Static lifetime requirement in boxed trait object (Box<dyn Trait>) is not clearly explained

[Tiwalun]

Code

struct Factory<'a> {
    value: &'a usize,
}

impl<'a> Factory<'a> {
    fn generate(&self) -> Box<dyn std::fmt::Debug + 'a> {
        Box::new(Value  { value: self.value })
    }
}


struct Owner {
    value: Box<dyn std::fmt::Debug>
}


#[derive(Debug)]
struct Value<'a> {
    value: &'a usize,
}


fn build_owner<'a,'b>(factory: &'b Factory<'a>) -> Owner {
    let value = factory.generate();

    Owner { value }
}

fn main() {
    let value = 10;

    let factory = Factory { value: &value };

    let _owner = build_owner(&factory);
}

Current output

Compiling playground v0.0.1 (/playground)

error: lifetime may not live long enough
  --> src/main.rs:29:13
   |
26 | fn build_owner<'a,'b>(factory: &'b Factory<'a>) -> Owner {
   |                -- lifetime `'a` defined here
...
29 |     Owner { value }
   |             ^^^^^ cast requires that `'a` must outlive `'static`

Desired output

error: lifetime may not live long enough
  --> src/main.rs:29:13
   |
26 | fn build_owner<'a,'b>(factory: &'b Factory<'a>) -> Owner {
   |                -- lifetime `'a` defined here
...
29 |     Owner { value }
   |             ^^^^^ cast requires that `'a` must outlive `'static`

note: value has type Box<dyn Test>, which means that a default lifetime requirement of 'static is added. 
note: Adding an explicit lifetime could help: Box<dyn Test +'a>

Rationale and extra context

I think it's not obvious that a boxed trait object means that there is a default lifetime requirement of 'static, and I've spent too much time looking for where the 'static requirement comes from. Would be great to get a hint from the compiler.

Other cases

No response

Anything else?

No response

[fmease]

@obeis You might've found this already but I think you can reuse or even generalize what maybe_suggest_constrain_dyn_trait_impl or add_static_impl_trait_suggestion in compiler/rustc_borrowck/src/diagnostics/region_errors.rs is doing (esp. with TraitObjectVisitor + HirTraitObjectVisitor). Just mentioning this information to you since I've already briefly looked at the code.

[Noratrieb]

@obeis sorry for stealing your issue, I didn't check whether there was one 😅
@rustbot claim

[obeis]

@Nilstrieb No problem

[Noratrieb]

I messed up the fixes in the PR description lol
#117835 has been merged

[Tiwalun]

Thanks for improving this! ❤️