Compiler asks for already present bound

[newpavlov]

While working on RustCrypto/stream-ciphers#195. I got roughly the following code:

use generic_array::{GenericArray, ArrayLength, typenum::{Unsigned, Quot, U16}};
use core::ops::Div;

pub trait Foo: Copy + Default {
    type Size: Unsigned;

    fn load<N>(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
    where
        N: ArrayLength<u8> + Div<Self::Size>,
        Quot<N, Self::Size>: ArrayLength<Self>
    {
        Default::default()
    }
}

#[derive(Copy, Clone, Default)]
#[repr(transparent)]
struct Bar(u128);

impl Foo for Bar {
    type Size = U16;
}

It compiles without issues. The default method impl will not be present in the final code, I just use it to show that the bounds are correct.

But if we are to overwrite the load method in the Bar impl by simply copying the default impl:

impl Foo for Bar {
    type Size = U16;

    fn load<N>(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
    where
        N: ArrayLength<u8> + Div<Self::Size>,
        Quot<N, Self::Size>: ArrayLength<Self>
    {
        Default::default()
    }
}

Playground

Compiler will return the following compilation errors:

Details

error[E0277]: cannot divide `N` by `UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>`
  --> src/lib.rs:23:5
   |
23 | /     fn load<N>(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
24 | |     where
25 | |         N: ArrayLength<u8> + Div<Self::Size>,
26 | |         Quot<N, Self::Size>: ArrayLength<Self>
   | |______________________________________________^ no implementation for `N / UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>`
   |
help: consider further restricting this bound
   |
25 |         N: ArrayLength<u8> + Div<Self::Size> + Div<UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>>,
   |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0277]: cannot divide `N` by `UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>`
  --> src/lib.rs:23:5
   |
23 | /     fn load<N>(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
24 | |     where
25 | |         N: ArrayLength<u8> + Div<Self::Size>,
26 | |         Quot<N, Self::Size>: ArrayLength<Self>
27 | |     {
28 | |         Default::default()
29 | |     }
   | |_____^ no implementation for `N / UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>`
   |
help: consider further restricting this bound
   |
25 |         N: ArrayLength<u8> + Div<Self::Size> + Div<UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>>,
   |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0277]: cannot divide `N` by `UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>`
  --> src/lib.rs:23:8
   |
23 |     fn load<N>(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
   |        ^^^^ no implementation for `N / UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>`
   |
help: consider further restricting this bound
   |
25 |         N: ArrayLength<u8> + Div<Self::Size> + Div<UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>>,
   |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 3 previous errors

Replacing Div<Self::Size> with Div<U16> or even with explicit Div<Uint<..>> does not fix the error.

[newpavlov]

I think the "needs repro" tag is wrong. The bug is reproduced in the playground link. It's not a minimal example, but it's a reproduction nevertheless.

[workingjubilee]

thanks for saying so! sorry, some of our automated triage steps recently involved some mistakes while we were trying to sift "things that lack an actually minimal example" from "things that lack any example at all".

[RodBurman]

The example code that "Compiles without issues" no longer does, I expect this is a change to GenericArray or a swap to a different supplier of that type. In the playground I get:
--Compiling playground v0.0.1 (/playground)
error[E0107]: trait takes 0 generic arguments but 1 generic argument was supplied
--> src/lib.rs:25:12
|
25 | N: ArrayLength + DivSelf::Size,
| ^^^^^^^^^^^ expected 0 generic arguments
|
note: trait defined here, with 0 generic parameters
--> /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/generic-array-1.1.0/src/lib.rs:179:18
|
179 | pub unsafe trait ArrayLength: Unsigned + 'static {
| ^^^^^^^^^^^
help: replace the generic bound with the associated type
|
25 | N: ArrayLength<ArrayType = u8> + DivSelf::Size,
| +++++++++++

error[E0107]: trait takes 0 generic arguments but 1 generic argument was supplied
--> src/lib.rs:26:30
|
26 | Quot<N, Self::Size>: ArrayLength
| ^^^^^^^^^^^ expected 0 generic arguments
|
note: trait defined here, with 0 generic parameters
--> /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/generic-array-1.1.0/src/lib.rs:179:18
|
179 | pub unsafe trait ArrayLength: Unsigned + 'static {
| ^^^^^^^^^^^
help: replace the generic bound with the associated type
|
26 | Quot<N, Self::Size>: ArrayLength<ArrayType = Self>
| +++++++++++

