This error indicates that a type or lifetime parameter has been declared
but not actually used. Here is an example that demonstrates the error:
```compile_fail,E0392
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:
```compile_fail,E0392
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.
[PhantomData]: https://doc.rust-lang.org/std/marker/struct.PhantomData.html