auto merge of #18105 : nikomatsakis/rust/issue-18055, r=pcwalton

auto merge of #18105 : nikomatsakis/rust/issue-18055, r=pcwalton

Check object lifetime bounds in coercions, not just trait bounds.  Fixes #18055.

r? @pcwalton

This is a [breaking change]. Change code like this:

    fn foo(v: &[u8]) -> Box<Clone+'static> { ... }

to make the lifetimes agree:

    // either...
    fn foo(v: &'static[u8]) -> Box<Clone+'static> { box v }

    // or ...
    fn foo<'a>(v: &'a [u8]) -> Box<Clone+'a> { box v }