error[E0038]: the trait `Bar` cannot be made into an object --> $DIR/issue-72410.rs:14:19 | LL | where for<'a> &'a mut [dyn Bar]: ; | ^^^^^^^^^^^^^^^^^ `Bar` cannot be made into an object | note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit --> $DIR/issue-72410.rs:13:8 | LL | pub trait Bar { | --- this trait cannot be made into an object... LL | fn map() | ^^^ ...because associated function `map` has no `self` parameter help: consider turning `map` into a method by giving it a `&self` argument | LL | fn map(&self) | +++++ help: alternatively, consider constraining `map` so it does not apply to trait objects | LL | where for<'a> &'a mut [dyn Bar]:, Self: Sized ; | +++++++++++++ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0038`.