lookupToken.lookup(
     nil,
-    with: LookupConfig(finishInSequentialScope: finishInSequentialScope, configuredRegions: configuredRegions)
+    with: LookupConfig(finishInSequentialScope: finishInSequentialScope, configuredRegions: configuredRegions),
+    cache: cache
   )
 
   // Early reordering of macro declaration parameters with its generic parameters.
@@ -413,7 +425,7 @@ private func flaggingPass(
       }
 
       // Check if next results at this position should be ignored. If so, set ignoreAt and omit this name.
-      if sllResult.ignoreNextFromHere && sllResult.position != ignoreAt {
+      if sllResult.ignoreNextFromHere {
         ignoreAt = sllResult.position
         sllResult.flags.insert(.shouldBeOmitted)
       }
diff --git a/lib/ASTGen/Sources/ASTGen/SourceFile.swift b/lib/ASTGen/Sources/ASTGen/SourceFile.swift
index d7692a4641607..4d3d4dd6cf0d3 100644
--- a/lib/ASTGen/Sources/ASTGen/SourceFile.swift
+++ b/lib/ASTGen/Sources/ASTGen/SourceFile.swift
@@ -16,6 +16,7 @@ import SwiftIfConfig
 @_spi(ExperimentalLanguageFeatures) import SwiftParser
 import SwiftParserDiagnostics
 @_spi(Compiler) import SwiftSyntax
+@_spi(Experimental) import SwiftLexicalLookup
 
 /// Describes a source file that has been "exported" to the C++ part of the
 /// compiler, with enough information to interface with the C++ layer.
@@ -42,6 +43,12 @@ public struct ExportedSourceFile {
   ///
   /// This is a cached value; access via configuredRegions(astContext:).
   var _configuredRegions: ConfiguredRegions? = nil
+  
+  /// Cache used by unqualified lookup
+  ///
+  /// Cached so we don't need to re-evaluate common parts
+  /// of the scope tree during successive lookups.
+  var lookupCache: LookupCache? = nil
 
   public func position(of location: BridgedSourceLoc) -> AbsolutePosition? {
     let sourceFileBaseAddress = UnsafeRawPointer(buffer.baseAddress!)