Static EXPLICIT_OUTLIVES_REQUIREMENTS 

pub static EXPLICIT_OUTLIVES_REQUIREMENTS: &'static Lint

The explicit_outlives_requirements lint detects unnecessary lifetime bounds that can be inferred.

Example

#![deny(explicit_outlives_requirements)]
#![deny(warnings)]

struct SharedRef<'a, T>
where
    T: 'a,
{
    data: &'a T,
}

{{produces}}

Explanation

If a struct, enum or union contains a reference, such as &'a T, the compiler requires that T outlives the lifetime 'a. This historically required writing an explicit lifetime bound to indicate this requirement. However, this can be overly explicit, causing clutter and unnecessary complexity. The language was changed to automatically infer some classes of lifetime bounds if they are not specified. Specifically, if a struct, enum or union contains a reference, directly or indirectly, to T with lifetime 'x and 'x refers to a lifetime parameter, then it will infer that T: 'x is a requirement.

See RFC 2093 for more details.

[!WARNING] Implicit lifetime bounds are not semantically equivalent to explicit ones since the latter may affect the implicit lifetime bound of trait object types that are passed as arguments to the overarching struct, enum or union. Rephrased, they participate in trait object lifetime defaulting.

Consider the following piece of code where removing bound T: 'a would lead to a lifetime error in function scope:

struct Ref<'a, T: ?Sized + 'a>(&'a T);

fn scope() {
    let buf = String::new();
    let str = buf.as_str();
    render(Ref(&str));
}

fn render(_: Ref<dyn std::fmt::Display>) {}

Consequently, removing explicit outlives-bounds on type parameters of publicly reachable types constitutes a breaking change if the lifetime refers to a lifetime parameter and the type parameter is not bounded by Sized (thereby admitting trait object types).