Unhelpful error message "ambiguous lifetime bound, explicit lifetime bound required"

[oberien]

Code

trait MyTrait<'a, 'b> where Self: 'a + 'b {}

pub struct Foo<'a, 'b> {
    expr: Box<dyn MyTrait<'a, 'b>>,
}
// Fix:
pub struct Bar<'all, 'a, 'b> where 'all: 'a, 'all: 'b {
    expr: Box<dyn MyTrait<'a, 'b> + 'all>,
}

Current output

error[E0227]: ambiguous lifetime bound, explicit lifetime bound required
 --> src/lib.rs:4:15
  |
4 |     expr: Box<dyn MyTrait<'a, 'b>>,
  |               ^^^^^^^^^^^^^^^^^^^

For more information about this error, try rustc --explain E0227.

Desired output

No response

Rationale and extra context

No response

Other cases

No response

Rust Version

$ rustc -Vv
rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: x86_64-unknown-linux-gnu
release: 1.81.0
LLVM version: 18.1.7

Anything else?

No response

[fmease]

See https://doc.rust-lang.org/reference/lifetime-elision.html#default-trait-object-lifetimes.

Trait object types can only have a single lifetime bound, so in dyn MyTrait<'a, 'b> one has to disambiguate since it's unclear if 'a or 'b is meant (looking at where Self: 'a + 'b). One does so via e.g. dyn MyTrait<'a, 'b> + 'a, dyn MyTrait<'a, 'b> + 'b or dyn MyTrait<'a, 'b> + 'static.

---

I guess we can add a structured suggestion to the diagnostic of the form explicitly specify the lifetime bound, dyn MyTrait<'a, 'b> + /* 'r */ (er, dunno how best to present this, there's room for improvement), would that be of help?

Additionally, we could highlight & label the bounds 'a and 'b in where Self: 'a + 'b, somehow explaining that one can't tell which one to choose.