Checked (and unchecked) slicing for strings?

What is this magic‽

5 files changed, 441 insertions, 56 deletions

diff --git a/src/libcollections/lib.rs b/src/libcollections/lib.rs
index 72e950bc91f..00448b6abb2 100644
--- a/src/libcollections/lib.rs
+++ b/src/libcollections/lib.rs
@@ -60,6 +60,7 @@
 #![feature(unicode)]
 #![feature(unique)]
 #![feature(untagged_unions)]
+#![cfg_attr(not(test), feature(str_checked_slicing))]
 #![cfg_attr(test, feature(rand, test))]
 
 #![no_std]
diff --git a/src/libcollections/slice.rs b/src/libcollections/slice.rs
index 5233887620a..424e175996e 100644
--- a/src/libcollections/slice.rs
+++ b/src/libcollections/slice.rs
@@ -362,7 +362,7 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn get<I>(&self, index: I) -> Option<&I::Output>
-        where I: SliceIndex<T>
+        where I: SliceIndex<Self>
     {
         core_slice::SliceExt::get(self, index)
     }
@@ -385,7 +385,7 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn get_mut<I>(&mut self, index: I) -> Option<&mut I::Output>
-        where I: SliceIndex<T>
+        where I: SliceIndex<Self>
     {
         core_slice::SliceExt::get_mut(self, index)
     }
@@ -405,7 +405,7 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub unsafe fn get_unchecked<I>(&self, index: I) -> &I::Output
-        where I: SliceIndex<T>
+        where I: SliceIndex<Self>
     {
         core_slice::SliceExt::get_unchecked(self, index)
     }
@@ -427,7 +427,7 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub unsafe fn get_unchecked_mut<I>(&mut self, index: I) -> &mut I::Output
-        where I: SliceIndex<T>
+        where I: SliceIndex<Self>
     {
         core_slice::SliceExt::get_unchecked_mut(self, index)
     }
diff --git a/src/libcollections/str.rs b/src/libcollections/str.rs
index 90e54a383d6..84b73090817 100644
--- a/src/libcollections/str.rs
+++ b/src/libcollections/str.rs
@@ -32,7 +32,7 @@ use borrow::{Borrow, ToOwned};
 use string::String;
 use std_unicode;
 use vec::Vec;
-use slice::SliceConcatExt;
+use slice::{SliceConcatExt, SliceIndex};
 use boxed::Box;
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -291,6 +291,114 @@ impl str {
         core_str::StrExt::as_ptr(self)
     }
 