error[E0107]: trait takes 0 generic arguments but 1 generic argument was supplied
--> src/lib.rs:9:12
|
9 | N: ArrayLength + DivSelf::Size,
| ^^^^^^^^^^^ expected 0 generic arguments
|
note: trait defined here, with 0 generic parameters
--> /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/generic-array-1.1.0/src/lib.rs:179:18
|
179 | pub unsafe trait ArrayLength: Unsigned + 'static {
| ^^^^^^^^^^^
help: replace the generic bound with the associated type
|
9 | N: ArrayLength<ArrayType = u8> + DivSelf::Size,
| +++++++++++

error[E0107]: trait takes 0 generic arguments but 1 generic argument was supplied
--> src/lib.rs:10:30
|
10 | Quot<N, Self::Size>: ArrayLength
| ^^^^^^^^^^^ expected 0 generic arguments
|
note: trait defined here, with 0 generic parameters
--> /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/generic-array-1.1.0/src/lib.rs:179:18
|
179 | pub unsafe trait ArrayLength: Unsigned + 'static {
| ^^^^^^^^^^^
help: replace the generic bound with the associated type
|
10 | Quot<N, Self::Size>: ArrayLength<ArrayType = Self>
| +++++++++++

error[E0277]: cannot divide N by UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>
--> src/lib.rs:23:5
|
23 | / fn load(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
24 | | where
25 | | N: ArrayLength + DivSelf::Size,
26 | | Quot<N, Self::Size>: ArrayLength
| |______________________________________________^ no implementation for N / UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>

error[E0276]: impl has stricter requirements than trait
--> src/lib.rs:25:30
|
7 | / fn load(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
8 | | where
9 | | N: ArrayLength + DivSelf::Size,
10 | | Quot<N, Self::Size>: ArrayLength
| |______________________________________________- definition of load from trait
...
25 | N: ArrayLength + DivSelf::Size,
| ^^^^^^^^^^^^^^^ impl has extra requirement N: Div<UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>>

error[E0277]: cannot divide N by UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>
--> src/lib.rs:23:67
|
23 | fn load(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
| ^^^^^^^^^^^^^^^^^^^ no implementation for N / UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>

error[E0277]: cannot divide N by UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>
--> src/lib.rs:27:5
|
27 | / {
28 | | Default::default()
29 | | }
| |_____^ no implementation for N / UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>

warning: unused variable: block
--> src/lib.rs:7:16
|
7 | fn load(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
| ^^^^^ help: if this is intentional, prefix it with an underscore: _block
|
= note: #[warn(unused_variables)] on by default

Some errors have detailed explanations: E0107, E0276, E0277.
For more information about an error, try rustc --explain E0107.
warning: playground (lib) generated 1 warning
error: could not compile playground (lib) due to 8 previous errors; 1 warning emitted

As a check I tried a build locally on my Mac using:
rod@Mac % cargo --version
cargo 1.81.0 (2dbb1af80 2024-08-20)
rod@Mac % cargo --version
cargo 1.81.0 (2dbb1af80 2024-08-20)

and I got the same errors.

[newpavlov]

I expect this is a change to GenericArray

Yes, the snippet targets generic-array v0.14, not v1.

[RodBurman]

Unfortunately the playground only has the options of versions 1.1.0 or 0.14.7 of GenericArray which both fail the "modern way", so the crate changes in one of the 0.14.x versions to make the original code non-viable.
If I use my local compiler and GenericArray version 0.14.0 the problem is reproducible with slightly different (expanded) compiler messages:

Compiling apb v0.1.0 (/Users/rod/code/rust/triage/apb)
error[E0277]: cannot divide N by UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>
--> src/lib.rs:23:5
|
23 | / fn load(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
24 | | where
25 | | N: ArrayLength + DivSelf::Size,
26 | | Quot<N, Self::Size>: ArrayLength
| |______________________________________________^ no implementation for N / UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>

error[E0276]: impl has stricter requirements than trait
--> src/lib.rs:25:30
|
7 | / fn load(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
8 | | where
9 | | N: ArrayLength + DivSelf::Size,
10 | | Quot<N, Self::Size>: ArrayLength
| |_____________________________________________- definition of load from trait
...
25 | N: ArrayLength + DivSelf::Size,
| ^^^^^^^^^^^^^^^ impl has extra requirement N: Div<UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>>

error[E0277]: cannot divide N by UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>
--> src/lib.rs:23:67
|
23 | fn load(block: &GenericArray<u8, N>) -> GenericArray<Self, Quot<N, Self::Size>>
| ^^^^^^^^^^^^^^^^^^^ no implementation for N / UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>

error[E0277]: cannot divide N by UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>
--> src/lib.rs:27:5
|
27 | / {
28 | | Default::default()
29 | | }
| |_____^ no implementation for N / UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B0>, B0>

Some errors have detailed explanations: E0276, E0277.
For more information about an error, try rustc --explain E0276.
error: could not compile apb (lib) due to 4 previous errors