diff --git a/pkgs/racket-test-core/tests/racket/optimize.rktl b/pkgs/racket-test-core/tests/racket/optimize.rktl
index 8af3769f99..8d938e8cf2 100644
--- a/pkgs/racket-test-core/tests/racket/optimize.rktl
+++ b/pkgs/racket-test-core/tests/racket/optimize.rktl
@@ -1396,6 +1396,50 @@
            '(lambda (w z) #t)
            #f)
 
+;Test types inference for string?
+(test-comp '(lambda (s) (fixnum? (string-length s)))
+           '(lambda (s) (string-length s) #t))
+(test-comp '(lambda (s) (string-length s) (string? s))
+           '(lambda (s) (string-length s) #t))
+(test-comp '(lambda (s p) (string-ref s p) (string? s))
+           '(lambda (s p) (string-ref s p) #t))
+(test-comp '(lambda (s p v) (string-set! s p v) (string? s))
+           '(lambda (s p v) (string-set! s p v) #t))
+(test-comp '(lambda (s1) (string-append s1) (string? s1))
+           '(lambda (s1) (string-append s1) #t))
+(test-comp '(lambda (s1 s2) (string-append s1 s2) (list (string? s1) (string? s2)))
+           '(lambda (s1 s2) (string-append s1 s2) (list #t #t)))
+(test-comp '(lambda (s1 s2 s3) (string-append s1 s2 s3) (list (string? s1) (string? s2) (string? s3)))
+           '(lambda (s1 s2 s3) (string-append s1 s2 s3) (list #t #t #t)))
+(test-comp '(lambda (s1) (string? (string-append s1)))
+           '(lambda (s1) (string-append s1) #t))
+(test-comp '(lambda (s1 s2) (string? (string-append s1 s2)))
+           '(lambda (s1 s2) (string-append s1 s2) #t))
+(test-comp '(lambda (s1 s2 s3) (string? (string-append s1 s2 s3)))
+           '(lambda (s1 s2 s3) (string-append s1 s2 s3) #t))
+
+;Test types inference for bytes?
+(test-comp '(lambda (s) (fixnum? (bytes-length s)))
+           '(lambda (s) (bytes-length s) #t))
+(test-comp '(lambda (s) (bytes-length s) (bytes? s))
+           '(lambda (s) (bytes-length s) #t))
+(test-comp '(lambda (s p) (bytes-ref s p) (bytes? s))
+           '(lambda (s p) (bytes-ref s p) #t))
+(test-comp '(lambda (s p v) (bytes-set! s p v) (bytes? s))
+           '(lambda (s p v) (bytes-set! s p v) #t))
+(test-comp '(lambda (s1) (bytes-append s1) (bytes? s1))
+           '(lambda (s1) (bytes-append s1) #t))
+(test-comp '(lambda (s1 s2) (bytes-append s1 s2) (list (bytes? s1) (bytes? s2)))
+           '(lambda (s1 s2) (bytes-append s1 s2) (list #t #t)))
+(test-comp '(lambda (s1 s2 s3) (bytes-append s1 s2 s3) (list (bytes? s1) (bytes? s2) (bytes? s3)))
+           '(lambda (s1 s2 s3) (bytes-append s1 s2 s3) (list #t #t #t)))
+(test-comp '(lambda (s1) (bytes? (bytes-append s1)))
+           '(lambda (s1) (bytes-append s1) #t))
+(test-comp '(lambda (s1 s2) (bytes? (bytes-append s1 s2)))
+           '(lambda (s1 s2) (bytes-append s1 s2) #t))
+(test-comp '(lambda (s1 s2 s3) (bytes? (bytes-append s1 s2 s3)))
+           '(lambda (s1 s2 s3) (bytes-append s1 s2 s3) #t))
+
 (test-comp '(lambda (w z)
               (let ([x (list* w z)]
                     [y (list* z w)])
@@ -3179,6 +3223,7 @@
   (test-use-unsafe 'mpair? 'mcdr 'unsafe-mcdr)
   (test-use-unsafe 'box? 'unbox 'unsafe-unbox)
   (test-use-unsafe 'vector? 'vector-length 'unsafe-vector-length)
+  (test-use-unsafe 'string? 'string-length 'unsafe-string-length)
   (test-use-unsafe 'bytes? 'bytes-length 'unsafe-bytes-length)
   (test-use-unsafe/savetype 'fixnum? 'bitwise-not 'unsafe-fxnot #f)
   (test-use-unsafe/savetype 'fixnum? 'fxnot 'unsafe-fxnot #f))
diff --git a/racket/src/racket/src/optimize.c b/racket/src/racket/src/optimize.c
index 7d31410d94..72591770d2 100644
--- a/racket/src/racket/src/optimize.c
+++ b/racket/src/racket/src/optimize.c
@@ -2617,9 +2617,6 @@ static Scheme_Object *rator_implies_predicate(Scheme_Object *rator, int argc)
       return scheme_list_pair_p_proc;
     else if (SAME_OBJ(rator, scheme_mcons_proc))
       return scheme_mpair_p_proc;
-    // XXX This could be implemented
-    // else if (SAME_OBJ(rator, scheme_make_byte_string_p))
-    //  return scheme_byte_string_p_proc;
     else if (SAME_OBJ(rator, scheme_list_proc)) {
       if (argc >= 1)
         return scheme_list_pair_p_proc;
@@ -2628,6 +2625,10 @@ static Scheme_Object *rator_implies_predicate(Scheme_Object *rator, int argc)
     } else if (SAME_OBJ(rator, scheme_list_star_proc)) {
       if (argc > 2)
         return scheme_pair_p_proc;
+    } else if (IS_NAMED_PRIM(rator, "string-append")) {
+        return scheme_string_p_proc;
+    } else if (IS_NAMED_PRIM(rator, "bytes-append")) {
+        return scheme_byte_string_p_proc;
     } else if (SAME_OBJ(rator, scheme_vector_proc)
                || SAME_OBJ(rator, scheme_vector_immutable_proc)
                || SAME_OBJ(rator, scheme_make_vector_proc)
@@ -2864,6 +2865,8 @@ static Scheme_Object *do_expr_implies_predicate(Scheme_Object *expr, Optimize_In
       return scheme_pair_p_proc;
     if (SCHEME_MPAIRP(expr))
       return scheme_mpair_p_proc;
+    if (SCHEME_CHAR_STRINGP(expr))
+      return scheme_string_p_proc;
     if (SCHEME_BYTE_STRINGP(expr))
       return scheme_byte_string_p_proc;
     if (SCHEME_VOIDP(expr))
@@ -3375,6 +3378,11 @@ static Scheme_Object *finish_optimize_application(Scheme_App_Rec *app, Optimize_
       check_known(info, app_o, rator, rand1, "andmap", scheme_procedure_p_proc, NULL);
       check_known(info, app_o, rator, rand1, "ormap", scheme_procedure_p_proc, NULL);
 
