diff --git a/nullability/inference/collect_evidence.cc b/nullability/inference/collect_evidence.cc
index 8ddc113..f2964cf 100644
--- a/nullability/inference/collect_evidence.cc
+++ b/nullability/inference/collect_evidence.cc
@@ -7,6 +7,7 @@
 #include <memory>
 #include <optional>
 #include <string>
+#include <string_view>
 #include <utility>
 #include <vector>
 
@@ -40,7 +41,6 @@
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/Specifiers.h"
 #include "clang/Index/USRGeneration.h"
-#include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/FunctionExtras.h"
 #include "llvm/ADT/STLFunctionalExtras.h"
@@ -51,13 +51,24 @@
 using ::clang::dataflow::DataflowAnalysisContext;
 using ::clang::dataflow::Environment;
 
+std::string_view getOrGenerateUSR(USRCache &Cache, const Decl &Decl) {
+  auto [It, Inserted] = Cache.try_emplace(&Decl);
+  if (Inserted) {
+    llvm::SmallString<128> USR;
+    if (!index::generateUSRForDecl(&Decl, USR)) It->second = USR.str();
+  }
+  return It->second;
+}
+
 llvm::unique_function<EvidenceEmitter> evidenceEmitter(
-    llvm::unique_function<void(const Evidence &) const> Emit) {
+    llvm::unique_function<void(const Evidence &) const> Emit,
+    nullability::USRCache &USRCache) {
   class EvidenceEmitterImpl {
    public:
     EvidenceEmitterImpl(
-        llvm::unique_function<void(const Evidence &) const> Emit)
-        : Emit(std::move(Emit)) {}
+        llvm::unique_function<void(const Evidence &) const> Emit,
+        nullability::USRCache &USRCache)
+        : Emit(std::move(Emit)), USRCache(USRCache) {}
 
     void operator()(const Decl &Target, Slot S, Evidence::Kind Kind,
                     SourceLocation Loc) const {
@@ -68,13 +79,9 @@
       E.set_slot(S);
       E.set_kind(Kind);
 
-      auto [It, Inserted] = USRCache.try_emplace(&Target);
-      if (Inserted) {
-        llvm::SmallString<128> USR;
-        if (!index::generateUSRForDecl(&Target, USR)) It->second = USR.str();
-      }
-      if (It->second.empty()) return;  // Can't emit without a USR
-      E.mutable_symbol()->set_usr(It->second);
+      std::string_view USR = getOrGenerateUSR(USRCache, Target);
+      if (USR.empty()) return;  // Can't emit without a USR
+      E.mutable_symbol()->set_usr(USR);
 
       // TODO: make collecting and propagating location information optional?
       auto &SM =
@@ -88,10 +95,10 @@
     }
 
    private:
-    mutable llvm::DenseMap<const Decl *, std::string> USRCache;
     llvm::unique_function<void(const Evidence &) const> Emit;
+    nullability::USRCache &USRCache;
   };
-  return EvidenceEmitterImpl(std::move(Emit));
+  return EvidenceEmitterImpl(std::move(Emit), USRCache);
 }
 
 namespace {
@@ -336,6 +343,7 @@
 
 llvm::Error collectEvidenceFromImplementation(
     const Decl &Decl, llvm::function_ref<EvidenceEmitter> Emit,
+    USRCache &USRCache,
     const llvm::DenseSet<SlotFingerprint> &PreviouslyInferredNullable,
     const llvm::DenseSet<SlotFingerprint> &PreviouslyInferredNonnull) {
   const FunctionDecl *Func = dyn_cast<FunctionDecl>(&Decl);
diff --git a/nullability/inference/collect_evidence.h b/nullability/inference/collect_evidence.h
index c516359..97a8578 100644
--- a/nullability/inference/collect_evidence.h
+++ b/nullability/inference/collect_evidence.h
@@ -5,12 +5,15 @@
 #ifndef CRUBIT_NULLABILITY_INFERENCE_COLLECT_EVIDENCE_H_
 #define CRUBIT_NULLABILITY_INFERENCE_COLLECT_EVIDENCE_H_
 
+#include <string>
+#include <string_view>
 #include <vector>
 
 #include "nullability/inference/inference.proto.h"
 #include "nullability/inference/slot_fingerprint.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/Basic/SourceLocation.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/FunctionExtras.h"
 #include "llvm/ADT/STLFunctionalExtras.h"
@@ -18,13 +21,17 @@
 
 namespace clang::tidy::nullability {
 
+using USRCache = llvm::DenseMap<const Decl *, std::string>;
+
+std::string_view getOrGenerateUSR(USRCache &Cache, const Decl &);
+
 // Callback used to report collected nullability evidence.
 using EvidenceEmitter = void(const Decl &Target, Slot, Evidence::Kind,
                              SourceLocation);
 // Creates an EvidenceEmitter that serializes the evidence as Evidence protos.
 // This emitter caches USR generation, and should be reused for the whole AST.
 llvm::unique_function<EvidenceEmitter> evidenceEmitter(
-    llvm::unique_function<void(const Evidence &) const>);
+    llvm::unique_function<void(const Evidence &) const>, USRCache &USRCache);
 
