Index: clangd/global-symbol-builder/GlobalSymbolBuilderMain.cpp
===================================================================
--- clangd/global-symbol-builder/GlobalSymbolBuilderMain.cpp
+++ clangd/global-symbol-builder/GlobalSymbolBuilderMain.cpp
@@ -17,8 +17,8 @@
 #include "index/Merge.h"
 #include "index/SymbolCollector.h"
 #include "index/SymbolYAML.h"
-#include "clang/Frontend/FrontendActions.h"
 #include "clang/Frontend/CompilerInstance.h"
+#include "clang/Frontend/FrontendActions.h"
 #include "clang/Index/IndexDataConsumer.h"
 #include "clang/Index/IndexingAction.h"
 #include "clang/Tooling/CommonOptionsParser.h"
@@ -29,6 +29,7 @@
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/ThreadPool.h"
+#include "llvm/Support/YAMLTraits.h"
 
 using namespace llvm;
 using namespace clang::tooling;
@@ -101,7 +102,8 @@
   Symbol::Details Scratch;
   Results->forEachResult([&](llvm::StringRef Key, llvm::StringRef Value) {
     Arena.Reset();
-    auto Sym = clang::clangd::SymbolFromYAML(Value, Arena);
+    llvm::yaml::Input Yin(Value, &Arena);
+    auto Sym = clang::clangd::SymbolFromYAML(Yin, Arena);
     clang::clangd::SymbolID ID;
     Key >> ID;
     if (const auto *Existing = UniqueSymbols.find(ID))
Index: clangd/index/SymbolYAML.h
===================================================================
--- clangd/index/SymbolYAML.h
+++ clangd/index/SymbolYAML.h
@@ -20,6 +20,7 @@
 
 #include "Index.h"
 #include "llvm/Support/Error.h"
+#include "llvm/Support/YAMLTraits.h"
 #include "llvm/Support/raw_ostream.h"
 
 namespace clang {
@@ -28,9 +29,10 @@
 // Read symbols from a YAML-format string.
 SymbolSlab SymbolsFromYAML(llvm::StringRef YAMLContent);
 
-// Read one symbol from a YAML-format string, backed by the arena.
-Symbol SymbolFromYAML(llvm::StringRef YAMLContent,
-                      llvm::BumpPtrAllocator &Arena);
+// Read one symbol from a YAML-stream.
+// The arena must be the Input's context! (i.e. yaml::Input Input(Text, &Arena))
+// The returned symbol is backed by both Input and Arena.
+Symbol SymbolFromYAML(llvm::yaml::Input &Input, llvm::BumpPtrAllocator &Arena);
 
 // Convert a single symbol to YAML-format string.
 // The YAML result is safe to concatenate.
Index: clangd/index/SymbolYAML.cpp
===================================================================
--- clangd/index/SymbolYAML.cpp
+++ clangd/index/SymbolYAML.cpp
@@ -12,7 +12,6 @@
 #include "llvm/ADT/Optional.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/MemoryBuffer.h"
-#include "llvm/Support/YAMLTraits.h"
 #include "llvm/Support/raw_ostream.h"
 
 LLVM_YAML_IS_DOCUMENT_LIST_VECTOR(clang::clangd::Symbol)
@@ -175,11 +174,11 @@
   return std::move(Syms).build();
 }
 
-Symbol SymbolFromYAML(llvm::StringRef YAMLContent,
-                      llvm::BumpPtrAllocator &Arena) {
-  llvm::yaml::Input Yin(YAMLContent, &Arena);
+Symbol SymbolFromYAML(llvm::yaml::Input &Input, llvm::BumpPtrAllocator &Arena) {
+  // We could grab Arena out of Input, but it'd be a huge hazard for callers.
+  assert(Input.getContext() == &Arena);
   Symbol S;
-  Yin >> S;
+  Input >> S;
   return S;
 }