+      check_known_all(info, app_o, "string-append", scheme_string_p_proc, scheme_true);
+      check_known_all(info, app_o, "bytes-append", scheme_byte_string_p_proc, scheme_true);
+      check_known(info, app_o, rator, rand1, "string-set!", scheme_string_p_proc, NULL);
+      check_known(info, app_o, rator, rand1, "bytes-set!", scheme_byte_string_p_proc, NULL);
+
       if (SCHEME_PRIM_PROC_OPT_FLAGS(rator) & SCHEME_PRIM_WANTS_REAL)
         check_known_all(info, app_o, NULL, scheme_real_p_proc,
                         (SCHEME_PRIM_PROC_OPT_FLAGS(rator) & SCHEME_PRIM_OMITTABLE_ON_GOOD_ARGS) ? scheme_true : NULL);
@@ -3790,13 +3798,17 @@ static Scheme_Object *finish_optimize_application2(Scheme_App2_Rec *app, Optimiz
       check_known(info, app_o, rator, rand, "unsafe-mcar", scheme_mpair_p_proc, NULL);
       check_known(info, app_o, rator, rand, "mcdr", scheme_mpair_p_proc, scheme_unsafe_mcdr_proc);
       check_known(info, app_o, rator, rand, "unsafe-mcdr", scheme_mpair_p_proc, NULL);
-      check_known(info, app_o, rator, rand, "bytes-length", scheme_byte_string_p_proc, scheme_unsafe_bytes_len_proc);
+      check_known(info, app_o, rator, rand, "string-length", scheme_string_p_proc, scheme_unsafe_string_length_proc);
+      check_known(info, app_o, rator, rand, "bytes-length", scheme_byte_string_p_proc, scheme_unsafe_byte_string_length_proc);
       /* It's not clear that these are useful, since a chaperone check is needed anyway: */
       check_known(info, app_o, rator, rand, "unbox", scheme_box_p_proc, scheme_unsafe_unbox_proc);
       check_known(info, app_o, rator, rand, "unsafe-unbox", scheme_box_p_proc, NULL);
       check_known(info, app_o, rator, rand, "unsafe-unbox*", scheme_box_p_proc, NULL);
       check_known(info, app_o, rator, rand, "vector-length", scheme_vector_p_proc, scheme_unsafe_vector_length_proc);
 
