Passing without bounds checks

compiler/builtins/bitcode/src/str.zig

@@ -1198,30 +1198,17 @@ fn fromUtf8(arg: RocList) FromUtf8Result {
 }
 
 pub fn fromUtf8RangeC(arg: RocList, countAndStart: CountAndStart, output: *FromUtf8Result) callconv(.C) void {
-    output.* = @call(.{ .modifier = always_inline }, fromUtf8Range, .{arg, countAndStart});
+    output.* = @call(.{ .modifier = always_inline }, fromUtf8Range, .{ arg, countAndStart });
 }
 
 fn fromUtf8Range(arg: RocList, countAndStart: CountAndStart) FromUtf8Result {
-    const bytes = @ptrCast([*]const u8, arg.bytes)[0..arg.length];
+    const bytes = @ptrCast([*]const u8, arg.bytes)[countAndStart.start..countAndStart.count];
 
     if (unicode.utf8ValidateSlice(bytes)) {
-        // the output will be correct. Now we need to take ownership of the input
-        if (arg.len() <= SMALL_STR_MAX_LENGTH) {
-            // turn the bytes into a small string
-            const string = RocStr.init(@ptrCast([*]u8, arg.bytes), arg.len());
+        // the output will be correct. Now we need to clone the input
+        const string = RocStr.init(@ptrCast([*]const u8, bytes), countAndStart.count);
 
-            // then decrement the input list
-            const data_bytes = arg.len();
-            utils.decref(arg.bytes, data_bytes, RocStr.alignment);
-
-            return FromUtf8Result{ .is_ok = true, .string = string, .byte_index = 0, .problem_code = Utf8ByteProblem.InvalidStartByte };
-        } else {
-            const byte_list = arg.makeUnique(RocStr.alignment, @sizeOf(u8));
-
-            const string = RocStr{ .str_bytes = byte_list.bytes, .str_len = byte_list.length };
-
-            return FromUtf8Result{ .is_ok = true, .string = string, .byte_index = 0, .problem_code = Utf8ByteProblem.InvalidStartByte };
-        }
+        return FromUtf8Result{ .is_ok = true, .string = string, .byte_index = 0, .problem_code = Utf8ByteProblem.InvalidStartByte };
     } else {
         const temp = errorToProblem(@ptrCast([*]u8, arg.bytes), arg.length);
 
@@ -1233,7 +1220,6 @@ fn fromUtf8Range(arg: RocList, countAndStart: CountAndStart) FromUtf8Result {
     }
 }
 
-
 fn errorToProblem(bytes: [*]u8, length: usize) struct { index: usize, problem: Utf8ByteProblem } {
     var index: usize = 0;
 

compiler/mono/src/borrow.rs

@@ -1015,7 +1015,7 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[bool] {
         StrStartsWith | StrEndsWith => arena.alloc_slice_copy(&[owned, borrowed]),
         StrStartsWithCodePoint => arena.alloc_slice_copy(&[borrowed, irrelevant]),
         StrFromUtf8 => arena.alloc_slice_copy(&[owned]),
-        StrFromUtf8Range => arena.alloc_slice_copy(&[owned, irrelevant]),
+        StrFromUtf8Range => arena.alloc_slice_copy(&[borrowed, irrelevant]),
         StrToBytes => arena.alloc_slice_copy(&[owned]),
         StrFromInt | StrFromFloat => arena.alloc_slice_copy(&[irrelevant]),
         Hash => arena.alloc_slice_copy(&[borrowed, irrelevant]),

compiler/test_gen/src/gen_str.rs

@@ -842,3 +842,39 @@ fn str_from_utf8_range() {
         RocStr
     );
 }
+
+#[test]
+fn str_from_utf8_range_slice() {
+    assert_evals_to!(
+        r#"
+        bytes = Str.toBytes "hello"
+        Str.fromUtf8Range bytes { start: 1, count: 4 }
+        "#,
+        RocStr::from("ello"),
+        RocStr
+    );
+}
+
+#[test]
+fn str_from_utf8_range_slice_not_end() {
+    assert_evals_to!(
+        r#"
+        bytes = Str.toBytes "hello"
+        Str.fromUtf8Range bytes { start: 1, count: 3 }
+        "#,
+        RocStr::from("ell"),
+        RocStr
+    );
+}
+
+// #[test]
+// fn str_from_utf8_range_slice() {
+//     assert_evals_to!(
+//         r#"
+//         bytes = Str.toBytes "hello"
+//         Str.fromUtf8Range bytes { start: 6, count: 5 }
+//         "#,
+//         RocStr::from("ello"),
+//         RocStr
+//     );
+// }