error using `Vec::<dyn Any>::new()` in a const fn

[droundy]

I tried this code:

const fn newv() -> Vec<Box<dyn std::any::Any>> {
    Vec::new()
}

(Playground link)

I expected this to compile, since Vec::new() is a const function, and no dyn anything needs to be created.

Instead, this happened:

Compiling playground v0.0.1 (/playground)
error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
 --> src/lib.rs:1:20
  |
1 | const fn newv() -> Vec<Box<dyn std::any::Any>> {
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
  = help: add `#![feature(const_fn)]` to the crate attributes to enable

Meta

rustc --version --verbose:

1.51.0 on the playground

[droundy]

BTW, if there is a workaround for this bug, I'd love to hear about it.

[Aaron1011]

This is unrelated to Vec::new(): the program const fn new(val: &Vec<Box<dyn std::any::Any>>) {} also does not compile.

Rust is conservatively disallowing all non-Sized traits in const fns, until there's been more design work around the interaction of traits and const fn. See rust-lang/rfcs#2632 for more discussion.

[RalfJung]

In particular, the very meaning of this type is up for discussion, so whether or not any values are being created is immaterial.

The open question is whether a dyn Trait returned by a const fn can have its methods called in another const fn, i.e., is there an implicit requirement here that the underlying type must implement const Trait or not? This has not been decided yet.

[larsfillmore]

BTW, if there is a workaround for this bug, I'd love to hear about it.

@droundy You've probably found a workaround for your specific case or do not need it any more, but I'll leave it here for others:

struct Wrapper {
    value: Box<dyn std::any::Any>,
}

const fn newv() -> Vec<Wrapper> {
    Vec::new()
}