+      check_known(info, app_o, rator, rand, "string-append", scheme_string_p_proc, scheme_true);
+      check_known(info, app_o, rator, rand, "bytes-append", scheme_byte_string_p_proc, scheme_true);
+
       if (SCHEME_PRIM_PROC_OPT_FLAGS(rator) & SCHEME_PRIM_WANTS_REAL)
         check_known(info, app_o, rator, rand, NULL, scheme_real_p_proc,
                     (SCHEME_PRIM_PROC_OPT_FLAGS(rator) & SCHEME_PRIM_OMITTABLE_ON_GOOD_ARGS) ? scheme_true : NULL);
@@ -4239,6 +4251,11 @@ static Scheme_Object *finish_optimize_application3(Scheme_App3_Rec *app, Optimiz
 
     rator = app->rator; /* in case it was updated */
 
+    check_known_both(info, app_o, rator, rand1, rand2, "string-append", scheme_string_p_proc, scheme_true);
+    check_known_both(info, app_o, rator, rand1, rand2, "bytes-append", scheme_byte_string_p_proc, scheme_true);
+    check_known(info, app_o, rator, rand1, "string-ref", scheme_string_p_proc, NULL);
+    check_known(info, app_o, rator, rand1, "bytes-ref", scheme_byte_string_p_proc, NULL);
+
     if (SCHEME_PRIM_PROC_OPT_FLAGS(rator) & SCHEME_PRIM_WANTS_REAL)
       check_known_both(info, app_o, rator, rand1, rand2, NULL, scheme_real_p_proc,
                        (SCHEME_PRIM_PROC_OPT_FLAGS(rator) & SCHEME_PRIM_OMITTABLE_ON_GOOD_ARGS) ? scheme_true : NULL);
@@ -4714,6 +4731,7 @@ static int relevant_predicate(Scheme_Object *pred)
           || SAME_OBJ(pred, scheme_box_p_proc)
           || SAME_OBJ(pred, scheme_list_p_proc)
           || SAME_OBJ(pred, scheme_list_pair_p_proc)
+          || SAME_OBJ(pred, scheme_string_p_proc)
           || SAME_OBJ(pred, scheme_byte_string_p_proc)
           || SAME_OBJ(pred, scheme_vector_p_proc)
           || SAME_OBJ(pred, scheme_procedure_p_proc)
diff --git a/racket/src/racket/src/schpriv.h b/racket/src/racket/src/schpriv.h
index d76bff8127..a117f10d84 100644
--- a/racket/src/racket/src/schpriv.h
+++ b/racket/src/racket/src/schpriv.h
@@ -547,8 +547,10 @@ extern Scheme_Object *scheme_unsafe_fxior_proc;
 extern Scheme_Object *scheme_unsafe_fxxor_proc;
 extern Scheme_Object *scheme_unsafe_fxrshift_proc;
 
+extern Scheme_Object *scheme_string_p_proc;
+extern Scheme_Object *scheme_unsafe_string_length_proc;
 extern Scheme_Object *scheme_byte_string_p_proc;
-extern Scheme_Object *scheme_unsafe_bytes_len_proc;
+extern Scheme_Object *scheme_unsafe_byte_string_length_proc;
 
 extern Scheme_Object *scheme_unsafe_real_add1_proc;
 extern Scheme_Object *scheme_unsafe_real_sub1_proc;
diff --git a/racket/src/racket/src/string.c b/racket/src/racket/src/string.c
index 11a86a317e..0a65ec4119 100644
--- a/racket/src/racket/src/string.c
+++ b/racket/src/racket/src/string.c
@@ -392,6 +392,7 @@ SHARED_OK static Scheme_Object *fs_change_props;
 
 READ_ONLY static Scheme_Object *complete_symbol, *continues_symbol, *aborts_symbol, *error_symbol;
 
+READ_ONLY Scheme_Object *scheme_string_p_proc;
 READ_ONLY Scheme_Object *scheme_byte_string_p_proc;
 
 void
@@ -484,10 +485,12 @@ scheme_init_string (Scheme_Env *env)
   banner_str = scheme_make_utf8_string(scheme_banner());
   SCHEME_SET_CHAR_STRING_IMMUTABLE(banner_str);
 
+  REGISTER_SO(scheme_string_p_proc);
   p = scheme_make_folding_prim(string_p, "string?", 1, 1, 1);
   SCHEME_PRIM_PROC_FLAGS(p) |= scheme_intern_prim_opt_flags(SCHEME_PRIM_IS_UNARY_INLINED
                                                             | SCHEME_PRIM_IS_OMITABLE);
   scheme_add_global_constant("string?", p, env);
+  scheme_string_p_proc = p;
 
   scheme_add_global_constant("make-string",
 			     scheme_make_immed_prim(make_string,
@@ -761,7 +764,7 @@ scheme_init_string (Scheme_Env *env)
 
