enum instrinsics

[philberty]

#[lang = "sized"]
pub trait Sized {}

/// Compiler-internal trait used to indicate the type of enum discriminants.
///
/// This trait is automatically implemented for every type and does not add any
/// guarantees to [`mem::Discriminant`]. It is **undefined behavior** to transmute
/// between `DiscriminantKind::Discriminant` and `mem::Discriminant`.
///
/// [`mem::Discriminant`]: crate::mem::Discriminant
#[unstable(
    feature = "discriminant_kind",
    issue = "none",
    reason = "this trait is unlikely to ever be stabilized, use `mem::discriminant` instead"
)]
#[lang = "discriminant_kind"]
pub trait DiscriminantKind {
    /// The type of the discriminant, which must satisfy the trait
    /// bounds required by `mem::Discriminant`.
    #[lang = "discriminant_type"]
    type Discriminant: Clone + Copy + Debug + Eq + PartialEq + Hash + Send + Sync + Unpin;
}

extern "rust-intrinsic" {
    /// Returns the value of the discriminant for the variant in 'v',
    /// cast to a `u64`; if `T` has no discriminant, returns 0.
    ///
    /// The stabilized version of this intrinsic is [`core::mem::discriminant`](crate::mem::discriminant).
    #[rustc_const_unstable(feature = "const_discriminant", issue = "69821")]
    pub fn discriminant_value<T>(v: &T) -> <T as DiscriminantKind>::Discriminant;

    /// Returns the number of variants of the type `T` cast to a `usize`;
    /// if `T` has no variants, returns 0. Uninhabited variants will be counted.
    ///
    /// The to-be-stabilized version of this intrinsic is [`mem::variant_count`].
    #[rustc_const_unstable(feature = "variant_count", issue = "73662")]
    pub fn variant_count<T>() -> usize;
}

enum BookFormat {
    Paperback,
    Hardback,
    Ebook,
}

pub fn main() {
    let a = BookFormat::Paperback;
    let b = Bookform::HardBack;

    unsafe {
        let c = discriminant_value(a);
        let d = discriminant_value(b);

        let e = variant_count(a);
        let f = variant_count(b);
    }
}

[philberty]

#![feature(intrinsics)]

#[lang = "sized"]
pub trait Sized {}

enum BookFormat {
    Paperback,
    Hardback,
    Ebook,
}

mod core {
    mod intrinsics {
        #[lang = "discriminant_kind"]
        pub trait DiscriminantKind {
            #[lang = "discriminant_type"]
            type Discriminant;
        }

        extern "rust-intrinsic" {
            pub fn discriminant_value<T>(v: &T) -> <T as DiscriminantKind>::Discriminant;
        }
    }
}

pub fn main() {
    let a = BookFormat::Paperback;

    unsafe {
        let c: isize = core::intrinsics::discriminant_value(&a);
    }
}

[philberty]

This compiler explorer link shows that the repr option does affect the type checking here the documentation around this intrinsic seems wrong to me in libcore: https://godbolt.org/z/x63Gr6xdK

[philberty]

#3425
#3426