 // Analyze code (such as a function body) to infer nullability.
 //
@@ -35,7 +42,7 @@
 // It is up to the caller to ensure the implementation is eligible for inference
 // (function has a body, is not dependent, etc).
 llvm::Error collectEvidenceFromImplementation(
-    const Decl &, llvm::function_ref<EvidenceEmitter>,
+    const Decl &, llvm::function_ref<EvidenceEmitter>, USRCache &USRCache,
     const llvm::DenseSet<SlotFingerprint> &PreviouslyInferredNullable = {},
     const llvm::DenseSet<SlotFingerprint> &PreviouslyInferredNonnull = {});
 
diff --git a/nullability/inference/collect_evidence_test.cc b/nullability/inference/collect_evidence_test.cc
index 5202fa7..5b08e1d 100644
--- a/nullability/inference/collect_evidence_test.cc
+++ b/nullability/inference/collect_evidence_test.cc
@@ -69,10 +69,13 @@
     llvm::StringRef Source) {
   std::vector<Evidence> Results;
   clang::TestAST AST(getInputsWithAnnotationDefinitions(Source));
+  USRCache usr_cache;
   auto Err = collectEvidenceFromImplementation(
       cast<FunctionDecl>(
           *dataflow::test::findValueDecl(AST.context(), "target")),
-      evidenceEmitter([&](const Evidence& E) { Results.push_back(E); }));
+      evidenceEmitter([&](const Evidence& E) { Results.push_back(E); },
+                      usr_cache),
+      usr_cache);
   if (Err) ADD_FAILURE() << toString(std::move(Err));
   return Results;
 }
@@ -80,9 +83,11 @@
 std::vector<Evidence> collectEvidenceFromTargetDecl(llvm::StringRef Source) {
   std::vector<Evidence> Results;
   clang::TestAST AST(getInputsWithAnnotationDefinitions(Source));
+  USRCache usr_cache;
   collectEvidenceFromTargetDeclaration(
       *dataflow::test::findValueDecl(AST.context(), "target"),
-      evidenceEmitter([&](const Evidence& E) { Results.push_back(E); }));
+      evidenceEmitter([&](const Evidence& E) { Results.push_back(E); },
+                      usr_cache));
   return Results;
 }
 
@@ -472,10 +477,13 @@
       "target", TargetInstantiationNodes);
   ASSERT_NE(InstantiationDecl, nullptr);
 
+  USRCache usr_cache;
   std::vector<Evidence> Results;
   auto Err = collectEvidenceFromImplementation(
       *InstantiationDecl,
-      evidenceEmitter([&](const Evidence& E) { Results.push_back(E); }));
+      evidenceEmitter([&](const Evidence& E) { Results.push_back(E); },
+                      usr_cache),
+      usr_cache);
   if (Err) ADD_FAILURE() << toString(std::move(Err));
   EXPECT_THAT(Results, IsEmpty());
 }
@@ -610,7 +618,8 @@
       dataflow::test::findValueDecl(AST.context(), "target");
   ASSERT_NE(TargetDecl, nullptr);
 
-  EXPECT_DEATH(evidenceEmitter([](const Evidence& e) {})(
+  USRCache usr_cache;
+  EXPECT_DEATH(evidenceEmitter([](const Evidence& e) {}, usr_cache)(
                    *TargetDecl, Slot{}, Evidence::ANNOTATED_UNKNOWN,
                    TargetDecl->getLocation()),
                "not an inference target");
diff --git a/nullability/inference/infer_tu.cc b/nullability/inference/infer_tu.cc
index cd783b2..2f9d616 100644
--- a/nullability/inference/infer_tu.cc
+++ b/nullability/inference/infer_tu.cc
@@ -11,6 +11,7 @@
 #include "nullability/inference/inference.proto.h"
 #include "nullability/inference/merge.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/DeclBase.h"
 #include "clang/Basic/SourceManager.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/STLExtras.h"
@@ -35,14 +36,17 @@
 
   // Collect all evidence.
   auto Sites = EvidenceSites::discover(Ctx);
-  auto Emitter = evidenceEmitter([&](auto& E) { AllEvidence.push_back(E); });
+  USRCache USRCache;
+  auto Emitter =
+      evidenceEmitter([&](auto& E) { AllEvidence.push_back(E); }, USRCache);
   for (const auto* Decl : Sites.Declarations) {
     if (Filter && !Filter(*Decl)) continue;
     collectEvidenceFromTargetDeclaration(*Decl, Emitter);
   }
   for (const auto* Impl : Sites.Implementations) {
     if (Filter && !Filter(*Impl)) continue;
-    if (auto Err = collectEvidenceFromImplementation(*Impl, Emitter)) {
+    if (auto Err =
+            collectEvidenceFromImplementation(*Impl, Emitter, USRCache)) {
       llvm::errs() << "Skipping function: " << toString(std::move(Err)) << "\n";
       Impl->print(llvm::errs());
     }