diff --git a/library/core/src/slice/index.rs b/library/core/src/slice/index.rs
index c295a0e0645..f0e5ea53d7d 100644
--- a/library/core/src/slice/index.rs
+++ b/library/core/src/slice/index.rs
@@ -371,12 +371,11 @@ fn get_mut(self, slice: &mut [T]) -> Option<&mut [T]> {
 
     #[inline]
     unsafe fn get_unchecked(self, slice: *const [T]) -> *const [T] {
-        let this = ops::Range { start: self.start, end: self.end };
+        let this = ops::Range { ..self };
         // SAFETY: the caller guarantees that `slice` is not dangling, so it
         // cannot be longer than `isize::MAX`. They also guarantee that
         // `self` is in bounds of `slice` so `self` cannot overflow an `isize`,
         // so the call to `add` is safe.
-
         unsafe {
             assert_unsafe_precondition!(
                 "slice::get_unchecked requires that the range is within the slice",
@@ -389,7 +388,7 @@ unsafe fn get_unchecked(self, slice: *const [T]) -> *const [T] {
 
     #[inline]
     unsafe fn get_unchecked_mut(self, slice: *mut [T]) -> *mut [T] {
-        let this = ops::Range { start: self.start, end: self.end };
+        let this = ops::Range { ..self };
         // SAFETY: see comments for `get_unchecked` above.
         unsafe {
             assert_unsafe_precondition!(
diff --git a/library/core/src/slice/mod.rs b/library/core/src/slice/mod.rs
index d93a3a57ecd..9926bf7fbbb 100644
--- a/library/core/src/slice/mod.rs
+++ b/library/core/src/slice/mod.rs
@@ -1679,7 +1679,13 @@ pub const fn split_at_mut(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
         let ptr = self.as_ptr();
 
         // SAFETY: Caller has to check that `0 <= mid <= self.len()`
-        unsafe { (from_raw_parts(ptr, mid), from_raw_parts(ptr.add(mid), len - mid)) }
+        unsafe {
+            assert_unsafe_precondition!(
+                "slice::split_at_unchecked requires the index to be within the slice",
+                (mid: usize, len: usize) => mid <= len
+            );
+            (from_raw_parts(ptr, mid), from_raw_parts(ptr.add(mid), len - mid))
+        }
     }
 
     /// Divides one mutable slice into two at an index, without doing bounds checking.
diff --git a/library/core/src/str/traits.rs b/library/core/src/str/traits.rs
index d3ed811b157..a7a2b03774a 100644
--- a/library/core/src/str/traits.rs
+++ b/library/core/src/str/traits.rs
@@ -1,6 +1,7 @@
 //! Trait implementations for `str`.
 
 use crate::cmp::Ordering;
+use crate::intrinsics::assert_unsafe_precondition;
 use crate::ops;
 use crate::ptr;
 use crate::slice::SliceIndex;
@@ -198,7 +199,15 @@ unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
         let slice = slice as *const [u8];
         // SAFETY: the caller guarantees that `self` is in bounds of `slice`
         // which satisfies all the conditions for `add`.
-        let ptr = unsafe { slice.as_ptr().add(self.start) };
+        let ptr = unsafe {
+            let this = ops::Range { ..self };
+            assert_unsafe_precondition!(
+                "str::get_unchecked requires that the range is within the string slice",
+                (this: ops::Range<usize>, slice: *const [u8]) =>
+                this.end >= this.start && this.end <= slice.len()
+            );
+            slice.as_ptr().add(self.start)
+        };
         let len = self.end - self.start;
         ptr::slice_from_raw_parts(ptr, len) as *const str
     }
@@ -206,7 +215,15 @@ unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
     unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
         let slice = slice as *mut [u8];
         // SAFETY: see comments for `get_unchecked`.
-        let ptr = unsafe { slice.as_mut_ptr().add(self.start) };
+        let ptr = unsafe {
+            let this = ops::Range { ..self };
+            assert_unsafe_precondition!(
+                "str::get_unchecked_mut requires that the range is within the string slice",
+                (this: ops::Range<usize>, slice: *mut [u8]) =>
+                this.end >= this.start && this.end <= slice.len()
+            );
+            slice.as_mut_ptr().add(self.start)
+        };
         let len = self.end - self.start;
         ptr::slice_from_raw_parts_mut(ptr, len) as *mut str
     }
@@ -276,15 +293,13 @@ fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
     }
     #[inline]
     unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
-        let slice = slice as *const [u8];
-        let ptr = slice.as_ptr();
-        ptr::slice_from_raw_parts(ptr, self.end) as *const str
+        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
+        unsafe { (0..self.end).get_unchecked(slice) }
     }
     #[inline]
     unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
-        let slice = slice as *mut [u8];
-        let ptr = slice.as_mut_ptr();
-        ptr::slice_from_raw_parts_mut(ptr, self.end) as *mut str
+        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
+        unsafe { (0..self.end).get_unchecked_mut(slice) }
     }
     #[inline]
     fn index(self, slice: &str) -> &Self::Output {
@@ -347,20 +362,15 @@ fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
     }
     #[inline]
     unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
-        let slice = slice as *const [u8];
-        // SAFETY: the caller guarantees that `self` is in bounds of `slice`
-        // which satisfies all the conditions for `add`.
-        let ptr = unsafe { slice.as_ptr().add(self.start) };
-        let len = slice.len() - self.start;
-        ptr::slice_from_raw_parts(ptr, len) as *const str
+        let len = (slice as *const [u8]).len();
+        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
+        unsafe { (self.start..len).get_unchecked(slice) }
     }
     #[inline]
     unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
-        let slice = slice as *mut [u8];
-        // SAFETY: identical to `get_unchecked`.
-        let ptr = unsafe { slice.as_mut_ptr().add(self.start) };
-        let len = slice.len() - self.start;
-        ptr::slice_from_raw_parts_mut(ptr, len) as *mut str
+        let len = (slice as *mut [u8]).len();
+        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
+        unsafe { (self.start..len).get_unchecked_mut(slice) }
     }
     #[inline]
     fn index(self, slice: &str) -> &Self::Output {
@@ -456,35 +466,29 @@ unsafe impl const SliceIndex<str> for ops::RangeToInclusive<usize> {
     type Output = str;
     #[inline]
     fn get(self, slice: &str) -> Option<&Self::Output> {
-        if self.end == usize::MAX { None } else { (..self.end + 1).get(slice) }
+        (0..=self.end).get(slice)
     }
     #[inline]
     fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
-        if self.end == usize::MAX { None } else { (..self.end + 1).get_mut(slice) }
+        (0..=self.end).get_mut(slice)
     }
     #[inline]
     unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
         // SAFETY: the caller must uphold the safety contract for `get_unchecked`.
-        unsafe { (..self.end + 1).get_unchecked(slice) }
+        unsafe { (0..=self.end).get_unchecked(slice) }
     }
     #[inline]
     unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
         // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`.
-        unsafe { (..self.end + 1).get_unchecked_mut(slice) }
+        unsafe { (0..=self.end).get_unchecked_mut(slice) }
     }
     #[inline]
     fn index(self, slice: &str) -> &Self::Output {
-        if self.end == usize::MAX {
-            str_index_overflow_fail();
-        }
-        (..self.end + 1).index(slice)
+        (0..=self.end).index(slice)
     }
     #[inline]
     fn index_mut(self, slice: &mut str) -> &mut Self::Output {
-        if self.end == usize::MAX {
-            str_index_overflow_fail();
-        }
-        (..self.end + 1).index_mut(slice)
+        (0..=self.end).index_mut(slice)
     }
 }