   REGISTER_SO(scheme_byte_string_p_proc);
   p = scheme_make_folding_prim(byte_string_p, "bytes?", 1, 1, 1);
-  SCHEME_PRIM_PROC_FLAGS(p) |= scheme_intern_prim_opt_flags(SCHEME_PRIM_IS_NARY_INLINED
+  SCHEME_PRIM_PROC_FLAGS(p) |= scheme_intern_prim_opt_flags(SCHEME_PRIM_IS_UNARY_INLINED
                                                             | SCHEME_PRIM_IS_OMITABLE);
   scheme_add_global_constant("bytes?", p, env);
   scheme_byte_string_p_proc = p;
diff --git a/racket/src/racket/src/vector.c b/racket/src/racket/src/vector.c
index 4423bd9cda..dbfcb6ca2a 100644
--- a/racket/src/racket/src/vector.c
+++ b/racket/src/racket/src/vector.c
@@ -35,7 +35,8 @@ READ_ONLY Scheme_Object *scheme_vector_ref_proc;
 READ_ONLY Scheme_Object *scheme_vector_set_proc;
 READ_ONLY Scheme_Object *scheme_list_to_vector_proc;
 READ_ONLY Scheme_Object *scheme_unsafe_vector_length_proc;
-READ_ONLY Scheme_Object *scheme_unsafe_bytes_len_proc;
+READ_ONLY Scheme_Object *scheme_unsafe_string_length_proc;
+READ_ONLY Scheme_Object *scheme_unsafe_byte_string_length_proc;
 
 /* locals */
 static Scheme_Object *vector_p (int argc, Scheme_Object *argv[]);
@@ -231,11 +232,13 @@ scheme_init_unsafe_vector (Scheme_Env *env)
   SCHEME_PRIM_PROC_FLAGS(p) |= scheme_intern_prim_opt_flags(SCHEME_PRIM_IS_NARY_INLINED);
   scheme_add_global_constant("unsafe-struct*-set!", p, env);  
 
+  REGISTER_SO(scheme_unsafe_string_length_proc);
   p = scheme_make_immed_prim(unsafe_string_len, "unsafe-string-length", 1, 1);
   SCHEME_PRIM_PROC_FLAGS(p) |= scheme_intern_prim_opt_flags(SCHEME_PRIM_IS_UNARY_INLINED
                                                             | SCHEME_PRIM_IS_UNSAFE_FUNCTIONAL
                                                             | SCHEME_PRIM_PRODUCES_FIXNUM);
   scheme_add_global_constant("unsafe-string-length", p, env);
+  scheme_unsafe_string_length_proc = p;
 
   p = scheme_make_immed_prim(unsafe_string_ref, "unsafe-string-ref", 2, 2);
   SCHEME_PRIM_PROC_FLAGS(p) |= scheme_intern_prim_opt_flags(SCHEME_PRIM_IS_BINARY_INLINED
@@ -247,13 +250,13 @@ scheme_init_unsafe_vector (Scheme_Env *env)
   SCHEME_PRIM_PROC_FLAGS(p) |= scheme_intern_prim_opt_flags(SCHEME_PRIM_IS_NARY_INLINED);
   scheme_add_global_constant("unsafe-string-set!", p, env);
 
-  REGISTER_SO(scheme_unsafe_bytes_len_proc);
+  REGISTER_SO(scheme_unsafe_byte_string_length_proc);
   p = scheme_make_immed_prim(unsafe_bytes_len, "unsafe-bytes-length", 1, 1);
   SCHEME_PRIM_PROC_FLAGS(p) |= scheme_intern_prim_opt_flags(SCHEME_PRIM_IS_UNARY_INLINED
                                                             | SCHEME_PRIM_IS_UNSAFE_FUNCTIONAL
                                                             | SCHEME_PRIM_PRODUCES_FIXNUM);
   scheme_add_global_constant("unsafe-bytes-length", p, env);
-  scheme_unsafe_bytes_len_proc = p;
+  scheme_unsafe_byte_string_length_proc = p;
 
   p = scheme_make_immed_prim(unsafe_bytes_ref, "unsafe-bytes-ref", 2, 2);
   SCHEME_PRIM_PROC_FLAGS(p) |= scheme_intern_prim_opt_flags(SCHEME_PRIM_IS_BINARY_INLINED