Test passing in debug but not release

compiler/can/src/builtins.rs

@@ -1551,7 +1551,55 @@ fn str_from_utf8_range(symbol: Symbol, var_store: &mut VarStore) -> Def {
         ),
     };
 
-    let body = LetNonRec(Box::new(def), Box::new(no_region(cont)), ret_var);
+    let roc_result = LetNonRec(Box::new(def), Box::new(no_region(cont)), ret_var);
+
+    // Only do the business with the let if we're in bounds!
+
+    let start_var = var_store.fresh();
+    let start_bool = var_store.fresh();
+
+    let body = If {
+        cond_var: start_bool,
+        branch_var: ret_var,
+        branches: vec![(
+            no_region(RunLowLevel {
+                op: LowLevel::NumLt,
+                args: vec![
+                    (
+                        start_var,
+                        Access {
+                            record_var: arg_record_var,
+                            ext_var: var_store.fresh(),
+                            field: "start".into(),
+                            field_var: var_store.fresh(),
+                            loc_expr: Box::new(no_region(Var(Symbol::ARG_2))),
+                        },
+                    ),
+                    (
+                        start_var,
+                        RunLowLevel {
+                            op: LowLevel::ListLen,
+                            args: vec![(bytes_var, Var(Symbol::ARG_1))],
+                            ret_var: start_var,
+                        },
+                    ),
+                ],
+                ret_var: start_bool,
+            }),
+            no_region(roc_result),
+        )],
+        final_else: Box::new(
+            // else-branch
+            no_region(
+                // Err
+                tag(
+                    "Err",
+                    vec![tag("OutOfBounds", Vec::new(), var_store)],
+                    var_store,
+                ),
+            ),
+        ),
+    };
 
     defn(
         symbol,

compiler/test_gen/src/gen_str.rs

@@ -895,18 +895,19 @@ fn str_from_utf8_range_order_does_not_matter() {
     );
 }
 
-// #[test]
+#[test]
-// fn str_from_utf8_range_out_of_bounds() {
+fn str_from_utf8_range_out_of_bounds() {
-//     assert_evals_to!(
+    assert_evals_to!(
-//        indoc!(
+        indoc!(
-//             r#"
+            r#"
-//             bytes = Str.toUtf8 "hello"
+            bytes = Str.toUtf8 "hello"
-//             when Str.fromUtf8Range bytes { start: 7,  count: 3 }  is
+            when Str.fromUtf8Range bytes { start: 7,  count: 3 }  is
-//                    Ok _ -> ""
+                   Ok _ -> ""
-//                    Err OutOfBounds -> "out of bounds"
+                   Err (BadUtf8 _ _) -> ""
-//             "#
+                   Err OutOfBounds -> "out of bounds"
-//         ),
+            "#
-//         RocStr::from("out of bounds"),
+        ),
-//         RocStr
+        RocStr::from("out of bounds"),
-//     );
+        RocStr
-// }
+    );
+}