Derive fails when using non-literal const generic default

[cmyr]

I imagine this is simplest with an illustration. The following code does not compile for me:

const MY_DEFAULT: usize = 4;

#[derive(serde::Deserialize)]
struct MyStruct<const N: usize = MY_DEFAULT>(bool);

but the following seems to be fine:

#[derive(serde::Deserialize)]
struct MyStruct<const N: usize = 4>(bool);

The error I see in the first case is that MY_DEFAULT is an unexpected token. I poked around the derive source here for a few minutes and didn't see anything jump out at me (it looks like syn parses this as an expression, not a literal, so it should at least parse?) but it looks like something isn't right.

If anyone has any insight, that would be much appreciated!

rustc 1.69.0 (84c898d65 2023-04-16)
serde v1.0.162

[oli-obk]

I think this is solely a syn bug