rust/src/librustc_error_codes/error_codes/E0178.md

In types, the + type operator has low precedence, so it is often necessary to use parentheses.

For example:

trait Foo {}

struct Bar<'a> {
    w: &'a Foo + Copy,   // error, use &'a (Foo + Copy)
    x: &'a Foo + 'a,     // error, use &'a (Foo + 'a)
    y: &'a mut Foo + 'a, // error, use &'a mut (Foo + 'a)
    z: fn() -> Foo + 'a, // error, use fn() -> (Foo + 'a)
}

More details can be found in RFC 438.