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 #72804.

r? @nikomatsakis

Clean up E0689 explanation

r? @Dylan-DPC

A way forward for pointer equality in const eval

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

cc #53020

 move leak-check to during coherence, candidate eval

Implementation of MCP https://github.com/rust-lang/compiler-team/issues/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 #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 #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
```

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 #68437

r? @varkor

impl ToSocketAddrs for (String, u16)

This adds a convenience impl of `ToSocketAddrs for (String, u16)`. When authoring HTTP services it's common to take command line options for `host` and `port` and parse them into `String` and `u16` respectively. Consider the following program:
```rust
#[derive(Debug, StructOpt)]
struct Config {
    host: String,
    port: u16,
}

async fn main() -> io::Result<()> {
    let config = Config::from_args();
    let stream = TcpStream::connect((&*config.host, config.port))?; // &* is not ideal
    // ...
}
```

Networking is a pretty common starting point for people new to Rust, and seeing `&*` in basic examples can be confusing. Even as someone that has experience with networking in Rust I tend to forget that `String` can't be passed directly there. Instead with this patch we can omit the `&*` conversion and pass `host` directly:
```rust
#[derive(Debug, StructOpt)]
struct Config {
    host: String,
    port: u16,
}

async fn main() -> io::Result<()> {
    let config = Config::from_args();
    let stream = TcpStream::connect((config.host, config.port))?; // no more conversions!
    // ...
}
```

I think should be an easy and small ergonomics improvement for networking. Thanks!

Revert PR #72389 - "Explain move errors that occur due to method calls involving `self"

r? @petrochenkov

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.

Motivation:

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

None of the tools seem to need syn 0.15.35, so we can just build syn
1.0.

This was causing an issue with clippy's `compile-test` program: since
multiple versions of `syn` would exist in the build directory, we would
non-deterministically pick one based on filesystem iteration order. If
the pre-1.0 version of `syn` was picked, a strange build error would
occur (see
https://github.com/rust-lang/rust/pull/73594#issuecomment-647671463)

To prevent this kind of issue from happening again, we now panic if we
find multiple versions of a crate in the build directly, instead of
silently picking the first version we find.

This reverts commit 372cb9b6, reversing
changes made to 5c61a8dc.

Update cargo

3 commits in 79c769c3d7b4c2cf6a93781575b7f592ef974255..089cbb80b73ba242efdcf5430e89f63fa3b5328d
2020-06-11 22:13:37 +0000 to 2020-06-15 14:38:34 +0000
- Support linker with -Zdoctest-xcompile. (rust-lang/cargo#8359)
- Fix doctests not running with --target=HOST. (rust-lang/cargo#8358)
- Allow passing a registry index url directly to `cargo install` (rust-lang/cargo#8344)

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).

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 #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

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.

This reverts commit 2e01db4b396a1e161f7a73933fff34bc9421dba0.

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`.

Rollup of 6 pull requests

Successful merges:

 - #71660 (impl PartialEq<Vec<B>> for &[A], &mut [A])
 - #72623 (Prefer accessible paths in 'use' suggestions)
 - #73502 (Add E0765)
 - #73580 (deprecate wrapping_offset_from)
 - #73582 (Miri: replace many bug! by span_bug!)
 - #73585 (Do not send a notification for P-high stable regressions)

Failed merges:

 - #73581 (Create 0766 error code)

r? @ghost

Do not send a notification for P-high stable regressions

This is kind of a hack to only match nightly and beta regressions, but not stable regressions. See my tests [on the playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6ff8a809162118aa2951f2ff12400067).

r? @spastorino cc @Mark-Simulacrum

Miri: replace many bug! by span_bug!

r? @oli-obk

deprecate wrapping_offset_from

As per https://github.com/rust-lang/rust/issues/41079#issuecomment-433140733 which seems like a consensus.

r? @Amanieu

Add E0765

Prefer accessible paths in 'use' suggestions

This PR addresses issue https://github.com/rust-lang/rust/issues/26454, where `use` suggestions are made for paths that don't work. For example:

```rust
mod foo {
    mod bar {
        struct X;
    }
}

fn main() { X; } // suggests `use foo::bar::X;`
```

impl PartialEq<Vec<B>> for &[A], &mut [A]

https://github.com/rust-lang/rfcs/issues/2917

Cache flags and escaping vars for predicates

With predicates becoming interned (rust-lang/compiler-team#285) this is now possible and could be a perf win. It would become an even larger win once we have recursive predicates.

cc @lcnr @nikomatsakis

r? @ghost

Add comment to clarify the pattern

Upgrade Chalk

Things done in this PR:
- Upgrade Chalk to `0.11.0`
- Added compare-mode=chalk
- Bump rustc-hash in `librustc_data_structures` to `1.1.0` to match Chalk
- Removed `RustDefId` since the builtin type support is there
- Add a few more `FIXME(chalk)`s for problem spots I hit when running all tests with chalk
- Added some more implementation code for some newer builtin Chalk types (e.g. `FnDef`, `Array`)
- Lower `RegionOutlives` and `ObjectSafe` predicates
- Lower `Dyn` without the region
- Handle `Int`/`Float` `CanonicalVarKind`s
- Uncomment some Chalk tests that actually work now
- Remove the revisions in `src/test/ui/coherence/coherence-subtyping.rs` since they aren't doing anything different

r? @nikomatsakis

This fixes an issue with the following sample:

    mod foo {
	mod inaccessible {
	    pub struct X;
	}
	pub mod avail {
	    pub struct X;
	}
    }

    fn main() { X; }

Instead of suggesting both `use crate::foo::inaccessible::X;` and `use
crate::foo::avail::X;`, it should only suggest the latter.

It is done by trimming the list of suggestions from inaccessible paths
if accessible paths are present.

Visibility is checked with `is_accessible_from` now instead of being
hard-coded.

-

Some tests fixes are trivial, and others require a bit more explaining,
here are my comments:

src/test/ui/issues/issue-35675.stderr: Only needs to make the enum
public to have the suggestion make sense.

src/test/ui/issues/issue-42944.stderr: Importing the tuple struct won't
help because its constructor is not visible, so the attempted
constructor does not work. In that case, it's better not to suggest it.
The case where the constructor is public is covered in `issue-26545.rs`.

Enable LLVM zlib

Compilers may generate ELF objects with compressed sections (although rustc currently doesn't do this). Currently, when linking these with `rust-lld`, you'll get this error:

`rust-lld: error: ...: contains a compressed section, but zlib is not available`

This enables zlib when building LLVM.