Support slicing in str_index_to_ptr, and fix a bounds error

py/objstr.c

@@ -360,8 +360,9 @@ uncomparable:
     return MP_OBJ_NULL; // op not supported
 }
 
-// Convert an index into a pointer to its lead byte, or raise IndexError if out of bounds
-STATIC const char *str_index_to_ptr(const char *self_data, uint self_len, mp_obj_t index) {
+// Convert an index into a pointer to its lead byte. Out of bounds indexing will raise IndexError or
+// be capped to the first/last character of the string, depending on is_slice.
+STATIC const char *str_index_to_ptr(const char *self_data, uint self_len, mp_obj_t index, bool is_slice) {
     machine_int_t i;
     // Copied from mp_get_index; I don't want bounds checking, just give me
     // the integer as-is. (I can't bounds-check without scanning the whole
@@ -377,6 +378,9 @@ STATIC const char *str_index_to_ptr(const char *self_data, uint self_len, mp_obj
         // Negative indexing is performed by counting from the end of the string.
         for (s = top - 1; i; --s) {
             if (s < self_data) {
+                if (is_slice) {
+                    return self_data;
+                }
                 nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_IndexError, "string index out of range"));
             }	
             if (!UTF8_IS_CONT(*s)) {
@@ -384,22 +388,27 @@ STATIC const char *str_index_to_ptr(const char *self_data, uint self_len, mp_obj
             }
         }
         ++s;
+    } else if (!i) {
+        return self_data; // Shortcut - str[0] is its base pointer
     } else {
         // Positive indexing, correspondingly, counts from the start of the string.
         // It's assumed that negative indexing will generally be used with small
         // absolute values (eg str[-1], not str[-1000000]), which means it'll be
         // more efficient this way.
-        for (s = self_data; i; ++s) {
+        for (s = self_data; true; ++s) {
             if (s >= top) {
+                if (is_slice) {
+                    while (UTF8_IS_CONT(*--s));
+                    return s;
+                }
                 nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_IndexError, "string index out of range"));
             }
-            if (!UTF8_IS_CONT(*s)) {
-                --i;
+            while (UTF8_IS_CONT(*s)) {
+                ++s;
+            }
+            if (!i--) {
+                return s;
             }
-        }
-        // Skip continuation bytes after the last lead byte
-        while (UTF8_IS_CONT(*s)) {
-            ++s;
         }
     }
     return s;
@@ -424,7 +433,7 @@ STATIC mp_obj_t str_subscr(mp_obj_t self_in, mp_obj_t index, mp_obj_t value) {
             uint index_val = mp_get_index(type, self_len, index, false);
             return MP_OBJ_NEW_SMALL_INT((mp_small_int_t)self_data[index_val]);
         }
-        const char *s = str_index_to_ptr((const char *)self_data, self_len, index);
+        const char *s = str_index_to_ptr((const char *)self_data, self_len, index, false);
         int len = 1;
         if (UTF8_IS_NONASCII(*s)) {
             // Count the number of 1 bits (after the first)