In Rust 1.3, the default object lifetime bounds are expected to change, as described in RFC 1156. You are getting a warning because the compiler thinks it is possible that this change will cause a compilation error in your code. It is possible, though unlikely, that this is a false alarm.
The heart of the change is that where &'a Box<SomeTrait> used to default to
&'a Box<SomeTrait+'a>, it now defaults to &'a Box<SomeTrait+'static> (here,
SomeTrait is the name of some trait type). Note that the only types which are
affected are references to boxes, like &Box<SomeTrait> or
&[Box<SomeTrait>]. More common types like &SomeTrait or Box<SomeTrait>
are unaffected.
To silence this warning, edit your code to use an explicit bound. Most of the
time, this means that you will want to change the signature of a function that
you are calling. For example, if the error is reported on a call like foo(x),
and foo is defined as follows:
fn foo(arg: &Box<SomeTrait>) { /* ... */ }You might change it to:
fn foo<'a>(arg: &'a Box<SomeTrait+'a>) { /* ... */ }This explicitly states that you expect the trait object SomeTrait to contain
references (with a maximum lifetime of 'a).