Custom vtables with integers

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2

Interesting. The 1414-rvalue_static_promotion - The Rust RFC Book RFC claims

These rules above should be consistent with the existing rvalue promotions in const initializer expressions:

// If this compiles:
const X: &'static T = &<constexpr foo>;

// Then this should compile as well:
let x: &'static T = &<constexpr foo>;

yet

fn foo() {
    const X: &'static u32 = &f(); // <- works
    let x: &'static u32 = &f(); // <- doesn't work
}

const fn f() -> u32 {
    42
}

The typical workaround for generic consts is to use traits, and that works here as well

use std::fmt::Display;

struct Vtable {
    to_string: unsafe fn(*const ()) -> String,
    value_size: usize,
}

const fn vtable_for_type<T: Display>() -> &'static Vtable {
    trait HasVTable {
        const VTABLE: &'static Vtable;
    }
    impl<T: Display> HasVTable for T {
        const VTABLE: &'static Vtable = &Vtable {
            to_string: to_string::<T>,
            value_size: compute_value_size::<T>(),
        };
    }
    T::VTABLE
}

const fn compute_value_size<T>() -> usize {
    std::mem::size_of::<T>()
}

unsafe fn to_string<T: Display>(the_value: *const ()) -> String {
    let my_ref = &*(the_value as *const T);
    my_ref.to_string()
}
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