We can never supply a meaningful implementation of this.
Instead, the follow up commits will create two intrinsics
that approximate comparisons:

* `ptr_maybe_eq`
* `ptr_maybe_ne`

The fact that `ptr_maybe_eq(a, b)` is not necessarily the same
value as `!ptr_maybe_ne(a, b)` is a symptom of this entire
problem.

…aining everything in the diagnostic

Fixes rust-lang#64731, rust-lang#73531.

See also rust-lang#64402#issuecomment-530852886

This commit fixes typos in the doc comments of 'librustc_mir/monomorphize/collector.rs'

…int.

typo fix

Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>

This commit fixes a regression introduced in rust-lang#73317 where an oversight
meant that `config.toml` was assumed to exist.

Signed-off-by: David Wood <david@davidtw.co>

Also, update the affected tests. This seems strictly better but it is
actually more permissive than I initially intended. In particular it
accepts this

```
forall<'a, 'b> {
  exists<'intersection> {
    'a: 'intersection,
    'b: 'intersection,
  }
}
```

and I'm not sure I want to accept that. It implies that we have a
`'empty` in the new universe intoduced by the `forall`.

…ers"

This reverts commit 2e01db4b396a1e161f7a73933fff34bc9421dba0.

In the new leak check, instead of getting a list of placeholders to
track, we look for any placeholder that is part of a universe which
was created during the snapshot.

We are looking for the following error patterns:

* P1: P2, where P1 != P2
* P1: R, where R is in some universe that cannot name P1

This new leak check is more precise than before, in that it accepts
this patterns:

* R: P1, even if R cannot name P1, because R = 'static is a valid
sol'n
* R: P1, R: P2, as above

Note that this leak check, when running during subtyping, is less
efficient than before in some sense because it is going to check and
re-check all the universes created since the snapshot. We're going to
move when the leak check runs to try and correct that.

In particular, it no longer occurs during the subtyping check. This is
important for enabling lazy normalization, because the subtyping check
will be producing sub-obligations that could affect its results.

Consider an example like

    for<'a> fn(<&'a as Mirror>::Item) =
      fn(&'b u8)

where `<T as Mirror>::Item = T` for all `T`. We will wish to produce a
new subobligation like

    <'!1 as Mirror>::Item = &'b u8

This will, after being solved, ultimately yield a constraint that `'!1
= 'b` which will fail. But with the leak-check being performed on
subtyping, there is no opportunity to normalize `<'!1 as
Mirror>::Item` (unless we invoke that normalization directly from
within subtyping, and I would prefer that subtyping and unification
are distinct operations rather than part of the trait solving stack).

The reason to keep the leak check during coherence and trait
evaluation is partly for backwards compatibility. The coherence change
permits impls for `fn(T)` and `fn(&T)` to co-exist, and the trait
evaluation change means that we can distinguish those two cases
without ambiguity errors. It also avoids recreating rust-lang#57639, where we
were incorrectly choosing a where clause that would have failed the
leak check over the impl which succeeds.

The other reason to keep the leak check in those places is that I
think it is actually close to the model we want. To the point, I think
the trait solver ought to have the job of "breaking down"
higher-ranked region obligation like ``!1: '2` into into region
obligations that operate on things in the root universe, at which
point they should be handed off to polonius. The leak check isn't
*really* doing that -- these obligations are still handed to the
region solver to process -- but if/when we do adopt that model, the
decision to pass/fail would be happening in roughly this part of the
code.

This change had somewhat more side-effects than I anticipated. It
seems like there are cases where the leak-check was not being enforced
during method proving and trait selection. I haven't quite tracked
this down but I think it ought to be documented, so that we know what
precisely we are committing to.

One surprising test was `issue-30786.rs`. The behavior there seems a
bit "fishy" to me, but the problem is not related to the leak check
change as far as I can tell, but more to do with the closure signature
inference code and perhaps the associated type projection, which
together seem to be conspiring to produce an unexpected
signature. Nonetheless, it is an example of where changing the
leak-check can have some unexpected consequences: we're now failing to
resolve a method earlier than we were, which suggests we might change
some method resolutions that would have been ambiguous to be
successful.

