Rollup merge of #149447 - theemathas:string-replace_range, r=Mark-Simulacrum

Rewrite `String::replace_range`

This simplifies the code, provides better panic messages, and avoids an integer overflow.

library/alloctests/tests/string.rs

@@ -616,8 +616,15 @@ fn test_replace_range() {
 }
 
 #[test]
-#[should_panic]
-fn test_replace_range_char_boundary() {
+#[should_panic = "start of range should be a character boundary"]
+fn test_replace_range_start_char_boundary() {
+    let mut s = "Hello, 世界!".to_owned();
+    s.replace_range(8.., "");
+}
+
+#[test]
+#[should_panic = "end of range should be a character boundary"]
+fn test_replace_range_end_char_boundary() {
     let mut s = "Hello, 世界!".to_owned();
     s.replace_range(..8, "");
 }
@@ -632,28 +639,32 @@ fn test_replace_range_inclusive_range() {
 }
 
 #[test]
-#[should_panic]
+#[should_panic = "range end index 6 out of range for slice of length 5"]
 fn test_replace_range_out_of_bounds() {
     let mut s = String::from("12345");
     s.replace_range(5..6, "789");
 }
 
 #[test]
-#[should_panic]
+#[should_panic = "range end index 5 out of range for slice of length 5"]
 fn test_replace_range_inclusive_out_of_bounds() {
     let mut s = String::from("12345");
     s.replace_range(5..=5, "789");
 }
 
+// The overflowed index value is target-dependent,
+// so we don't check for its exact value in the panic message
 #[test]
-#[should_panic]
+#[should_panic = "out of range for slice of length 3"]
 fn test_replace_range_start_overflow() {
     let mut s = String::from("123");
     s.replace_range((Excluded(usize::MAX), Included(0)), "");
 }
 
+// The overflowed index value is target-dependent,
+// so we don't check for its exact value in the panic message
 #[test]
-#[should_panic]
+#[should_panic = "out of range for slice of length 3"]
 fn test_replace_range_end_overflow() {
     let mut s = String::from("456");
     s.replace_range((Included(0), Included(usize::MAX)), "");