+    /// Returns a subslice of `str`.
+    ///
+    /// This is the non-panicking alternative to indexing the `str`. Returns `None` whenever
+    /// equivalent indexing operation would panic.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(str_checked_slicing)]
+    /// let v = "🗻∈🌏";
+    /// assert_eq!(Some("🗻"), v.get(0..4));
+    /// assert!(v.get(1..).is_none());
+    /// assert!(v.get(..8).is_none());
+    /// assert!(v.get(..42).is_none());
+    /// ```
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    #[inline]
+    pub fn get<I: SliceIndex<str>>(&self, i: I) -> Option<&I::Output> {
+        core_str::StrExt::get(self, i)
+    }
+
+    /// Returns a mutable subslice of `str`.
+    ///
+    /// This is the non-panicking alternative to indexing the `str`. Returns `None` whenever
+    /// equivalent indexing operation would panic.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(str_checked_slicing)]
+    /// let mut v = String::from("🗻∈🌏");
+    /// assert_eq!(Some("🗻"), v.get_mut(0..4).map(|v| &*v));
+    /// assert!(v.get_mut(1..).is_none());
+    /// assert!(v.get_mut(..8).is_none());
+    /// assert!(v.get_mut(..42).is_none());
+    /// ```
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    #[inline]
+    pub fn get_mut<I: SliceIndex<str>>(&mut self, i: I) -> Option<&mut I::Output> {
+        core_str::StrExt::get_mut(self, i)
+    }
+
+    /// Returns a unchecked subslice of `str`.
+    ///
+    /// This is the unchecked alternative to indexing the `str`.
+    ///
+    /// # Safety
+    ///
+    /// Callers of this function are responsible that these preconditions are
+    /// satisfied:
+    ///
+    /// * The starting index must come before the ending index;
+    /// * Indexes must be within bounds of the original slice;
+    /// * Indexes must lie on UTF-8 sequence boundaries.
+    ///
+    /// Failing that, the returned string slice may reference invalid memory or
+    /// violate the invariants communicated by the `str` type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(str_checked_slicing)]
+    /// let v = "🗻∈🌏";
+    /// unsafe {
+    ///     assert_eq!("🗻", v.get_unchecked(0..4));
+    ///     assert_eq!("∈", v.get_unchecked(4..7));
+    ///     assert_eq!("🌏", v.get_unchecked(7..11));
+    /// }
+    /// ```
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    #[inline]
+    pub unsafe fn get_unchecked<I: SliceIndex<str>>(&self, i: I) -> &I::Output {
+        core_str::StrExt::get_unchecked(self, i)
+    }
+
+    /// Returns a mutable, unchecked subslice of `str`.
+    ///
+    /// This is the unchecked alternative to indexing the `str`.
+    ///
+    /// # Safety
+    ///
+    /// Callers of this function are responsible that these preconditions are
+    /// satisfied:
+    ///
+    /// * The starting index must come before the ending index;
+    /// * Indexes must be within bounds of the original slice;
+    /// * Indexes must lie on UTF-8 sequence boundaries.
+    ///
+    /// Failing that, the returned string slice may reference invalid memory or
+    /// violate the invariants communicated by the `str` type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(str_checked_slicing)]
+    /// let mut v = String::from("🗻∈🌏");
+    /// unsafe {
+    ///     assert_eq!("🗻", v.get_unchecked_mut(0..4));
+    ///     assert_eq!("∈", v.get_unchecked_mut(4..7));
+    ///     assert_eq!("🌏", v.get_unchecked_mut(7..11));
+    /// }
+    /// ```
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    #[inline]
+    pub unsafe fn get_unchecked_mut<I: SliceIndex<str>>(&mut self, i: I) -> &mut I::Output {
+        core_str::StrExt::get_unchecked_mut(self, i)
+    }
+
     /// Creates a string slice from another string slice, bypassing safety
     /// checks.
     ///
diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index 6f8b199f886..4e56fa80cd9 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -97,8 +97,7 @@ pub trait SliceExt {
 
     #[stable(feature = "core", since = "1.6.0")]
     fn get<I>(&self, index: I) -> Option<&I::Output>
-        where I: SliceIndex<Self::Item>;
-
+        where I: SliceIndex<Self>;
     #[stable(feature = "core", since = "1.6.0")]
     fn first(&self) -> Option<&Self::Item>;
 
@@ -113,8 +112,7 @@ pub trait SliceExt {
 
     #[stable(feature = "core", since = "1.6.0")]
     unsafe fn get_unchecked<I>(&self, index: I) -> &I::Output
-        where I: SliceIndex<Self::Item>;
-
+        where I: SliceIndex<Self>;
     #[stable(feature = "core", since = "1.6.0")]
     fn as_ptr(&self) -> *const Self::Item;
 
@@ -141,8 +139,7 @@ pub trait SliceExt {
 
     #[stable(feature = "core", since = "1.6.0")]
     fn get_mut<I>(&mut self, index: I) -> Option<&mut I::Output>
-        where I: SliceIndex<Self::Item>;
-
+        where I: SliceIndex<Self>;
     #[stable(feature = "core", since = "1.6.0")]
     fn iter_mut(&mut self) -> IterMut<Self::Item>;
 
@@ -184,8 +181,7 @@ pub trait SliceExt {
 
     #[stable(feature = "core", since = "1.6.0")]
     unsafe fn get_unchecked_mut<I>(&mut self, index: I) -> &mut I::Output
-        where I: SliceIndex<Self::Item>;
-
+        where I: SliceIndex<Self>;
     #[stable(feature = "core", since = "1.6.0")]
     fn as_mut_ptr(&mut self) -> *mut Self::Item;
 
