negative_impls impl changed for tuple

[wangyanci]

Code

I tried this code:

#![feature(negative_impls, auto_traits)]

auto trait NotEqual {}

impl<T> ! NotEqual for (T, T) {} // expected: NotEqual is not implemented only and only if for tuple(a, b) a and b are of the same type

fn call<T>(_: T) where T: NotEqual {} // expected: only tuple(a, b) a and b are of the same type compile err; but a and b are of the different type compile ok

fn main() {
    // compile ok in all nightly;  it's exactly what i expected
    call(1);
    call((1,1.2, 1.3));

    // compile not ok in all nightly;  yes, it's exactly what i expected
    // call((1,1));

    // compile ok in nightly-2023-07-28(cmd: cargo +nightly-2023-07-28 run);  yes, it's exactly what i expected
    // compile not ok after nightly-2023-07-29 version(cmd: cargo +nightly-2023-07-29 run);  it's not exactly what i expected! why? for what reasons did it change?
    call((1,1.2));
}

I expected to see this happen:
just compile ok like before nightly-2023-07-28 version

$ cargo +nightly-2023-07-28 run
   Compiling test_negative_impls v0.1.0 (/Users/wangyanci/workspace_rust/test_negative_impls)
    Finished dev [unoptimized + debuginfo] target(s) in 0.20s
     Running `target/debug/test_negative_impls`

Instead, this happened:
compile error after nightly-2023-07-29 version

Compiling test_negative_impls v0.1.0 (/Users/wangyanci/workspace_rust/test_negative_impls)
error[E0277]: the trait bound `({integer}, {float}): NotEqual` is not satisfied
  --> src/main.rs:15:10
   |
15 |     call((1,1.2));
   |     ---- ^^^^^^^ the trait `NotEqual` is not implemented for `({integer}, {float})`
   |     |
   |     required by a bound introduced by this call
   |
note: required by a bound in `call`
  --> src/main.rs:7:27
   |
7  | fn call<T>(_: T) where T: NotEqual {}
   |                           ^^^^^^^^ required by this bound in `call`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `test_negative_impls` (bin "test_negative_impls") due to previous error

Version it worked on

It most recently worked on: nightly-2023-07-28

$rustup install nightly-2023-07-28
info: syncing channel updates for 'nightly-2023-07-28-x86_64-apple-darwin'

  nightly-2023-07-28-x86_64-apple-darwin unchanged - rustc 1.73.0-nightly (500647fd8 2023-07-27)

info: checking for self-update

Version with regression

rustc --version --verbose:
nightly-2023-07-28

$ rustup override set nightly-2023-07-28 && rustc --version --verbose
info: using existing install for 'nightly-2023-07-28-x86_64-apple-darwin'
info: override toolchain for '/Users/wangyanci/workspace_rust/test_negative_impls' set to 'nightly-2023-07-28-x86_64-apple-darwin'

  nightly-2023-07-28-x86_64-apple-darwin unchanged - rustc 1.73.0-nightly (500647fd8 2023-07-27)

rustc 1.73.0-nightly (500647fd8 2023-07-27)
binary: rustc
commit-hash: 500647fd8138cc09e87edb08d62f81654fbf6ef8
commit-date: 2023-07-27
host: x86_64-apple-darwin
release: 1.73.0-nightly
LLVM version: 16.0.5

nightly-2023-07-28

rustup override set nightly-2023-07-29 && rustc --version --verbose
info: using existing install for 'nightly-2023-07-29-x86_64-apple-darwin'
info: override toolchain for '/Users/wangyanci/workspace_rust/test_negative_impls' set to 'nightly-2023-07-29-x86_64-apple-darwin'

$  nightly-2023-07-29-x86_64-apple-darwin unchanged - rustc 1.73.0-nightly (04abc370b 2023-07-28)

rustc 1.73.0-nightly (04abc370b 2023-07-28)
binary: rustc
commit-hash: 04abc370b9f3855b28172b65a7f7d5a433f41412
commit-date: 2023-07-28
host: x86_64-apple-darwin
release: 1.73.0-nightly
LLVM version: 16.0.5

Backtrace

Backtrace

$ cargo +nightly-2023-07-29 run
error[E0277]: the trait bound `({integer}, {float}): NotEqual` is not satisfied
  --> src/main.rs:15:10
   |
15 |     call((1,1.2));
   |     ---- ^^^^^^^ the trait `NotEqual` is not implemented for `({integer}, {float})`
   |     |
   |     required by a bound introduced by this call
   |
note: required by a bound in `call`
  --> src/main.rs:7:27
   |
7  | fn call<T>(_: T) where T: NotEqual {}
   |                           ^^^^^^^^ required by this bound in `call`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `test_negative_impls` (bin "test_negative_impls") due to previous error

I'd like to know what considerations changed it or is it a problem in itself

[compiler-errors]

This is intentional. This broke in #113312 -- see the FCP in #113312 (comment), which decided that we would adjust the way that impls (positive or negative) interact with auto traits to fix unsoundness.

[wangyanci]

This is intentional. This broke in #113312 -- see the FCP in #113312 (comment), which decided that we would adjust the way that impls (positive or negative) interact with auto traits to fix unsoundness.

OK， thanks