Rust error E0398: #### Note: this error code is no longer emitted by the compiler

The compiler team's explanation

Note: this error code is no longer emitted by the compiler.

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).

From rustc --explain E0398 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.