rkyv in Rust 1.56: type annotations needed

[xobs]

Prior to Rust 1.56, I could use const usize values when defining rkyv structs. However, in Rust 1.56, this is no longer allowed.

Code

I tried this code:

const ARRAY_SIZE: usize = 32;

#[derive(rkyv::Archive, rkyv::Deserialize)]
struct SomeStruct1 {
    buffer_one: Option<[u8; ARRAY_SIZE]>,
    buffer_two: Option<[u8; ARRAY_SIZE]>,
}

fn main() {
    println!("Pass");
}

I expected to see this happen: Prior to Rust 1.56, this compiled.

Instead, this happened: The following error appears:

error[E0284]: type annotations needed: cannot satisfy `<Option<[u8; 32]> as Archive>::Archived == _`
 --> src\main.rs:3:25
  |
3 | #[derive(rkyv::Archive, rkyv::Deserialize)]
  |                         ^^^^^^^^^^^^^^^^^ cannot satisfy `<Option<[u8; 32]> as Archive>::Archived == _`
  |
  = note: this error originates in the derive macro `rkyv::Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0284`.
error: could not compile `rkyv-test` due to previous error

Version it worked on

It most recently worked on: Rust 1.55

Version with regression

rustc --version --verbose:

rustc 1.56.0 (09c42c458 2021-10-18)
binary: rustc
commit-hash: 09c42c45858d5f3aedfa670698275303a3d19afa
commit-date: 2021-10-18
host: x86_64-pc-windows-msvc
release: 1.56.0
LLVM version: 13.0.0

[xobs]

Further information: This is using the rkyv crate. We use an older 0.4.0, however this still occurs with the latest 0.7 version.

[hkratz]

Bisected to #87280.

@rustbot label +E-needs-mcve

[hkratz]

Reproducer without macros, but still dependent on rkyv:

#![allow(dead_code)]
#![allow(unused_variables)]

use rkyv::{Archive, Archived, Deserialize, Fallible};

const ARRAY_SIZE: usize = 32;
struct SomeStruct1 {
    buffer_one: Option<[u8; ARRAY_SIZE]>,
    buffer_two: Option<[u8; ARRAY_SIZE]>,
}

struct ArchivedSomeStruct1
where
    Option<[u8; ARRAY_SIZE]>: ::rkyv::Archive,
    Option<[u8; ARRAY_SIZE]>: ::rkyv::Archive,
{
    buffer_one: ::rkyv::Archived<Option<[u8; ARRAY_SIZE]>>,
    buffer_two: ::rkyv::Archived<Option<[u8; ARRAY_SIZE]>>,
}

struct SomeStruct1Resolver
where
    Option<[u8; ARRAY_SIZE]>: ::rkyv::Archive,
    Option<[u8; ARRAY_SIZE]>: ::rkyv::Archive,
{
    buffer_one: ::rkyv::Resolver<Option<[u8; ARRAY_SIZE]>>,
    buffer_two: ::rkyv::Resolver<Option<[u8; ARRAY_SIZE]>>,
}

impl Archive for SomeStruct1
where
    Option<[u8; ARRAY_SIZE]>: ::rkyv::Archive,
    Option<[u8; ARRAY_SIZE]>: ::rkyv::Archive,
{
    type Archived = ArchivedSomeStruct1;
    type Resolver = SomeStruct1Resolver;
    #[allow(clippy::unit_arg)]
    #[inline]
    unsafe fn resolve(&self, pos: usize, resolver: Self::Resolver, out: *mut Self::Archived) {
        unreachable!();
    }
}

impl<__D: Fallible + ?Sized> Deserialize<SomeStruct1, __D> for Archived<SomeStruct1>
where
    Option<[u8; ARRAY_SIZE]>: Archive,
    Archived<Option<[u8; ARRAY_SIZE]>>: Deserialize<Option<[u8; ARRAY_SIZE]>, __D>,
    Option<[u8; ARRAY_SIZE]>: Archive,
    Archived<Option<[u8; ARRAY_SIZE]>>: Deserialize<Option<[u8; ARRAY_SIZE]>, __D>,
{
    fn deserialize(
        &self,
        deserializer: &mut __D,
    ) -> ::core::result::Result<SomeStruct1, __D::Error> {
        unreachable!();
    }
}

fn main() {}

[SNCPlay42]

Reduced:

pub trait Archive {
    type Archived;
}

impl<T> Archive for Option<T> {
    type Archived = ();
}
pub type Archived<T> = <T as Archive>::Archived;

pub trait Deserialize<D> {}

const ARRAY_SIZE: usize = 32;
impl<__D> Deserialize<__D> for ()
where
    Option<[u8; ARRAY_SIZE]>: Archive,
    Option<[u8; ARRAY_SIZE]>: Archive,
    Archived<Option<[u8; ARRAY_SIZE]>>: Deserialize<__D>,
{
}

Also, if you remove one of the Option<[u8; ARRAY_SIZE]>: Archive, lines, the code still compiles on 1.55 but the error on 1.56 changes to:

error[E0275]: overflow evaluating the requirement `(): Deserialize<__D>`
  --> <source>:13:11
   |
13 | impl<__D> Deserialize<__D> for ()
   |           ^^^^^^^^^^^^^^^^
   |
note: required by a bound in `Deserialize`
  --> <source>:10:1
   |
10 | pub trait Deserialize<D> {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Deserialize`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0275`.

@rustbot label regression-from-stable-to-stable

(not sure what happened with rustbot and the labels above)

[SNCPlay42]

@rustbot label -E-needs-mcve

[yanok]

@rustbot claim

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-high +T-compiler

[yanok]

Ok, I know what is the problem and have a fix.

Apparently when looking for Archive implementation for Option<[u8; ARRAY_SIZE]> compiler considers candidates: one for top-level impl and two ParamCandidates.

Before #87280 candidate_should_be_dropped_in_favor_of was using TypeFoldable::is_global() that was enough to discard the two ParamCandidates. But #87280 changed it to use TypeFoldable::is_know_global() instead, which is pessimistic, so now the compiler drops the global impl instead (because is_known_global is not sure) and then can't decide between the two ParamCandidates.

[yanok]

Funny thing is it seems like it's fixed already in the master branch... Trying to bisect what fixed it to better understand if my change is needed after all.

[yanok]

c6b6901 also fixes it.