Nonsensical help message in error E0277

[ElectrifyPro]

I tried this code (while debugging):

async fn handle_binary(server_ref: &Ref<State>, uuid: Uuid, bytes: Bytes) -> Result<(), SendError<Packet>> {
    if uuid.as_bytes() == [0xaa, 0x2a, 0x15, 0xf6, 0xca, 0xb3, 0x46, 0xdf, 0xa8, 0x28, 0x25, 0x4d, 0x96, 0x4d, 0xd8, 0x57] {
        // give user fake lag
        tokio::time::sleep(tokio::time::Duration::from_millis(1000)).await;
    }
}

Where Uuid is the corresponding type from the uuid crate (and everything else is my types). It has an associated function as_bytes, which returns a 16-byte slice containing the UUID. Because the right-hand-side of the comparison is not a byte slice but an array, the two expressions can't be compared and I get error E0277, shown below. Of interest is the help message at the end, which has the correct solution, but proposes a fix with invalid syntax. It should suggest placing & before [0xaa,....

error[E0277]: can't compare `&[u8; 16]` with `[{integer}; 16]`
  --> src/api/ws.rs:68:24
   |
68 |     if uuid.as_bytes() == [0xaa, 0x2a, 0x15, 0xf6, 0xca, 0xb3, 0x46, 0xdf, 0xa8, 0x28, 0x25, 0x4d, 0x96, 0x4d, 0xd8, 0x57] {
   |                        ^^ no implementation for `&[u8; 16] == [{integer}; 16]`
   |
   = help: the trait `PartialEq<[{integer}; 16]>` is not implemented for `&[u8; 16]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             <&[B] as PartialEq<[A; N]>>
             <&[FormatItem<'_>] as PartialEq<FormatItem<'_>>>
             <&[OwnedFormatItem] as PartialEq<OwnedFormatItem>>
             <&[T] as PartialEq<Vec<U, A>>>
             <&[u8] as PartialEq<BytesMut>>
             <&[u8] as PartialEq<bytes::Bytes>>
             <&mut [B] as PartialEq<[A; N]>>
             <&mut [T] as PartialEq<Vec<U, A>>>
           and 11 others
help: convert the array to a `&[u8]` slice instead
   |
68 |     if uuid.as_bytes() &==[..] [0xaa, 0x2a, 0x15, 0xf6, 0xca, 0xb3, 0x46, 0xdf, 0xa8, 0x28, 0x25, 0x4d, 0x96, 0x4d, 0xd8, 0x57] {
   |                        +  ++++

Meta

rustc --version --verbose:

rustc 1.71.0-beta.4 (dbf31f17d 2023-06-24)
binary: rustc
commit-hash: dbf31f17d020475885f50fb7d5fc15ff239843d8
commit-date: 2023-06-24
host: x86_64-unknown-linux-gnu
release: 1.71.0-beta.4
LLVM version: 16.0.5

I could not test on nightly, as I got a different compile error, apparently related to procedural macros:

Error

warning: some crates are on edition 2021 which defaults to `resolver = "2"`, but virtual workspaces default to `resolver = "1"`
note: to keep the current resolver, specify `workspace.resolver = "1"` in the workspace root's manifest
note: to use the edition 2021 resolver, specify `workspace.resolver = "2"` in the workspace root's manifest
   Compiling proc-macro2 v1.0.48
   Compiling cc v1.0.78
   Compiling parking_lot_core v0.9.5
   Compiling lock_api v0.4.9
   Compiling slab v0.4.7
   Compiling tokio v1.23.0
    Checking futures-sink v0.3.25
   Compiling generic-array v0.14.6
    Checking mio v0.8.5
    Checking socket2 v0.4.7
error[E0422]: cannot find struct, variant or union type `LineColumn` in crate `proc_macro`
   --> /home/me/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.48/src/wrapper.rs:479:33
    |
479 |                 let proc_macro::LineColumn { line, column } = s.start();
    |                                 ^^^^^^^^^^ not found in `proc_macro`
    |
help: consider importing one of these items
    |
1   + use crate::LineColumn;
    |
1   + use crate::fallback::LineColumn;
    |
help: if you import `LineColumn`, refer to it directly
    |
479 -                 let proc_macro::LineColumn { line, column } = s.start();
479 +                 let LineColumn { line, column } = s.start();
    |

error[E0422]: cannot find struct, variant or union type `LineColumn` in crate `proc_macro`
   --> /home/me/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.48/src/wrapper.rs:496:33
    |
496 |                 let proc_macro::LineColumn { line, column } = s.end();
    |                                 ^^^^^^^^^^ not found in `proc_macro`
    |
help: consider importing one of these items
    |
1   + use crate::LineColumn;
    |
1   + use crate::fallback::LineColumn;
    |
help: if you import `LineColumn`, refer to it directly
    |
496 -                 let proc_macro::LineColumn { line, column } = s.end();
496 +                 let LineColumn { line, column } = s.end();
    |

error[E0635]: unknown feature `proc_macro_span_shrink`
  --> /home/me/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.48/src/lib.rs:92:30
   |
92 |     feature(proc_macro_span, proc_macro_span_shrink)
   |                              ^^^^^^^^^^^^^^^^^^^^^^

    Checking signal-hook-registry v1.4.0
    Checking num_cpus v1.14.0
    Checking parking_lot v0.12.1

[jieyouxu]

For the proc_macro_span_shrink error, see #113152.

It is fixed in proc-macro2 1.0.60+ dtolnay/proc-macro2#391

Could you try to bump your proc-macro2 version? Also, could you maybe provide either the full code or a smaller reproduction of the suggestion (if possible), I can't seem to reproduce the suggestion for your pattern on stable.

[ElectrifyPro]

Thanks for the notice. I just bumped proc-macro2 to 1.0.63 and retried on nightly:

rustc 1.72.0-nightly (85bf07972 2023-07-06)
binary: rustc
commit-hash: 85bf07972a1041b9e25393b803d0e006bec3eaaf
commit-date: 2023-07-06
host: x86_64-unknown-linux-gnu
release: 1.72.0-nightly
LLVM version: 16.0.5

Unfortunately the exact same misleading help message still appears.

Also, could you maybe provide either the full code or a smaller reproduction of the suggestion (if possible), I can't seem to reproduce the suggestion for your pattern on stable.

I managed to reproduce it here: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=138ae39d401a51710bb27978cf730f49

The bytes::Bytes import or Bytes argument is actually necessary to cause the bug. If you remove both, the help message doesn't appear. Perhaps this is related to the uuid crate, which I noticed uses a type alias called Bytes (pub type Bytes = [u8; 16]), unrelated to bytes::Bytes.

[jieyouxu]

Smaller repro but still requires bytes dependency:

use bytes::Bytes;

fn main() {
    &[0u8] == [0xAA];
}

Am investigating how to get rid of the bytes dependency.

[jieyouxu]

Repro without dependencies:

pub struct Bytes;

impl Bytes {
    pub fn as_slice(&self) -> &[u8] {
        todo!()
    }
}

impl PartialEq<[u8]> for Bytes {
    fn eq(&self, other: &[u8]) -> bool {
        self.as_slice() == other
    }
}

impl PartialEq<Bytes> for &[u8] {
    fn eq(&self, other: &Bytes) -> bool {
        *other == **self
    }
}

fn main() {
    &[0u8] == [0xAA];
}