Add panic-safe slicing methods

text/0000-panic-safe-slicing.md

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+- Feature Name: panic_safe_slicing
+- Start Date: 2015-10-16
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Add "panic-safe" or "total" alternatives to the existing panicking slicing syntax.
+
+# Motivation
+
+`SliceExt::get` and `SliceExt::get_mut` can be thought as non-panicking versions of the simple
+slicing syntax, `a[idx]`. However, there is no such equivalent for `a[start..end]`, `a[start..]`,
+or `a[..end]`. This RFC proposes such methods to fill the gap.
+
+# Detailed design
+
+Add `get_range`, `get_range_mut`, `get_range_unchecked`, `get_range_unchecked_mut` to `SliceExt`.
+
+`get_range` and `get_range_mut` may be implemented roughly as follows:
+
+```rust
+use std::ops::{RangeFrom, RangeTo, Range};
+use std::slice::from_raw_parts;
+use core::slice::SliceExt;
+
+trait Rangeable<T: ?Sized> {
+    fn start(&self, slice: &T) -> usize;
+    fn end(&self, slice: &T) -> usize;
+}
+
+impl<T: SliceExt + ?Sized> Rangeable<T> for RangeFrom<usize> {
+    fn start(&self, _: &T) -> usize { self.start }
+    fn end(&self, slice: &T) -> usize { slice.len() }
+}
+
+impl<T: SliceExt + ?Sized> Rangeable<T> for RangeTo<usize> {
+    fn start(&self, _: &T) -> usize { 0 }
+    fn end(&self, _: &T) -> usize { self.end }
+}
+
+impl<T: SliceExt + ?Sized> Rangeable<T> for Range<usize> {
+    fn start(&self, _: &T) -> usize { self.start }
+    fn end(&self, _: &T) -> usize { self.end }
+}
+
+trait GetRangeExt: SliceExt {
+    fn get_range<R: Rangeable<Self>>(&self, range: R) -> Option<&[Self::Item]>;
+}
+
+impl<T> GetRangeExt for [T] {
+    fn get_range<R: Rangeable<Self>>(&self, range: R) -> Option<&[T]> {
+        let start = range.start(self);
+        let end = range.end(self);
+
+        if start > end { return None; }
+        if end > self.len() { return None; }
+
+        unsafe { Some(from_raw_parts(self.as_ptr().offset(start as isize), end - start)) }
+    }
+}
+
+fn main() {
+    let a = [1, 2, 3, 4, 5];
+
+    assert_eq!(a.get_range(1..), Some(&a[1..]));
+    assert_eq!(a.get_range(..3), Some(&a[..3]));
+    assert_eq!(a.get_range(2..5), Some(&a[2..5]));
+    assert_eq!(a.get_range(..6), None);
+    assert_eq!(a.get_range(4..2), None);
+}
+```
+
+`get_range_unchecked` and `get_range_unchecked_mut` should be the unchecked versions of the methods
+above.
+
+# Drawbacks
+
+- Are these methods worth adding to `std`? Are such use cases common to justify such extention?
+
+# Alternatives
+
+- Stay as is.
+- Could there be any other (and better!) total functions that serve the similar purpose?
+
+# Unresolved questions
+
+- Naming, naming, naming: Is `get_range` the most suitable name? How about `get_slice`, or just
+  `slice`? Or any others?