New problem with const_trait_impl

[leonardo-m]

This code:

#![feature(const_trait_impl, const_fn_trait_bound, const_panic)]

pub trait Foo {
    fn bar(self) -> usize;
    const MAX: Self;
}
impl const Foo for usize {
    fn bar(self) -> usize { self }
    const MAX: Self = Self::MAX;
}

const fn spam<T: Foo>(n: usize) {
    assert!(n <= T::MAX.bar());
}

fn main() {}

Using:

rustc 1.56.0-nightly (ac50a5335 2021-08-27)
binary: rustc
commit-hash: ac50a53359328a5d7f2f558833e63d59d372e4f7
commit-date: 2021-08-27
host: x86_64-pc-windows-gnu
release: 1.56.0-nightly
LLVM version: 13.0.0

Gives:

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
  --> ...\test.rs:13:18
   |
13 |     assert!(n <= T::MAX.bar());
   |                  ^^^^^^^^^^^^

I think this used to compile up to the precedent Nightly.

[fee1-dead]

You need ~const on every bound when you require a const impl in a const context.

#![feature(const_trait_impl, const_fn_trait_bound, const_panic)]

pub trait Foo {
    fn bar(self) -> usize;
    const MAX: Self;
}
impl const Foo for usize {
    fn bar(self) -> usize { self }
    const MAX: Self = Self::MAX;
}

const fn spam<T: ~const Foo>(n: usize) {
    assert!(n <= T::MAX.bar());
}

fn main() {}

[leonardo-m]

I see. Thank you. So do I convert this into a diagnostic enhancement request asking for a helpful error message (or help message), or do I close this issue down and open another one for that?

[leonardo-m]

Here I have a problem:

#![feature(const_fn_trait_bound, const_trait_impl)]
#![allow(dead_code)]

const fn foo<T: ~const Default + Copy>() -> T {
    T::default()
}

fn main() {
    const X1: u64 = foo(); // OK
    const X2: (u8, u64) = foo(); // Err
}

That gives me:

error[E0277]: the trait bound `(u8, u64): Default` is not satisfied
  --> ...\test1.rs:10:27
   |
10 |     const X2: (u8, u64) = foo(); // Err
   |                           ^^^ the trait `Default` is not implemented for `(u8, u64)`
   |
note: required by a bound in `foo`
  --> ...\test1.rs:4:17
   |
4  | const fn foo<T: ~const Default + Copy>() -> T {
   |                 ^^^^^^^^^^^^^^ required by this bound in `foo`
help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
   |
8  | fn main() where (u8, u64): Default {
   |           ++++++++++++++++++++++++

A little more complex situation:

#![feature(
    const_evaluatable_checked,
    const_fn_trait_bound,
    const_generics,
    const_panic,
    const_trait_impl,
)]

#![allow(incomplete_features)]

pub trait ToFromUsize {
    fn to_usize(self) -> usize;
    fn from_usize(x: usize) -> Self;
    const MAX: Self;
}
impl const ToFromUsize for usize {
    fn to_usize(self) -> usize { self }
    fn from_usize(x: usize) -> Self { x }
    const MAX: Self = Self::MAX;
}

pub const fn assert_nonzero(n: usize) -> usize {
    assert!(n > 0);
    n
}

pub const fn is_contained(n: usize, m: usize) -> usize {
    assert!(n <= m);
    n
}

pub const fn is_representable<Ti: ~const ToFromUsize>(n: usize) -> usize {
    assert!(n <= Ti::MAX.to_usize());
    n
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Default)]
#[repr(transparent)]
pub struct Foo<Ti: ToFromUsize + Copy, const N: usize>(Ti)
where [(); assert_nonzero(N)]:,
      [(); is_representable::<Ti>(N - 1)]:;

impl<Ti: ToFromUsize + Copy, const N: usize> Foo<Ti, N>
where [(); assert_nonzero(N)]:,
      [(); is_representable::<Ti>(N - 1)]: {
    pub const fn new(i: Ti) -> Option<Self> {
        if i.to_usize() < N {
            Some(Self(i))
        } else {
            None
        }
    }
}

fn main() {}

That gives:

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
  --> ...\test2.rs:47:12
   |
47 |         if i.to_usize() < N {
   |            ^^^^^^^^^^^^

[leonardo-m]

See also: https://users.rust-lang.org/t/help-for-the-new-const-trait-checks/64237

[fee1-dead]

I think you can just move the bound to the const fn's where clause.

pub const fn new(i: Ti) -> Option<Self> where Ti: ~const ToFromUsize

[leonardo-m]

I think you can just move the bound to the const fn's where clause.

