stabilize const mem::discriminant

[lcnr]

Changes mem::discriminant to be const stable starting at version 1.46.0

closes #69821

[rust-highfive]

r? @shepmaster

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[oli-obk]

While I see no reason not to do this, I think the stabilization would benefit from at least mentioning a real world use case that would benefit from this or be possible only with this.

[lcnr]

Now that matches! is stable, this is not as important anymore and cases where this might be actually needed are fairly rare. Apart from "this just feels like it should be const", which isn't really a good argument, it helps in cases where a match statement is not useable and the enum variant is hard to construct. We now have do it only once by using a constant.

enum Whatever {
    Variant(Foo, Bar, StructWithWierdInvariants),
    OtherVariant,
}

const WHATEVER_KIND_DISCR: Discriminant<Whatever> =
    mem::discriminant(&Whatever::Kind(...));

fn this_is_a_function(input: Whatever) {
    other_function_requiring_discriminant(input, WHATEVER_KIND_DISCR);
}

[oli-obk]

Apart from "this just feels like it should be const", which isn't really a good argument

That argument is enough for me :D

But I think we should keep waiting for a real use case, though I worry we won't get real use cases without it being stable :/

[bors]

☔ The latest upstream changes (presumably #72437) made this pull request unmergeable. Please resolve the merge conflicts.