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A parameter type is missing an explicit lifetime bound and may not live long
enough.
Erroneous code example:
```compile_fail,E0309
// This won't compile because the applicable impl of
// `SomeTrait` (below) requires that `T: 'a`, but the struct does
// not have a matching where-clause.
struct Foo<'a, T> {
foo: <T as SomeTrait<'a>>::Output,
}
trait SomeTrait<'a> {
type Output;
}
impl<'a, T> SomeTrait<'a> for T
where
T: 'a,
{
type Output = u32;
}
```
The type definition contains some field whose type requires an outlives
annotation. Outlives annotations (e.g., `T: 'a`) are used to guarantee that all
the data in `T` is valid for at least the lifetime `'a`. This scenario most
commonly arises when the type contains an associated type reference like
`<T as SomeTrait<'a>>::Output`, as shown in the previous code.
There, the where clause `T: 'a` that appears on the impl is not known to be
satisfied on the struct. To make this example compile, you have to add a
where-clause like `T: 'a` to the struct definition:
```
struct Foo<'a, T>
where
T: 'a,
{
foo: <T as SomeTrait<'a>>::Output
}
trait SomeTrait<'a> {
type Output;
}
impl<'a, T> SomeTrait<'a> for T
where
T: 'a,
{
type Output = u32;
}
```
|
