Add read-core feature gate

[nbdd0121]

Fix #581

[nbdd0121]

It seems that

#[derive(Debug)]
struct Foo<#[cfg(foo)] T, #[cfg(not(foo))] T>(T);

only works in Rust 1.48+. However it doesn't seem to be mentioned in the release notes.

[nbdd0121]

Bisection shows that it's changed in rust-lang/rust#76010, which fixes rust-lang/rust#75930. I could manually implement these traits, or perhaps we can bump MSRV to 1.48.

[philipc]

When does the type checking for type parameters occur? It seems defaulting to StoreOnHeap always compiles, even if StoreOnHeap is missing the storage implementation? If so, it's not ideal to have a default that won't work, but for working code it makes no difference.

Otherwise, 1.48 is okay, it's getting close to a year old.

[nbdd0121]

It seems that the checking is lazy if the bound depends on generic parameter:

trait Bar<U> {}

struct Foo<U, T: Bar<U> = ()>(U, T);
//^ this is lazy, since () may implement Bar<U> for some U, but not necessary all Us

and this is eagerly checked:

trait Bar {}

struct Foo<T: Bar = ()>(T);
//^ this is eager, so will raise an error

[philipc]

Thanks!

[philipc]

Does this all look good for a new release now?

[nbdd0121]

No PRs left to submit from my side!