feat: String.Pos.isValid (#3959)

This adds a function that can be used to check whether a position is on
a UTF-8 byte boundary.

src/Init/Data/String/Basic.lean

@@ -44,6 +44,16 @@ def append : String → (@& String) → String
 def toList (s : String) : List Char :=
   s.data
 
+/-- Returns true if `p` is a valid UTF-8 position in the string `s`, meaning that `p ≤ s.endPos`
+and `p` lies on a UTF-8 character boundary. This has an O(1) implementation in the runtime. -/
+@[extern "lean_string_is_valid_pos"]
+def Pos.isValid (s : @&String) (p : @& Pos) : Bool :=
+  go s.data 0
+where
+  go : List Char → Pos → Bool
+  | [],    i => i = p
+  | c::cs, i => if i = p then true else go cs (i + c)
+
 def utf8GetAux : List Char → Pos → Pos → Char
   | [],    _, _ => default
   | c::cs, i, p => if i = p then c else utf8GetAux cs (i + c) p

src/runtime/object.cpp

@@ -1922,6 +1922,19 @@ static inline bool is_utf8_first_byte(unsigned char c) {
     return (c & 0x80) == 0 || (c & 0xe0) == 0xc0 || (c & 0xf0) == 0xe0 || (c & 0xf8) == 0xf0;
 }
 
+extern "C" LEAN_EXPORT uint8 lean_string_is_valid_pos(b_obj_arg s, b_obj_arg i0) {
+    if (!lean_is_scalar(i0)) {
+        /* See comment at string_utf8_get */
+        return false;
+    }
+    usize i = lean_unbox(i0);
+    usize sz = lean_string_size(s) - 1;
+    if (i > sz) return false;
+    if (i == sz) return true;
+    char const * str = lean_string_cstr(s);
+    return is_utf8_first_byte(str[i]);
+}
+
 extern "C" LEAN_EXPORT obj_res lean_string_utf8_extract(b_obj_arg s, b_obj_arg b0, b_obj_arg e0) {
     if (!lean_is_scalar(b0) || !lean_is_scalar(e0)) {
         /* See comment at string_utf8_get */