Auto merge of #50039 - ExpHP:quick-50002, r=alexcrichton

smaller PR just to fix #50002

I pulled this out of #50010 to make it easier to backport to beta if necessary, considering that inclusive range syntax is stabilizing soon (?).

It fixes a bug in `<str>::index_mut` with `(..=end)` ranges (#50002), which prior to this fix was not only unusable but also UB in the cases where it "worked" (it gave improperly truncated UTF-8).

(not that I can imagine why anybody would *use* `<str>::index_mut`... but I'm not here to judge)

1 files changed, 30 insertions, 0 deletions

diff --git a/src/liballoc/tests/str.rs b/src/liballoc/tests/str.rs
index a3f4c385fe2..a03b61ec97e 100644
--- a/src/liballoc/tests/str.rs
+++ b/src/liballoc/tests/str.rs
@@ -402,6 +402,36 @@ fn test_str_get_maxinclusive() {
 }
 
 #[test]
+fn test_str_slice_rangetoinclusive_ok() {
+    let s = "abcαβγ";
+    assert_eq!(&s[..=2], "abc");
+    assert_eq!(&s[..=4], "abcα");
+}
+
+#[test]
+#[should_panic]
+fn test_str_slice_rangetoinclusive_notok() {
+    let s = "abcαβγ";
+    &s[..=3];
+}
+
+#[test]
+fn test_str_slicemut_rangetoinclusive_ok() {
+    let mut s = "abcαβγ".to_owned();
+    let s: &mut str = &mut s;
+    assert_eq!(&mut s[..=2], "abc");
+    assert_eq!(&mut s[..=4], "abcα");
+}
+
+#[test]
+#[should_panic]
+fn test_str_slicemut_rangetoinclusive_notok() {
+    let mut s = "abcαβγ".to_owned();
+    let s: &mut str = &mut s;
+    &mut s[..=3];
+}
+
+#[test]
 fn test_is_char_boundary() {
     let s = "ศไทย中华Việt Nam β-release 🐱123";
     assert!(s.is_char_boundary(0));