Why does adding a generic type to a trait affect the lifetime of trait objects and associated types?

Question

I have following code:

trait T<GT> {
    type AT;

    fn foo(&self);
}

struct AbstractT<GT, AT> {
    t: Box<dyn T<GT, AT = AT>>,
}

impl<GT, AT> T<GT> for AbstractT<GT, AT> {
    type AT = AT;

    fn foo(&self) {
        self.t.foo();
    }
}

fn boxed_abstract<GT, TT: T<GT> + 'static>(tt: TT) -> Box<dyn T<GT, AT = TT::AT>> {
    Box::new(AbstractT { t: Box::new(tt) })
}

playground

Which throws these errors:

error[E0310]: the associated type `<TT as T<GT>>::AT` may not live long enough
  --> src/lib.rs:20:5
   |
20 |     Box::new(AbstractT { t: Box::new(tt) })
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: consider adding an explicit lifetime bound `<TT as T<GT>>::AT: 'static`...
note: ...so that the type `AbstractT<GT, <TT as T<GT>>::AT>` will meet its required lifetime bounds
  --> src/lib.rs:20:5
   |
20 |     Box::new(AbstractT { t: Box::new(tt) })
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0310]: the parameter type `GT` may not live long enough
  --> src/lib.rs:20:5
   |
19 | fn boxed_abstract<GT, TT: T<GT> + 'static>(tt: TT) -> Box<dyn T<GT, AT = TT::AT>> {
   |                   -- help: consider adding an explicit lifetime bound...: `GT: 'static`
20 |     Box::new(AbstractT { t: Box::new(tt) })
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: ...so that the type `AbstractT<GT, <TT as T<GT>>::AT>` will meet its required lifetime bounds
  --> src/lib.rs:20:5
   |
20 |     Box::new(AbstractT { t: Box::new(tt) })
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If I remove GT from everywhere, it compiles just fine, but with GT it fails with a bunch of lifetime errors. It seems that existence of GT should not affect lifetimes of either dyn T or T::AT (because it's not used in them), but it apparently does. Likewise, lifetime of dyn T should not depend lifetimes of GT or AT but it apparently does.

Am I missing something or is it a lifetime inference problem?

Answer 1

You might be missing something. Rust automatically infers lifetime bounds for trait objects, in the case of boxed trait objects the automatically inferred lifetime bounds is 'static. For example, this what the Rust compiler sees when it looks at your code:

trait T<GT> {
    type AT;

    fn foo(&self);
}

struct AbstractT<GT, AT> {
    // notice the added "+ 'static" below
    t: Box<dyn T<GT, AT = AT> + 'static>,
}

impl<GT, AT> T<GT> for AbstractT<GT, AT> {
    type AT = AT;

    fn foo(&self) {
        self.t.foo();
    }
}

// notice the added "+ 'static" in the return type
fn boxed_abstract<GT, TT: T<GT> + 'static>(tt: TT) -> Box<dyn T<GT, AT = TT::AT> + 'static> {
    Box::new(AbstractT { t: Box::new(tt) })
}

To make it compile we just need to add more explicit 'static bounds on all your generic types like so:

trait T<GT> {
    type AT;

    fn foo(&self);
}

struct AbstractT<GT, AT> {
    t: Box<dyn T<GT, AT = AT>>,
}

impl<GT, AT> T<GT> for AbstractT<GT, AT> {
    type AT = AT;

    fn foo(&self) {
        self.t.foo();
    }
}

fn boxed_abstract<GT, TT>(tt: TT) -> Box<dyn T<GT, AT = TT::AT>>
    where TT: T<GT> + 'static, GT: 'static
{
    Box::new(AbstractT { t: Box::new(tt) })
}

playground

The reason why we need all these 'static bounds is simple: a container type can only be bound by 'static if all of the types inside of it are bound by 'static and so on recursively.

Further reading:

• Default Trait Object Lifetimes

Update

If you don't like the 'static requirement you can make AbstractT generic over lifetimes by adding an explicit lifetime annotation to its type, like so:

trait T<GT> {
    type AT;

    fn foo(&self);
}

struct AbstractT<'a, GT, AT> {
    t: Box<dyn T<GT, AT = AT> + 'a>,
}

impl<'a, GT, AT> T<GT> for AbstractT<'a, GT, AT> {
    type AT = AT;

    fn foo(&self) {
        self.t.foo();
    }
}

fn boxed_abstract<'a, GT, TT>(tt: TT) -> Box<dyn T<GT, AT = TT::AT> + 'a>
    where TT: T<GT> + 'a, GT: 'a
{
    Box::new(AbstractT { t: Box::new(tt) })
}

playground