Rust error E0311: the parameter type `T` may not live long enough

The compiler team's explanation

This error occurs when there is an unsatisfied outlives bound involving an elided region and a generic type parameter or associated type.

Erroneous code example:

fn no_restriction<T>(x: &()) -> &() {
    with_restriction::<T>(x)
}

fn with_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
    x
}

Why doesn't this code compile? It helps to look at the lifetime bounds that are automatically added by the compiler. For more details see the documentation for [lifetime elision]( https://doc.rust-lang.org/reference/lifetime-elision.html).

The compiler elides the lifetime of x and the return type to some arbitrary lifetime 'anon in no_restriction(). The only information available to the compiler is that 'anon is valid for the duration of the function. When calling with_restriction(), the compiler requires the completely unrelated type parameter T to outlive 'anon because of the T: 'a bound in with_restriction(). This causes an error because T is not required to outlive 'anon in no_restriction().

If no_restriction() were to use &T instead of &() as an argument, the compiler would have added an implied bound, causing this to compile.

This error can be resolved by explicitly naming the elided lifetime for x and then explicitly requiring that the generic parameter T outlives that lifetime:

fn no_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
    with_restriction::<T>(x)
}

fn with_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
    x
}

From rustc --explain E0311 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.