Allow comparing Boxes with different allocators

[gootorov]

Currently, comparing Boxes over different allocators is not allowed:

error[E0308]: mismatched types
  --> library/alloc/tests/boxed.rs:22:20
   |
22 |     assert_eq!(b1, b2);
   |                    ^^ expected `Box<{integer}, ConstAllocator>`, found `Box<{integer}, AnotherAllocator>`
   |
   = note: expected struct `Box<{integer}, ConstAllocator>`
              found struct `Box<{integer}, AnotherAllocator>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `alloc` (test "collectionstests") due to previous error

This PR lifts this limitation

[rustbot]

r? @joshtriplett

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[gootorov]

Corresponding ACP: rust-lang/libs-team#241

[joshtriplett]

@bors r+ rollup

[bors]

📌 Commit 001b081 has been approved by joshtriplett

It is now in the queue for this repository.

[smoelius]

This PR appears to have introduced a regression. The following program compiles before this PR, but not after:

#[derive(Debug, PartialEq)]
struct Expr;

#[allow(dead_code)]
fn from_exprs(exprs: Vec<Box<Expr>>) {
    debug_assert_ne!(exprs, vec![], "exprs must not be empty");
}

E.g.:

$ cargo clean && cargo +353dd71d73d build
   Compiling a v0.1.0 (/tmp/a)
    Finished dev [unoptimized + debuginfo] target(s) in 0.08s
$ cargo clean && cargo +448d2a84179 build
   Compiling a v0.1.0 (/tmp/a)
error[E0282]: type annotations needed
 --> src/lib.rs:6:5
  |
6 |     debug_assert_ne!(exprs, vec![], "exprs must not be empty");
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
  |
  = note: this error originates in the macro `$crate::assert_ne` which comes from the expansion of the macro `debug_assert_ne` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0282`.
error: could not compile `a` (lib) due to previous error

I noticed this while trying to build swc_ecma_ast (which does not currently buiid with nightly):

error[E0282]: type annotations needed
   --> crates/swc_ecma_ast/src/expr.rs:210:9
    |
210 |         debug_assert_ne!(exprs, vec![], "exprs must not be empty");
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
    |
    = note: this error originates in the macro `$crate::assert_ne` which comes from the expansion of the macro `debug_assert_ne` (in Nightly builds, run with -Z macro-backtrace for more info)