This error indicates that a type or lifetime parameter has been declared but not actually used. Here is an example that demonstrates the error:
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:
enum Foo { Bar, }
Alternatively, if the type parameter was intentionally inserted, it must be used. A simple fix is shown below:
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:
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
:
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.