@@ -337,7 +333,7 @@ impl<T> SliceExt for [T] {
 
     #[inline]
     fn get<I>(&self, index: I) -> Option<&I::Output>
-        where I: SliceIndex<T>
+        where I: SliceIndex<[T]>
     {
         index.get(self)
     }
@@ -365,7 +361,7 @@ impl<T> SliceExt for [T] {
 
     #[inline]
     unsafe fn get_unchecked<I>(&self, index: I) -> &I::Output
-        where I: SliceIndex<T>
+        where I: SliceIndex<[T]>
     {
         index.get_unchecked(self)
     }
@@ -406,7 +402,7 @@ impl<T> SliceExt for [T] {
 
     #[inline]
     fn get_mut<I>(&mut self, index: I) -> Option<&mut I::Output>
-        where I: SliceIndex<T>
+        where I: SliceIndex<[T]>
     {
         index.get_mut(self)
     }
@@ -538,7 +534,7 @@ impl<T> SliceExt for [T] {
 
     #[inline]
     unsafe fn get_unchecked_mut<I>(&mut self, index: I) -> &mut I::Output
-        where I: SliceIndex<T>
+        where I: SliceIndex<[T]>
     {
         index.get_unchecked_mut(self)
     }
@@ -631,7 +627,7 @@ impl<T> SliceExt for [T] {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_on_unimplemented = "slice indices are of type `usize` or ranges of `usize`"]
 impl<T, I> ops::Index<I> for [T]
-    where I: SliceIndex<T>
+    where I: SliceIndex<[T]>
 {
     type Output = I::Output;
 
@@ -644,7 +640,7 @@ impl<T, I> ops::Index<I> for [T]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_on_unimplemented = "slice indices are of type `usize` or ranges of `usize`"]
 impl<T, I> ops::IndexMut<I> for [T]
-    where I: SliceIndex<T>
+    where I: SliceIndex<[T]>
 {
     #[inline]
     fn index_mut(&mut self, index: I) -> &mut I::Output {
@@ -667,37 +663,37 @@ fn slice_index_order_fail(index: usize, end: usize) -> ! {
 /// A helper trait used for indexing operations.
 #[unstable(feature = "slice_get_slice", issue = "35729")]
 #[rustc_on_unimplemented = "slice indices are of type `usize` or ranges of `usize`"]
-pub trait SliceIndex<T> {
+pub trait SliceIndex<T: ?Sized> {
     /// The output type returned by methods.
     type Output: ?Sized;
 
     /// Returns a shared reference to the output at this location, if in
     /// bounds.
-    fn get(self, slice: &[T]) -> Option<&Self::Output>;
+    fn get(self, slice: &T) -> Option<&Self::Output>;
 
     /// Returns a mutable reference to the output at this location, if in
     /// bounds.
-    fn get_mut(self, slice: &mut [T]) -> Option<&mut Self::Output>;
+    fn get_mut(self, slice: &mut T) -> Option<&mut Self::Output>;
 
     /// Returns a shared reference to the output at this location, without
     /// performing any bounds checking.
-    unsafe fn get_unchecked(self, slice: &[T]) -> &Self::Output;
+    unsafe fn get_unchecked(self, slice: &T) -> &Self::Output;
 
     /// Returns a mutable reference to the output at this location, without
     /// performing any bounds checking.
-    unsafe fn get_unchecked_mut(self, slice: &mut [T]) -> &mut Self::Output;
+    unsafe fn get_unchecked_mut(self, slice: &mut T) -> &mut Self::Output;
 
     /// Returns a shared reference to the output at this location, panicking
     /// if out of bounds.
-    fn index(self, slice: &[T]) -> &Self::Output;
+    fn index(self, slice: &T) -> &Self::Output;
 
     /// Returns a mutable reference to the output at this location, panicking
     /// if out of bounds.
-    fn index_mut(self, slice: &mut [T]) -> &mut Self::Output;
+    fn index_mut(self, slice: &mut T) -> &mut Self::Output;
 }
 
