Error code E0392
A type or lifetime parameter has been declared but is not actually used.
Erroneous code example:
#![allow(unused)] fn main() { enum Foo<T> { Bar, } }
If the type parameter was included by mistake, this error can be fixed by simply removing the type parameter, as shown below:
#![allow(unused)] fn main() { enum Foo { Bar, } }
Alternatively, if the type parameter was intentionally inserted, it must be used. A simple fix is shown below:
#![allow(unused)] fn main() { enum Foo<T> { Bar(T), } }
This error may also commonly be found when working with unsafe code. For example, when using raw pointers one may wish to specify the lifetime for which the pointed-at data is valid. An initial attempt (below) causes this error:
#![allow(unused)] fn main() { struct Foo<'a, T> { x: *const T, } }
We want to express the constraint that Foo should not outlive 'a
, because
the data pointed to by T
is only valid for that lifetime. The problem is
that there are no actual uses of 'a
. It's possible to work around this
by adding a PhantomData type to the struct, using it to tell the compiler
to act as if the struct contained a borrowed reference &'a T
:
#![allow(unused)] fn main() { use std::marker::PhantomData; struct Foo<'a, T: 'a> { x: *const T, phantom: PhantomData<&'a T> } }
PhantomData can also be used to express information about unused type parameters.