From b74d6922ff15d9f3bf39d317ccd7141518f4f5ec Mon Sep 17 00:00:00 2001
From: Michael Lamparski <diagonaldevice@gmail.com>
Date: Mon, 16 Apr 2018 16:34:09 -0400
Subject: [PATCH 1/2] smaller PR just to fix #50002

---
 src/liballoc/tests/str.rs | 16 ++++++++++++++++
 src/libcore/str/mod.rs    |  9 ++-------
 2 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/src/liballoc/tests/str.rs b/src/liballoc/tests/str.rs
index a3f4c385fe23b..e3f198f736232 100644
--- a/src/liballoc/tests/str.rs
+++ b/src/liballoc/tests/str.rs
@@ -401,6 +401,22 @@ fn test_str_get_maxinclusive() {
     }
 }
 
+#[test]
+fn test_str_slicemut_rangetoinclusive_ok() {
+    let mut s = "abcαβγ".to_owned();
+    let s: &mut str = &mut s;
+    &mut s[..=3]; // before alpha
+    &mut s[..=5]; // after alpha
+}
+
+#[test]
+#[should_panic]
+fn test_str_slicemut_rangetoinclusive_notok() {
+    let mut s = "abcαβγ".to_owned();
+    let s: &mut str = &mut s;
+    &mut s[..=4]; // middle of alpha, which is 2 bytes long
+}
+
 #[test]
 fn test_is_char_boundary() {
     let s = "ศไทย中华Việt Nam β-release 🐱123";
diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index f1fe23092de93..5b52119d0310a 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -2100,18 +2100,13 @@ mod traits {
         fn index(self, slice: &str) -> &Self::Output {
             assert!(self.end != usize::max_value(),
                 "attempted to index str up to maximum usize");
-            let end = self.end + 1;
-            self.get(slice).unwrap_or_else(|| super::slice_error_fail(slice, 0, end))
+            (..self.end+1).index(slice)
         }
         #[inline]
         fn index_mut(self, slice: &mut str) -> &mut Self::Output {
             assert!(self.end != usize::max_value(),
                 "attempted to index str up to maximum usize");
-            if slice.is_char_boundary(self.end) {
-                unsafe { self.get_unchecked_mut(slice) }
-            } else {
-                super::slice_error_fail(slice, 0, self.end + 1)
-            }
+            (..self.end+1).index_mut(slice)
         }
     }
 

From 90b361b3a748e9fb01cd9aec7b83edca2d9e996e Mon Sep 17 00:00:00 2001
From: Michael Lamparski <diagonaldevice@gmail.com>
Date: Wed, 18 Apr 2018 16:48:34 -0400
Subject: [PATCH 2/2] fix my unit test that was horrendously wrong

and add one for non-mut slicing since I touched that method too
---
 src/liballoc/tests/str.rs | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/src/liballoc/tests/str.rs b/src/liballoc/tests/str.rs
index e3f198f736232..a03b61ec97e51 100644
--- a/src/liballoc/tests/str.rs
+++ b/src/liballoc/tests/str.rs
@@ -401,12 +401,26 @@ 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;
-    &mut s[..=3]; // before alpha
-    &mut s[..=5]; // after alpha
+    assert_eq!(&mut s[..=2], "abc");
+    assert_eq!(&mut s[..=4], "abcα");
 }
 
 #[test]
@@ -414,7 +428,7 @@ fn test_str_slicemut_rangetoinclusive_ok() {
 fn test_str_slicemut_rangetoinclusive_notok() {
     let mut s = "abcαβγ".to_owned();
     let s: &mut str = &mut s;
-    &mut s[..=4]; // middle of alpha, which is 2 bytes long
+    &mut s[..=3];
 }
 
 #[test]