Insufficiently clear explanation of E0310

[gnzlbg]

The following program (Playground):

pub struct Foo<T>(T, &'static str);

pub trait Bar {
    fn baz(self) -> Box<dyn Baz>;
}

pub struct Bazzzz<T>(T, &'static str);

pub trait Baz {}
impl<T> Baz for Bazzzz<T> {}

impl<T> Bar for Foo<T> {
    fn baz(self) -> Box<dyn Baz> {
        Box::new(Bazzzz(self.0, self.1))
    }
}

fn main() {}

errors with

error[E0310]: the parameter type `T` may not live long enough
  --> src/main.rs:14:9
   |
12 | impl<T> Bar for Foo<T> {
   |      - help: consider adding an explicit lifetime bound `T: 'static`...
13 |     fn baz(self) -> Box<dyn Baz> {
14 |         Box::new(Bazzzz(self.0, self.1))
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: ...so that the type `Bazzzz<T>` will meet its required lifetime bounds
  --> src/main.rs:14:9
   |
14 |         Box::new(Bazzzz(self.0, self.1))
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0310`.
error: Could not compile `playground`.

From reading the explanation of E0310, which only mentions the situation in which T is used behind a static reference, e.g., foo: &'static T, I really had no idea what was going on. It seems that other people have had similar issues with this (e.g. https://stackoverflow.com/questions/40053550/the-compiler-suggests-i-add-a-static-lifetime-because-the-parameter-type-may-no ).

The explanation of E0310 should be improved to make it more clear why T: 'static is necessary, and what alternatives are there.

What confused me most is that T was not used in any reference type of the example above, but it was only used behind a reference in the example of the error message explanation.

[pnkfelix]

Here's what I think is a more reduced example that still shows the poor diagnostic (play)

pub struct Foo<T>(T);

pub struct Bazzzz<T>(T);

pub trait Baz {}
impl<T> Baz for Bazzzz<T> {}

impl<T> Foo<T> {
    fn baz(self) -> Box<dyn Baz> {
        Box::new(Bazzzz(self.0))
    }
}

fn main() {}

[pnkfelix]

In particular, it would probably be a good idea to somehow indicate that Box<dyn Baz> implicitly stands for Box<dyn Baz + 'static>, and that is what imposes the requirement that Bazzzz<T> itself outlive the 'static lifetime...

[estebank]

Current output

error[E0310]: the parameter type `T` may not live long enough
  --> src/main.rs:14:9
   |
14 |         Box::new(Bazzzz(self.0, self.1))
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds
   |
help: consider adding an explicit lifetime bound...
   |
12 | impl<T: 'static> Bar for Foo<T> {
   |       +++++++++

Might be a duplicate of #41966