 #[stable(feature = "slice-get-slice-impls", since = "1.15.0")]
-impl<T> SliceIndex<T> for usize {
+impl<T> SliceIndex<[T]> for usize {
     type Output = T;
 
     #[inline]
@@ -746,7 +742,7 @@ impl<T> SliceIndex<T> for usize {
 }
 
 #[stable(feature = "slice-get-slice-impls", since = "1.15.0")]
-impl<T> SliceIndex<T> for  ops::Range<usize> {
+impl<T> SliceIndex<[T]> for  ops::Range<usize> {
     type Output = [T];
 
     #[inline]
@@ -807,7 +803,7 @@ impl<T> SliceIndex<T> for  ops::Range<usize> {
 }
 
 #[stable(feature = "slice-get-slice-impls", since = "1.15.0")]
-impl<T> SliceIndex<T> for ops::RangeTo<usize> {
+impl<T> SliceIndex<[T]> for ops::RangeTo<usize> {
     type Output = [T];
 
     #[inline]
@@ -842,7 +838,7 @@ impl<T> SliceIndex<T> for ops::RangeTo<usize> {
 }
 
 #[stable(feature = "slice-get-slice-impls", since = "1.15.0")]
-impl<T> SliceIndex<T> for ops::RangeFrom<usize> {
+impl<T> SliceIndex<[T]> for ops::RangeFrom<usize> {
     type Output = [T];
 
     #[inline]
@@ -877,7 +873,7 @@ impl<T> SliceIndex<T> for ops::RangeFrom<usize> {
 }
 
 #[stable(feature = "slice-get-slice-impls", since = "1.15.0")]
-impl<T> SliceIndex<T> for ops::RangeFull {
+impl<T> SliceIndex<[T]> for ops::RangeFull {
     type Output = [T];
 
     #[inline]
@@ -913,7 +909,7 @@ impl<T> SliceIndex<T> for ops::RangeFull {
 
 
 #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
-impl<T> SliceIndex<T> for ops::RangeInclusive<usize> {
+impl<T> SliceIndex<[T]> for ops::RangeInclusive<usize> {
     type Output = [T];
 
     #[inline]
@@ -976,7 +972,7 @@ impl<T> SliceIndex<T> for ops::RangeInclusive<usize> {
 }
 
 #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
-impl<T> SliceIndex<T> for ops::RangeToInclusive<usize> {
+impl<T> SliceIndex<[T]> for ops::RangeToInclusive<usize> {
     type Output = [T];
 
     #[inline]
diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index cf3e8a684df..aecfaa7ee02 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -22,7 +22,7 @@ use convert::TryFrom;
 use fmt;
 use iter::{Map, Cloned, FusedIterator};
 use mem;
-use slice;
+use slice::{self, SliceIndex};
 
 pub mod pattern;
 
@@ -1408,6 +1408,8 @@ Section: Trait implementations
 mod traits {
     use cmp::Ordering;
     use ops;
+    use mem;
+    use slice::{self, SliceIndex};
     use str::eq_slice;
 
     /// Implements ordering of strings.
@@ -1490,14 +1492,7 @@ mod traits {
         type Output = str;
         #[inline]
         fn index(&self, index: ops::Range<usize>) -> &str {
-            // is_char_boundary checks that the index is in [0, .len()]
-            if index.start <= index.end &&
-               self.is_char_boundary(index.start) &&
-               self.is_char_boundary(index.end) {
-                unsafe { self.slice_unchecked(index.start, index.end) }
-            } else {
-                super::slice_error_fail(self, index.start, index.end)
-            }
+            index.index(self)
         }
     }
 
@@ -1519,14 +1514,7 @@ mod traits {
     impl ops::IndexMut<ops::Range<usize>> for str {
         #[inline]
         fn index_mut(&mut self, index: ops::Range<usize>) -> &mut str {
-            // is_char_boundary checks that the index is in [0, .len()]
-            if index.start <= index.end &&
-               self.is_char_boundary(index.start) &&
-               self.is_char_boundary(index.end) {
-                unsafe { self.slice_mut_unchecked(index.start, index.end) }
-            } else {
-                super::slice_error_fail(self, index.start, index.end)
-            }
+            index.index_mut(self)
         }
     }
 
