auto merge of #16453 : nikomatsakis/rust/type-bounds-3, r=pcwalton

Implements https://github.com/rust-lang/rfcs/pull/192.

In particular:

1. type parameters can have lifetime bounds and objects can close over borrowed values, presuming that they have suitable bounds.
2. objects must have a bound, though it may be derived from the trait itself or from a `Send` bound.
3. all types must be well-formed.
4. type parameters and lifetime parameters may themselves have lifetimes as bounds. Something like `T:'a` means "the type T outlives 'a`" and something like `'a:'b`" means "'a outlives 'b". Outlives here means "all borrowed data has a lifetime at least as long".

This is a [breaking-change]. The most common things you have to fix after this change are:

1. Introduce lifetime bounds onto type parameters if your type (directly or indirectly) contains a reference. Thus a struct like `struct Ref<'a, T> { x: &'a T }` would be changed to `struct Ref<'a, T:'a> { x: &'a T }`.
2. Introduce lifetime bounds onto lifetime parameters if your type contains a double reference. Thus a type like `struct RefWrapper<'a, 'b> { r: &'a Ref<'b, int> }` (where `Ref` is defined as before) would need to be changed to `struct RefWrapper<'a, 'b:'a> { ... }`.
2. Explicitly give object lifetimes in structure definitions. Most commonly, this means changing something like `Box<Reader>` to `Box<Reader+'static>`, so as to indicate that this is a reader without any borrowed data. (Note: you may wish to just change to `Box<Reader+Send>` while you're at it; it's a more restrictive type, technically, but means you can send the reader between threads.)

The intuition for points 1 and 2 is that a reference must never outlive its referent (the thing it points at). Therefore, if you have a type `&'a T`, we must know that `T` (whatever it is) outlives `'a`. And so on.

Closes #5723.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 6a3e1fa2862..13d4a0a1f0a 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -105,9 +105,9 @@ pub trait BoxAny {
 }
 
 #[stable]
-impl BoxAny for Box<Any> {
+impl BoxAny for Box<Any+'static> {
     #[inline]
-    fn downcast<T: 'static>(self) -> Result<Box<T>, Box<Any>> {
+    fn downcast<T: 'static>(self) -> Result<Box<T>, Box<Any+'static>> {
         if self.is::<T>() {
             unsafe {
                 // Get the raw representation of the trait object
@@ -132,7 +132,7 @@ impl<T: fmt::Show> fmt::Show for Box<T> {
     }
 }
 
-impl fmt::Show for Box<Any> {
+impl fmt::Show for Box<Any+'static> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.pad("Box<Any>")
     }