Unfortunately it gives me a "error: ~const is not allowed here". Followed by "note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions"

[fee1-dead]

Hmmm, looks like I will have to allow const fns in inherent impls having ~const bounds in a new PR.

[leonardo-m]

The latest Nightly solves most of my problems, but I have two further problems. (Tell me if you prefer me to move them to new issues).
Here I don't how to define the const values at the bottom:

#![feature(
    const_fn_trait_bound,
    const_panic,
    const_trait_impl,
    generic_const_exprs,
)]

#![allow(incomplete_features)]

pub trait ToFromUsize {
    fn to_usize(self) -> usize;
    fn from_usize(x: usize) -> Self;
    const MAX: Self;
}
impl const ToFromUsize for usize {
    fn to_usize(self) -> usize { self }
    fn from_usize(x: usize) -> Self { x }
    const MAX: Self = Self::MAX;
}

pub const fn assert_nonzero(n: usize) -> usize {
    assert!(n > 0);
    n
}

pub const fn is_contained(n: usize, m: usize) -> usize {
    assert!(n <= m);
    n
}

pub const fn is_representable<Ti: ~const ToFromUsize>(n: usize) -> usize {
    assert!(n <= Ti::MAX.to_usize());
    n
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Default)]
#[repr(transparent)]
pub struct Foo<Ti: ToFromUsize + Copy, const N: usize>(Ti)
where [(); assert_nonzero(N)]:,
      [(); is_representable::<Ti>(N - 1)]:;

impl<Ti: ToFromUsize + Copy, const N: usize> Foo<Ti, N>
where [(); assert_nonzero(N)]:,
      [(); is_representable::<Ti>(N - 1)]: {

    pub const unsafe fn new_unchecked(i: Ti) -> Self where Ti: ~const ToFromUsize {
        Self(i)
    }

    pub const FIRST: Self = unsafe { Self::new_unchecked(Ti::from_usize(0)) };
    pub const LAST: Self = unsafe { Self::new_unchecked(Ti::from_usize(N - 1)) };
}

fn main() {}

It gives me:

error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
  --> ...\test.rs:50:58
   |
50 |     pub const FIRST: Self = unsafe { Self::new_unchecked(Ti::from_usize(0)) };
   |                                                          ^^^^^^^^^^^^^^^^^

error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
  --> ...\test.rs:51:57
   |
51 |     pub const LAST: Self = unsafe { Self::new_unchecked(Ti::from_usize(N - 1)) };
   |                                                         ^^^^^^^^^^^^^^^^^^^^^

The other problem is an enhancement request. If I remove a "const" from the signature of new_unchecked the compiler doesn't give me much help to see the problem:

#![feature(
    const_fn_trait_bound,
    const_panic,
    const_trait_impl,
    generic_const_exprs,
)]

#![allow(incomplete_features)]

pub trait ToFromUsize {
    fn to_usize(self) -> usize;
    fn from_usize(x: usize) -> Self;
    const MAX: Self;
}
impl const ToFromUsize for usize {
    fn to_usize(self) -> usize { self }
    fn from_usize(x: usize) -> Self { x }
    const MAX: Self = Self::MAX;
}

pub const fn assert_nonzero(n: usize) -> usize {
    assert!(n > 0);
    n
}

pub const fn is_contained(n: usize, m: usize) -> usize {
    assert!(n <= m);
    n
}

pub const fn is_representable<Ti: ~const ToFromUsize>(n: usize) -> usize {
    assert!(n <= Ti::MAX.to_usize());
    n
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Default)]
#[repr(transparent)]
pub struct Foo<Ti: ToFromUsize + Copy, const N: usize>(Ti)
where [(); assert_nonzero(N)]:,
      [(); is_representable::<Ti>(N - 1)]:;

impl<Ti: ToFromUsize + Copy, const N: usize> Foo<Ti, N>
where [(); assert_nonzero(N)]:,
      [(); is_representable::<Ti>(N - 1)]: {

    pub unsafe fn new_unchecked(i: Ti) -> Self where Ti: ~const ToFromUsize {
        Self(i)
    }
}

fn main() {}

It gives:

error: `~const` is not allowed here
  --> ...\test2.rs:46:58
   |
46 |     pub unsafe fn new_unchecked(i: Ti) -> Self where Ti: ~const ToFromUsize {
   |                                                          ^^^^^^^^^^^^^^^^^^
   |
   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions

[fee1-dead]

1. To accept that would require more design work.
2. Make a new issue for it. (should have a more helpful message)

[leonardo-m]

Thank you :)
Opened #89007 for the diagnostic issue.