Const generics and structural match.

[crlf0710]

Not sure whether this has been reported before.
I tried this code:

#![feature(const_generics)]
#![feature(structural_match)]

use std::marker::{StructuralPartialEq, StructuralEq};

struct Container<T: StructuralPartialEq + StructuralEq, const V: T>;

fn main() {
    let container1: Container<i32, {42}> = Container;
    let container2: Container<&'static str, "Hello world"> = Container;
}

I expected to see this happen: The compile passes.

Instead, this happened:

error[E0741]: the types of const generic parameters must derive `PartialEq` and `Eq`
 --> src/main.rs:6:66
  |
6 | struct Container<T: StructuralPartialEq + StructuralEq, const V: T>;
  |                                                                  ^ `T` doesn't derive both `PartialEq` and `Eq`

Meta

rustc --version --verbose:

Nightly channel
Build using the Nightly version: 1.44.0-nightly

(2020-04-21 45d050cde277b22a7558)

Backtrace

<backtrace>

@rustbot modify labels to +A-const generics F-const generics T-compiler requires-nightly

[rustbot]

Error: Label generics can only be set by Rust team members

Please let @rust-lang/release know if you're having trouble with this bot.

[crlf0710]

@rustbot modify labels to +A-const-generics F-const_generics T-compiler requires-nightly

[crlf0710]

#![feature(const_generics)]
#![feature(structural_match)]

use std::marker::{StructuralPartialEq, StructuralEq};

struct Container<T: StructuralPartialEq + StructuralEq + PartialEq + Eq, const V: T>;

fn main() {
    let container1: Container<i32, {42}> = Container;
    let container2: Container<&'static str, "Hello world"> = Container;
}

This also doesn't work.

[varkor]

I'm pretty sure StructuralPartialEq and StructuralEq are not supposed to be public like this… it's not valid to claim that an arbitrary type is structural_match. I don't expect this to work.

cc @pnkfelix @eddyb

[lcnr]

We don't allow const paramters with generic types at all afaik. So while the error message is misleading, I don't think that this will work soon.

[varkor]

We don't allow const paramters with generic types at all afaik. So while the error message is misleading, I don't think that this will work soon.

Right. This will actually be improved by #70845, which makes this explicit.

[crlf0710]

Thanks. Is this a language limitation or implementation limitation? Will lazy normalization allow this to be written?

[varkor]

It won't be supported after lazy normalisation specifically, but we would like to be able to use const parameters with generic types eventually.

[vmarkushin]

Also consider this example

#![feature(const_generics)]

#[derive(PartialEq, Eq)]
enum Nat {
    Z,
    S(Box<Nat>)
}

fn foo<const N: Nat>() {}

error[E0741]: `Nat` must be annotated with `#[derive(PartialEq, Eq)]` to be used as the type of a const parameter
 --> src/lib.rs:9:17
  |
9 | fn foo<const N: Nat>() {}
  |                 ^^^ `Nat` doesn't derive both `PartialEq` and `Eq`

[lcnr]

@vmarkushin can you open a separate issue for this as your example seems distinct enough to be tracked separately?