A text's ASCII check is its char count against its length, so pinning a text never changes how it counts, and a char writes through a dyn view where its code point fits.

runtime/flan_dyn.c

@@ -767,23 +767,20 @@ static void char_spell(uint32_t cp, char buf[16]) {
 }
 
 /* A text is immutable, so its char count is taken once, when it is made, and
- * kept in the header's [u.i], which a text does not otherwise use; [gen]
- * says whether every byte is ASCII, which makes an index a byte offset. */
-#define TEXT_ASCII 1u
-
+ * kept in the header's [u.i], which a text does not otherwise use. A count
+ * equal to the byte length means every byte is ASCII, and an index is then a
+ * byte offset. [gen] is not touched: on a text it is [pin_text]'s stamp. */
 static void text_measure(flan_obj *o) {
   const uint8_t *p = obj_text_bytes(o);
   int64_t i = 0, n = 0;
-  int w, ascii = 1;
+  int w;
   while (i < o->len) {
     if (p[i] < 0x80) { i++; n++; continue; }
-    ascii = 0;
     utf8_decode(p + i, o->len - i, &w);
     i += w;
     n++;
   }
   o->u.i = n;
-  o->gen = ascii ? TEXT_ASCII : 0;
 }
 
 /* The byte offset of char [k], 0 <= k <= the char count. */
@@ -791,7 +788,7 @@ static int64_t text_offset(flan_obj *o, int64_t k) {
   const uint8_t *p = obj_text_bytes(o);
   int64_t i = 0;
   int w;
-  if (o->gen & TEXT_ASCII) return k;
+  if (o->u.i == o->len) return k;
   while (k > 0 && i < o->len) {
     utf8_decode(p + i, o->len - i, &w);
     i += w;
@@ -1681,8 +1678,9 @@ static void mark_desc(char *base, const flan_desc *d) {
  * allocator put there next.
  *
  * A text is pinned once per stamp, however often it crosses: the stamp's
- * number is written into the text's [gen], which nothing else reads for a
- * text, so a program that never calls free-temp and passes the same texts
+ * number is written into the text's [gen], which nothing else reads or
+ * writes for a text — its char count is in [u.i], and whether it is ASCII is
+ * that count against its length ([text_measure]) — so a program that never calls free-temp and passes the same texts
  * in a loop keeps a flat list. Distinct texts cost one pointer each here,
  * and each keeps its own object alive until the stamp ends — heavier than
  * i64->bytes's few bytes of arena, since the object has a header. The pins
@@ -4067,6 +4065,27 @@ static void view_write(const uint8_t *loc, int64_t loclen, const char *op,
   desc_spell(d, ty, sizeof ty);
   if (int_range(*d, &lo, &hi)) {
     int64_t n;
+    /* A char is written as its code point where it fits, [into_put]'s rule:
+       into a byte only when ASCII, since a byte past ASCII is not that char
+       in UTF-8. */
+    if (flan_dyn_tag(x) == FLAN_DYN_TAG_CHAR) {
+      char cs[16];
+      int byte = *d == 'b' || *d == 'B';
+      n = (int64_t)dyn_payload(x);
+      if (n <= (byte ? 127 : hi)) goto store;
+      char_spell((uint32_t)n, cs);
+      if (byte)
+        snprintf(why, sizeof why, ", and the char %s is more than one byte in "
+                 "UTF-8. Take its code point as an i32", cs);
+      else
+        snprintf(why, sizeof why, ", which holds %lld to %lld, and the char "
+                 "%s, code point %lld, does not fit", (long long)lo,
+                 (long long)hi, cs, (long long)n);
+      if (field) field_refuse(loc, loclen, op, v, key, d, x, "DynRange", why);
+      flan_say(loc, loclen, "dyn %s: this element is %s %s%s", op, an(ty), ty,
+               why);
+      dyn_trap((const uint8_t *)"DynRange", 8);
+    }
     if (flan_dyn_tag(x) != FLAN_DYN_TAG_INT) {
       if (field) {
         value_is(why, sizeof why, x);
@@ -4098,6 +4117,7 @@ static void view_write(const uint8_t *loc, int64_t loclen, const char *op,
                  (long long)hi);
       dyn_trap((const uint8_t *)"DynRange", 8);
     }
+  store:
     switch (*d) {
     case 'b': case 'B': { uint8_t b = (uint8_t)n; memcpy(p, &b, 1); return; }
     case 'h': case 'H': { uint16_t h = (uint16_t)n; memcpy(p, &h, 2); return; }

test/programs/dyn-char-pinned.flan

@@ -0,0 +1,25 @@
+;;;; A non-ASCII text that has crossed into a str still counts characters,
+;;;; however many times it crosses; and a char is written through a dyn view of
+;;;; typed storage as its code point where it fits. With an argument, \é
+;;;; written into a view of bytes traps.
+
+(defn byte-len [s str] i32 (length s))
+
+(defn main [args [str]] i32
+  (let [t (the dyn "é日😀")]
+    (dotimes [i 3] (println (byte-len t)))
+    (println (length t))
+    (println (at t 1))
+    (println (slice t 0 2)))
+  (let [a (the [3 i32] [1 2 3])
+        v (the dyn a)]
+    (set (at v 0) \z)
+    (set (at v 1) \日)
+    (println (at a 0) (at a 1) (at a 2)))
+  (let [b (the [2 u8] [1 2])
+        w (the dyn b)]
+    (set (at w 0) \a)
+    (println (at b 0) (at b 1))
+    (when (> (length args) 1)
+      (set (at w 1) \é)))
+  0)

test/test_acceptance.ml

@@ -5447,6 +5447,29 @@ level "1"
     outputs "dyn: chars" "programs/dyn-char.flan" dyn_char_out;
     outputs ~opt:"-O0" "dyn: chars, -O0" "programs/dyn-char.flan" dyn_char_out;
     outputs ~x86:true "dyn: chars, --x86" "programs/dyn-char.flan" dyn_char_out;
+    (* A text pinned by crossing into a str keeps counting characters (the
+       pin's stamp and the text's measure live in different header fields),
+       and a char writes through a view of typed storage. *)
+    let pinned_out = "9\n9\n9\n3\n\\日\né日\n122 26085 3\n97 2\n" in
+    outputs "dyn: a pinned text counts chars" "programs/dyn-char-pinned.flan"
+      pinned_out;
+    outputs ~opt:"-O0" "dyn: a pinned text counts chars, -O0"
+      "programs/dyn-char-pinned.flan" pinned_out;
+    outputs ~x86:true "dyn: a pinned text counts chars, --x86"
+      "programs/dyn-char-pinned.flan" pinned_out;
+    List.iter
+      (fun x86 ->
+         let exe = compile ~x86 "programs/dyn-char-pinned.flan" in
+         let code, text = run exe (Some "x") in
+         let want = "programs/dyn-char-pinned.flan:24:7: dyn set-at: this \
+                     element is a u8, and the char \\é is more than one byte" in
+         if code <> 134 || not (contains text want) then begin
+           incr failures;
+           Printf.printf "FAIL  dyn: \\é into a byte view traps%s\n        \
+                          got: %S (exit %d)\n"
+             (if x86 then ", --x86" else "") text code
+         end)
+      [ false; true ];
     (* A dyn into every integer width: both edges pass, a char passes where
        it fits, a cast takes a char and wraps as the typed cast beside it
        does; then one trap per width past its range, a char too wide, and a