@@ -1694,8 +1682,276 @@ mod traits {
             self.index_mut(0...index.end)
         }
     }
+
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    impl SliceIndex<str> for ops::RangeFull {
+        type Output = str;
+        #[inline]
+        fn get(self, slice: &str) -> Option<&Self::Output> {
+            Some(slice)
+        }
+        #[inline]
+        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
+            Some(slice)
+        }
+        #[inline]
+        unsafe fn get_unchecked(self, slice: &str) -> &Self::Output {
+            slice
+        }
+        #[inline]
+        unsafe fn get_unchecked_mut(self, slice: &mut str) -> &mut Self::Output {
+            slice
+        }
+        #[inline]
+        fn index(self, slice: &str) -> &Self::Output {
+            slice
+        }
+        #[inline]
+        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
+            slice
+        }
+    }
+
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    impl SliceIndex<str> for ops::Range<usize> {
+        type Output = str;
+        #[inline]
+        fn get(self, slice: &str) -> Option<&Self::Output> {
+            if self.start <= self.end &&
+               slice.is_char_boundary(self.start) &&
+               slice.is_char_boundary(self.end) {
+                Some(unsafe { self.get_unchecked(slice) })
+            } else {
+                None
+            }
+        }
+        #[inline]
+        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
+            if self.start <= self.end &&
+               slice.is_char_boundary(self.start) &&
+               slice.is_char_boundary(self.end) {
+                Some(unsafe { self.get_unchecked_mut(slice) })
+            } else {
+                None
+            }
+        }
+        #[inline]
+        unsafe fn get_unchecked(self, slice: &str) -> &Self::Output {
+            let ptr = slice.as_ptr().offset(self.start as isize);
+            let len = self.end - self.start;
+            super::from_utf8_unchecked(slice::from_raw_parts(ptr, len))
+        }
+        #[inline]
+        unsafe fn get_unchecked_mut(self, slice: &mut str) -> &mut Self::Output {
+            let ptr = slice.as_ptr().offset(self.start as isize);
+            let len = self.end - self.start;
+            mem::transmute(slice::from_raw_parts_mut(ptr as *mut u8, len))
+        }
+        #[inline]
+        fn index(self, slice: &str) -> &Self::Output {
+            let (start, end) = (self.start, self.end);
+            self.get(slice).unwrap_or_else(|| super::slice_error_fail(slice, start, end))
+        }
+        #[inline]
+        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
+            // is_char_boundary checks that the index is in [0, .len()]
+            // canot reuse `get` as above, because of NLL trouble
+            if self.start <= self.end &&
+               slice.is_char_boundary(self.start) &&
+               slice.is_char_boundary(self.end) {
+                unsafe { self.get_unchecked_mut(slice) }
+            } else {
+                super::slice_error_fail(slice, self.start, self.end)
+            }
+        }
+    }
+
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    impl SliceIndex<str> for ops::RangeTo<usize> {
+        type Output = str;
+        #[inline]
+        fn get(self, slice: &str) -> Option<&Self::Output> {
+            if slice.is_char_boundary(self.end) {
+                Some(unsafe { self.get_unchecked(slice) })
+            } else {
+                None
+            }
+        }
+        #[inline]
+        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
+            if slice.is_char_boundary(self.end) {
+                Some(unsafe { self.get_unchecked_mut(slice) })
+            } else {
+                None
+            }
+        }
+        #[inline]
+        unsafe fn get_unchecked(self, slice: &str) -> &Self::Output {
+            let ptr = slice.as_ptr();
+            super::from_utf8_unchecked(slice::from_raw_parts(ptr, self.end))
+        }
+        #[inline]
+        unsafe fn get_unchecked_mut(self, slice: &mut str) -> &mut Self::Output {
+            let ptr = slice.as_ptr();
+            mem::transmute(slice::from_raw_parts_mut(ptr as *mut u8, self.end))
+        }
+        #[inline]
+        fn index(self, slice: &str) -> &Self::Output {
+            let end = self.end;
+            self.get(slice).unwrap_or_else(|| super::slice_error_fail(slice, 0, end))
+        }
+        #[inline]
+        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
+            if slice.is_char_boundary(self.end) {
+                unsafe { self.get_unchecked_mut(slice) }
+            } else {
+                super::slice_error_fail(slice, 0, self.end)
+            }
+        }
+    }
+
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    impl SliceIndex<str> for ops::RangeFrom<usize> {
+        type Output = str;
+        #[inline]
+        fn get(self, slice: &str) -> Option<&Self::Output> {
+            if slice.is_char_boundary(self.start) {
+                Some(unsafe { self.get_unchecked(slice) })
