Minor improvements

* Prefer compile_fail over ignore, to catch if the code accidentally
  starts compiling in future versions.
* Make an example compile by providing mocked environment around it in
  case it should be correct.
* Links into std.

src/borrow-splitting.md

@@ -26,7 +26,7 @@ println!("{} {} {} {}", a, b, c, c2);
 However borrowck doesn't understand arrays or slices in any way, so this doesn't
 work:
 
-```rust,ignore
+```rust,compile_fail
 let mut x = [1, 2, 3];
 let a = &mut x[0];
 let b = &mut x[1];
@@ -60,7 +60,12 @@ the left of the index, and one for everything to the right. Intuitively we know
 this is safe because the slices don't overlap, and therefore alias. However
 the implementation requires some unsafety:
 
-```rust,ignore
+```rust
+# use std::slice::from_raw_parts_mut;
+# struct FakeSlice<T>(T);
+# impl<T> FakeSlice<T> {
+# fn len(&self) -> usize { unimplemented!() }
+# fn as_mut_ptr(&mut self) -> *mut T { unimplemented!() }
 fn split_at_mut(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
     let len = self.len();
     let ptr = self.as_mut_ptr();
@@ -70,6 +75,7 @@ fn split_at_mut(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
          from_raw_parts_mut(ptr.offset(mid as isize), len - mid))
     }
 }
+# }
 ```
 
 This is actually a bit subtle. So as to avoid ever making two `&mut`'s to the

src/checked-uninit.md

@@ -4,7 +4,7 @@ Like C, all stack variables in Rust are uninitialized until a value is
 explicitly assigned to them. Unlike C, Rust statically prevents you from ever
 reading them until you do:
 
-```rust,ignore
+```rust,compile_fail
 fn main() {
     let x: i32;
     println!("{}", x);
@@ -39,7 +39,7 @@ fn main() {
 
 but this doesn't:
 
-```rust,ignore
+```rust,compile_fail
 fn main() {
     let x: i32;
     if true {

src/phantom-data.md

@@ -5,7 +5,7 @@ types or lifetimes are logically associated with a struct, but not actually
 part of a field. This most commonly occurs with lifetimes. For instance, the
 `Iter` for `&'a [T]` is (approximately) defined as follows:
 
-```rust,ignore
+```rust,compile_fail
 struct Iter<'a, T: 'a> {
     ptr: *const T,
     end: *const T,

src/transmutes.md

@@ -6,9 +6,10 @@ really can't emphasize that you should deeply think about finding Another Way
 than the operations covered in this section. This is really, truly, the most
 horribly unsafe thing you can do in Rust. The railguards here are dental floss.
 
-`mem::transmute<T, U>` takes a value of type `T` and reinterprets it to have
-type `U`. The only restriction is that the `T` and `U` are verified to have the
-same size. The ways to cause Undefined Behavior with this are mind boggling.
+[`mem::transmute<T, U>`][transmute] takes a value of type `T` and reinterprets
+it to have type `U`. The only restriction is that the `T` and `U` are verified
+to have the same size. The ways to cause Undefined Behavior with this are mind
+boggling.
 
 * First and foremost, creating an instance of *any* type with an invalid state
   is going to cause arbitrary chaos that can't really be predicted.
@@ -23,13 +24,16 @@ same size. The ways to cause Undefined Behavior with this are mind boggling.
 * Transmuting to a reference without an explicitly provided lifetime
   produces an [unbounded lifetime]
 
-`mem::transmute_copy<T, U>` somehow manages to be *even more* wildly unsafe than
-this. It copies `size_of<U>` bytes out of an `&T` and interprets them as a `U`.
-The size check that `mem::transmute` has is gone (as it may be valid to copy
-out a prefix), though it is Undefined Behavior for `U` to be larger than `T`.
+[`mem::transmute_copy<T, U>`][transmute_copy] somehow manages to be *even more*
+wildly unsafe than this. It copies `size_of<U>` bytes out of an `&T` and
+interprets them as a `U`.  The size check that `mem::transmute` has is gone (as
+it may be valid to copy out a prefix), though it is Undefined Behavior for `U`
+to be larger than `T`.
 
 Also of course you can get most of the functionality of these functions using
 pointer casts.
 
 
 [unbounded lifetime]: unbounded-lifetimes.html
+[transmute]: ../std/mem/fn.transmute.html
+[transmute_copy]: ../std/mem/fn.transmute.html