Hello, I'm a bit confused by the following Rust error: Rust Playground trait Trait {} struct Impl; impl Trait for Impl {} struct Holder<'r> { r: &'r mut dyn Trait, } fn static_holder() -> Holder<'static> { Holder{ r: Box::leak(Box::new(Impl)) } } impl<'r> Default for Holder<'r> { fn default() -> Holder<'r> { static_holder() } } I'm getting: error: lifetime may not live long enough --> src/lib.rs:16:9 | 14 | impl<'r> Default for Holder<'r> { | -- lifetime `'r` defined here 15 | fn default() -> Holder<'r> { 16 | static_holder() | ^^^^^^^^^^^^^^^ returning this value requires that `'r` must outlive `'static` | = note: requirement occurs because of the type `Holder<'_>`, which makes the generic argument `'_` invariant = note: the struct `Holder<'r>` is invariant over the parameter `'r` = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance Why is Holder<'r> invariant over r ? The linked nomicon says &'r mut T is covariant over 'r . Why does it work when I remove the dyn Trait and instead hardcode Impl ? Rust Playground Why does it work when I manually deconstruct static_holder and reconstruct a new Holder instance? Rust Playground Thanks! Jonathan 4 posts - 3 participants Read full topic