+            } else {
+                None
+            }
+        }
+        #[inline]
+        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
+            if slice.is_char_boundary(self.start) {
+                Some(unsafe { self.get_unchecked_mut(slice) })
+            } else {
+                None
+            }
+        }
+        #[inline]
+        unsafe fn get_unchecked(self, slice: &str) -> &Self::Output {
+            let ptr = slice.as_ptr().offset(self.start as isize);
+            let len = slice.len() - self.start;
+            super::from_utf8_unchecked(slice::from_raw_parts(ptr, len))
+        }
+        #[inline]
+        unsafe fn get_unchecked_mut(self, slice: &mut str) -> &mut Self::Output {
+            let ptr = slice.as_ptr().offset(self.start as isize);
+            let len = slice.len() - self.start;
+            mem::transmute(slice::from_raw_parts_mut(ptr as *mut u8, len))
+        }
+        #[inline]
+        fn index(self, slice: &str) -> &Self::Output {
+            let (start, end) = (self.start, slice.len());
+            self.get(slice).unwrap_or_else(|| super::slice_error_fail(slice, start, end))
+        }
+        #[inline]
+        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
+            if slice.is_char_boundary(self.start) {
+                unsafe { self.get_unchecked_mut(slice) }
+            } else {
+                super::slice_error_fail(slice, self.start, slice.len())
+            }
+        }
+    }
+
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    impl SliceIndex<str> for ops::RangeInclusive<usize> {
+        type Output = str;
+        #[inline]
+        fn get(self, slice: &str) -> Option<&Self::Output> {
+            match self {
+                ops::RangeInclusive::Empty { .. } => 0..0,
+                ops::RangeInclusive::NonEmpty { start, end } => start..end+1,
+            }.get(slice)
+        }
+        #[inline]
+        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
+            match self {
+                ops::RangeInclusive::Empty { .. } => 0..0,
+                ops::RangeInclusive::NonEmpty { start, end } => start..end+1,
+            }.get_mut(slice)
+        }
+        #[inline]
+        unsafe fn get_unchecked(self, slice: &str) -> &Self::Output {
+            match self {
+                ops::RangeInclusive::Empty { .. } => 0..0,
+                ops::RangeInclusive::NonEmpty { start, end } => start..end+1,
+            }.get_unchecked(slice)
+        }
+        #[inline]
+        unsafe fn get_unchecked_mut(self, slice: &mut str) -> &mut Self::Output {
+            match self {
+                ops::RangeInclusive::Empty { .. } => 0..0,
+                ops::RangeInclusive::NonEmpty { start, end } => start..end+1,
+            }.get_unchecked_mut(slice)
+        }
+        #[inline]
+        fn index(self, slice: &str) -> &Self::Output {
+            match self {
+                ops::RangeInclusive::Empty { .. } => 0..0,
+                ops::RangeInclusive::NonEmpty { start, end } => start..end+1,
+            }.index(slice)
+        }
+        #[inline]
+        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
+            match self {
+                ops::RangeInclusive::Empty { .. } => 0..0,
+                ops::RangeInclusive::NonEmpty { start, end } => start..end+1,
+            }.index_mut(slice)
+        }
+    }
+
+
+
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    impl SliceIndex<str> for ops::RangeToInclusive<usize> {
+        type Output = str;
+        #[inline]
+        fn get(self, slice: &str) -> Option<&Self::Output> {
+            if slice.is_char_boundary(self.end + 1) {
+                Some(unsafe { self.get_unchecked(slice) })
+            } else {
+                None
+            }
+        }
+        #[inline]
+        fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
+            if slice.is_char_boundary(self.end + 1) {
+                Some(unsafe { self.get_unchecked_mut(slice) })
+            } else {
+                None
+            }
+        }
+        #[inline]
+        unsafe fn get_unchecked(self, slice: &str) -> &Self::Output {
+            let ptr = slice.as_ptr();
+            super::from_utf8_unchecked(slice::from_raw_parts(ptr, self.end + 1))
+        }
+        #[inline]
+        unsafe fn get_unchecked_mut(self, slice: &mut str) -> &mut Self::Output {
+            let ptr = slice.as_ptr();
+            mem::transmute(slice::from_raw_parts_mut(ptr as *mut u8, self.end + 1))
+        }
+        #[inline]
+        fn index(self, slice: &str) -> &Self::Output {
+            let end = self.end + 1;
+            self.get(slice).unwrap_or_else(|| super::slice_error_fail(slice, 0, end))
+        }
+        #[inline]
+        fn index_mut(self, slice: &mut str) -> &mut Self::Output {
+            if slice.is_char_boundary(self.end) {
+                unsafe { self.get_unchecked_mut(slice) }
+            } else {
+                super::slice_error_fail(slice, 0, self.end + 1)
+            }
+        }
+    }
+
 }
 