TODO:

* figure out remainig test failures
* add new coherence tests for the patterns we ARE disallowing

Revert the code that states that upcasting traits requires full
equality and change to require that the source type is a subtype of
the target type, as one would expect. As the comment states, this was
an old bug that we didn't want to fix yet as it interacted poorly with
the old leak-check. This fixes the old-lub-glb-object test, which was
previously reporting too many errors (i.e., in the previous commit).

Motivation:

- we want to use leak-check sparingly, first off
- these calls were essentially the same as doing the check during subtyping

The bug was revealed by the behavior of the old-lub-glb-hr-noteq1.rs
test. The old-lub-glb-hr-noteq2 test shows the current 'order dependent'
behavior of coercions around higher-ranked functions, at least when
running with `-Zborrowck=mir`.

Also, run compare-mode=nll.

Improve compiler error message for wrong generic parameter order

- Added optional "help" parameter that shows a help message on the compiler error if required.
- Added a simple ordered parameter as a sample help.

@varkor will make more changes as required. Let me know if I'm heading in the right direction.

Fixes rust-lang#68437

r? @varkor

…thewjasper

 move leak-check to during coherence, candidate eval

Implementation of MCP rust-lang/compiler-team#295.

I'd like to do a crater run on this.

Note to @rust-lang/lang: This PR is a breaking change (bugfix). It causes tests like the following to go from a future-compatibility warning rust-lang#56105 to a hard error:

```rust
trait Trait {}
impl Trait for for<'a, 'b> fn(&'a u32, &'b u32) {}
impl Trait for for<'c> fn(&'c u32, &'c u32) {} // now rejected, used to warn
```

I am not aware of any instances of this code in the wild, but that is why we are doing a crater run. The reason for this change is that those two types are, in fact, the same type, and hence the two impls are overlapping.

There will still be impls that trigger rust-lang#56105 after this lands, however -- I hope that we will eventually just accept those impls without warning, for the most part. One example of such an impl is this pattern, which is used by wasm-bindgen and other crates as well:

```rust
trait Trait {}
impl<T> Trait for fn(&T) { }
impl<T> Trait for fn(T) { } // still accepted, but warns
```

…,RalfJung,nagisa

A way forward for pointer equality in const eval

r? @varkor on the first commit and @RalfJung on the second commit

cc rust-lang#53020

…an-DPC

Clean up E0689 explanation

r? @Dylan-DPC

…, r=nikomatsakis

Account for multiple impl/dyn Trait in return type when suggesting `'_`

Make `impl` and `dyn` Trait lifetime suggestions a bit more resilient.

Follow up to rust-lang#72804.

r? @nikomatsakis

…s, r=oli-obk

Add second message for LiveDrop errors

This is an attempt to fix rust-lang#72907 by adding a second message to the `LiveDrop` diagnostics. Changing from this
```
error[E0493]: destructors cannot be evaluated at compile-time
 --> src/lib.rs:7:9
  |
7 |     let mut always_returned = None;
  |         ^^^^^^^^^^^^^^^^^^^ constants cannot evaluate destructors

error: aborting due to previous error
```
to this
```
error[E0493]: destructors cannot be evaluated at compile-time
  --> foo.rs:6:9
   |
6  |     let mut always_returned = None;
   |         ^^^^^^^^^^^^^^^^^^^ constants cannot evaluate destructors
...
10 |         always_returned = never_returned;
   |         --------------- value is dropped here

error: aborting due to previous error
```
r? @RalfJung @ecstatic-morse

Clarify --extern documentation.

Fixes rust-lang#64731, rust-lang#73531.

See also rust-lang#64402 (comment)

Fix typos in doc comments

Hello 🦀 ,

This commit fixes typos in the doc comments of 'librustc_mir/monomorphize/collector.rs'

Thank you for reviewing this PR 👍

…var, r=Mark-Simulacrum

bootstrap: no `config.toml` exists regression

Fixes rust-lang#73574.

This PR fixes a regression introduced in rust-lang#73317 where an oversight meant that `config.toml` was assumed to exist.