rkyv in Rust 1.56: type annotations needed

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

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.

add maintenance notes around the fix for rust-lang/rust#90195

@rustbot

Bisected to #87280.

@rustbot label +E-needs-mcve

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() {}

@rustbot

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)

@rustbot

@rustbot label -E-needs-mcve

@rustbot

@rustbot claim

@rustbot

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

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

Use `is_global` in `candidate_should_be_dropped_in_favor_of`

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.

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.

c6b6901 also fixes it.

Rollup merge of rust-lang#90375 - yanok:master, r=lcnr

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rkyv in Rust 1.56: type annotations needed #90195

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rkyv in Rust 1.56: type annotations needed #90195

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xobs commented on Oct 23, 2021

hkratz commented on Oct 23, 2021

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yanok commented on Oct 28, 2021

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