+
 /// Methods for string slices
 #[allow(missing_docs)]
 #[doc(hidden)]
@@ -1745,6 +2001,14 @@ pub trait StrExt {
     #[rustc_deprecated(since = "1.6.0", reason = "use lines() instead now")]
     #[allow(deprecated)]
     fn lines_any(&self) -> LinesAny;
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    fn get<I: SliceIndex<str>>(&self, i: I) -> Option<&I::Output>;
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    fn get_mut<I: SliceIndex<str>>(&mut self, i: I) -> Option<&mut I::Output>;
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    unsafe fn get_unchecked<I: SliceIndex<str>>(&self, i: I) -> &I::Output;
+    #[unstable(feature = "str_checked_slicing", issue = "0")]
+    unsafe fn get_unchecked_mut<I: SliceIndex<str>>(&mut self, i: I) -> &mut I::Output;
     #[stable(feature = "core", since = "1.6.0")]
     unsafe fn slice_unchecked(&self, begin: usize, end: usize) -> &str;
     #[stable(feature = "core", since = "1.6.0")]
@@ -1935,17 +2199,33 @@ impl StrExt for str {
     }
 
     #[inline]
+    fn get<I: SliceIndex<str>>(&self, i: I) -> Option<&I::Output> {
+        i.get(self)
+    }
+
+    #[inline]
+    fn get_mut<I: SliceIndex<str>>(&mut self, i: I) -> Option<&mut I::Output> {
+        i.get_mut(self)
+    }
+
+    #[inline]
+    unsafe fn get_unchecked<I: SliceIndex<str>>(&self, i: I) -> &I::Output {
+        i.get_unchecked(self)
+    }
+
+    #[inline]
+    unsafe fn get_unchecked_mut<I: SliceIndex<str>>(&mut self, i: I) -> &mut I::Output {
+        i.get_unchecked_mut(self)
+    }
+
+    #[inline]
     unsafe fn slice_unchecked(&self, begin: usize, end: usize) -> &str {
-        let ptr = self.as_ptr().offset(begin as isize);
-        let len = end - begin;
-        from_utf8_unchecked(slice::from_raw_parts(ptr, len))
+        (begin..end).get_unchecked(self)
     }
 
     #[inline]
     unsafe fn slice_mut_unchecked(&mut self, begin: usize, end: usize) -> &mut str {
-        let ptr = self.as_ptr().offset(begin as isize);
-        let len = end - begin;
-        mem::transmute(slice::from_raw_parts_mut(ptr as *mut u8, len))
+        (begin..end).get_unchecked_mut(self)
     }
 
     #[inline]