I want to have a very generic top-level HashMap.

static mut cache: HashMap<std::hash::Hash   Eq, MyBox<_>> = HashMap::new();

MyBox is a smart pointer.

I get all these errors:

the placeholder _ is not allowed within types on item signatures for static variables

only auto traits can be used as additional traits in a trait object
help: consider creating a new trait with all of these as supertraits and using that trait here instead: trait NewTrait: Hash Eq {}

the trait Eq cannot be made into an object

And also a warning "trait objects without an explicit dyn are deprecated"

How can I create a static mut HashMap that allows any keys and any boxed values?

CodePudding user response：

The entire thing seems like a very dubious xy problem, mutable statics are frowned upon to start with. What are you actually trying to achieve here?

How can I create a static mut HashMap that allows any keys and any boxed values?

You can't create a hashmap which allows "any key". As the last error message indicates, Eq is not object-safe, so you can't create a trait object anywhere near Eq, which means you can't create a "dyn any" key which is equatable.

By comparison the placeholder issue can be solved relatively easily, by making the values Box<dyn Any>, though what you'd do with that I can't tell you.