diff --git a/libgrust/libproc_macro/literal.cc b/libgrust/libproc_macro/literal.cc
index 4ad45c90331..8e1e0a4a116 100644
--- a/libgrust/libproc_macro/literal.cc
+++ b/libgrust/libproc_macro/literal.cc
@@ -28,7 +28,7 @@ namespace ProcMacro {
 
 extern "C" {
 bool
-Literal__from_string (const unsigned char *str, std::uint64_t len, Literal *lit)
+Literal__from_string (FFIString str, Literal *lit)
 {
   // FIXME: implement this function with lexer
   std::abort ();
diff --git a/libgrust/libproc_macro/literal.h b/libgrust/libproc_macro/literal.h
index e1b7079451a..fb8a8b442e2 100644
--- a/libgrust/libproc_macro/literal.h
+++ b/libgrust/libproc_macro/literal.h
@@ -105,8 +105,7 @@ public:
 
 extern "C" {
 bool
-Literal__from_string (const unsigned char *str, std::uint64_t len,
-		      Literal *lit);
+Literal__from_string (FFIString str, Literal *lit);
 }
 } // namespace ProcMacro
 
diff --git a/libgrust/libproc_macro/tokenstream.cc b/libgrust/libproc_macro/tokenstream.cc
index c3502f1c7b7..5d9abb69a2d 100644
--- a/libgrust/libproc_macro/tokenstream.cc
+++ b/libgrust/libproc_macro/tokenstream.cc
@@ -103,11 +103,10 @@ TokenSream__push (TokenStream *stream, TokenTree tree)
 }
 
 extern "C" bool
-TokenStream__from_string (unsigned char *str, std::uint64_t len,
-			  TokenStream *ts)
+TokenStream__from_string (FFIString str, TokenStream *ts)
 {
   bool result;
-  auto source = std::string (reinterpret_cast<const char *> (str), len);
+  auto source = str.to_string ();
 
   *ts = TokenStream::make_tokenstream (source, result);
   return result;
diff --git a/libgrust/libproc_macro/tokenstream.h b/libgrust/libproc_macro/tokenstream.h
index 8b2432b9a68..0fde5ea5397 100644
--- a/libgrust/libproc_macro/tokenstream.h
+++ b/libgrust/libproc_macro/tokenstream.h
@@ -27,6 +27,8 @@
 #include <vector>
 #include <string>
 
+#include "ffistring.h"
+
 namespace ProcMacro {
 struct TokenTree;
 
@@ -59,8 +61,7 @@ extern "C" void
 TokenSream__push (TokenStream *stream, TokenTree tree);
 
 extern "C" bool
-TokenStream__from_string (unsigned char *str, std::uint64_t len,
-			  TokenStream *ts);
+TokenStream__from_string (FFIString str, TokenStream *ts);
 
 extern "C" TokenStream
 TokenStream__clone (const TokenStream *ts);