fix borrow-splitting

Author: Gankra

src/doc/tarpl/SUMMARY.md

@@ -17,7 +17,7 @@
 	* [Subtyping and Variance](subtyping.md)
 	* [Drop Check](dropck.md)
 	* [PhantomData](phantom-data.md)
-	* [Splitting Lifetimes](lifetime-splitting.md)
+	* [Splitting Borrows](borrow-splitting.md)
 * [Type Conversions](conversions.md)
 	* [Coercions](coercions.md)
 	* [The Dot Operator](dot-operator.md)

src/doc/tarpl/lifetime-splitting.md renamed to src/doc/tarpl/borrow-splitting.md

@@ -1,4 +1,4 @@
-% Splitting Lifetimes
+% Splitting Borrows
 
 The mutual exclusion property of mutable references can be very limiting when
 working with a composite structure. The borrow checker understands some basic
@@ -67,9 +67,8 @@ fn split_at_mut(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
 }
 ```
 
-This is pretty plainly dangerous. We use transmute to duplicate the slice with
-an *unbounded* lifetime, so that it can be treated as disjoint from the other
-until we unify them when we return.
+This is actually a bit subtle. So as to avoid ever making two `&mut`'s to the
+same value, we explicitly construct brand-new slices through raw pointers.
 
 However more subtle is how iterators that yield mutable references work.
 The iterator trait is defined as follows:

src/doc/tarpl/unchecked-uninit.md

@@ -15,11 +15,11 @@ initialization.
 
 Unfortunately, this opens us up to all kinds of problems. Assignment has a
 different meaning to Rust based on whether it believes that a variable is
-initialized or not. If it's uninitialized, then Rust will semantically just
-memcopy the bits over the uninitialized ones, and do nothing else. However if Rust
-believes a value to be initialized, it will try to `Drop` the old value!
-Since we've tricked Rust into believing that the value is initialized, we
-can no longer safely use normal assignment.
+initialized or not. If it's believed uninitialized, then Rust will semantically
+just memcopy the bits over the uninitialized ones, and do nothing else. However
+if Rust believes a value to be initialized, it will try to `Drop` the old value!
+Since we've tricked Rust into believing that the value is initialized, we can no
+longer safely use normal assignment.
 
 This is also a problem if you're working with a raw system allocator, which
 returns a pointer to uninitialized memory.