Rollup of 9 pull requests

Successful merges:

 - #69251 (#[track_caller] in traits)
 - #69880 (miri engine: turn error sanity checks into assertions)
 - #70207 (Use getentropy(2) on macos)
 - #70227 (Only display definition when suggesting a typo)
 - #70236 (resolve: Avoid "self-confirming" import resolutions in one more case)
 - #70248 (parser: simplify & remove unused field)
 - #70249 (handle ConstKind::Unresolved after monomorphizing)
 - #70269 (remove redundant closures (clippy::redundant_closure))
 - #70270 (Clean up E0449 explanation)

Failed merges:

r? @ghost

Clean up E0449 explanation

r? @Dylan-DPC

remove redundant closures (clippy::redundant_closure)

handle ConstKind::Unresolved after monomorphizing

fixes #70125

r? @bjorn3

parser: simplify & remove unused field

r? @petrochenkov

resolve: Avoid "self-confirming" import resolutions in one more case

So the idea behind "blacklisted bindings" is that we must ignore some name definitions during resolution because otherwise they cause infinite cycles.
E.g. import
```rust
use my_crate;
```
would refer to itself (on 2018 edition) without this blacklisting, because `use my_crate;` is the first name in scope when we are resolving `my_crate` here.

In this PR we are doing this blacklisting for the case
```rust
use same::same;
```
, namely blacklisting the second `same` when resolving the first `same`.
This was previously forgotten.

Fixes https://github.com/rust-lang/rust/issues/62767

Only display definition when suggesting a typo

Closes #70206
r? @Centril

Use getentropy(2) on macos

resolves #70179

miri engine: turn error sanity checks into assertions

We had these as debug assertions so far to make sure our test suite is clean, but really these are conditions that should never arise and also @EddyB told me to turn non-performance-critical debug assertions into full assertions so here we go. ;)

I propose that we do a check-only crater run to make sure this does not actually happen in practice.

r? @oli-obk

#[track_caller] in traits

Per https://github.com/rust-lang/rust/issues/47809#issuecomment-572791760, this allows the `#[track_caller]` attribute on trait methods.

Includes tests for `#[track_caller]` with:

* "regular" trait impls
* default trait impls
* "blanket-tracked" trait impls, where the annotation is in the trait definition and is inherited by "regular" impls of the trait

Liberate `rustc_ast_lowering` from `rustc`

The whole point of this PR is the very last commit, in which we remove `rustc` as one of `rustc_ast_lowering`'s dependencies, thereby improving `./x.py` parallelism and working towards https://github.com/rust-lang/rust/issues/65031.

Noteworthy:
- From `rustc::arena` we move logic into `arena`, in particular `declare_arena!`. This is then used in `rustc_ast_lowering` so that lowering has its own separate arena.
- Some linting code is unfortunately moved to `rustc_session::lint` cause its used both in `rustc_lint` and `rustc_ast_lowering`, and this is their common dependency.
- `rustc_session::CrateDisambiguator` is moved into `rustc_ast` so that `rustc::hir::map::definitions` can be moved into `rustc_hir`, so that `rustc_ast_lowering` can stop referring to `rustc::hir`.

r? @Zoxc

We can do this now that opt_associated_item doesn't have any panicking paths.

The codegen implementation already works for this, so we're:

* propagating track_caller attr from trait def to impl
* relaxing errors
* adding tests

Approved in a recent lang team meeting:
https://github.com/rust-lang/lang-team/blob/master/minutes/2020-01-09.md

perf(dep_graph): Avoid allocating a set on when the number reads are …

…small

`reserve_and_rehash` takes up 1.4% of the runtime on the `packed-simd`
benchmark which I believe is due to the number of reads are very low in
many cases (see https://github.com/rust-lang/rust/pull/50565 for
instance).

This avoids allocating the set until we start allocating the `reads`
`SmallVec` but it is possible that a lower limit might be better (not
tested since the improvement will be hard to spot either way).

Allow calculating the layout behind a pointer

There was some discussion around allowing this previously.

This does make the requirement for raw pointers to have valid metadata exposed as part of the std API (as a safety invariant, not validity invariant), though I think this is not strictly necessarily required as of current. cc @rust-lang/wg-unsafe-code-guidelines

Naming is hard; I picked the best "obvious" name I could come up with.

If it's agreed that this is actually a desired API surface, I'll file a tracking issue and update the attributes.

Rollup of 10 pull requests

Successful merges:

 - #68099 (Amend Rc/Arc::from_raw() docs regarding unsafety)
 - #70172 (parse/lexer: support `StringReader::retokenize` called on external files.)
 - #70209 (parser: recover on `for<'a> |...| body` closures)
 - #70223 (fix type of const params in associated types.)
 - #70229 (more clippy fixes)
 - #70240 (Return NonZeroU64 from ThreadId::as_u64.)
 - #70250 (Remove wrong entry from RELEASES.md)
 - #70253 (Remove another wrong entry from RELEASES.md)
 - #70254 (couple more clippy fixes (let_and_return, if_same_then_else))
 - #70266 (proc_macro_harness: Use item header spans for errors)

Failed merges:

r? @ghost

proc_macro_harness: Use item header spans for errors

Addresses https://github.com/rust-lang/rust/pull/70233#discussion_r396043004.

couple more clippy fixes (let_and_return, if_same_then_else)

* summarize if-else-code with identical blocks (clippy::if_same_then_else)
* don't create variable bindings just to return the bound value immediately (clippy::let_and_return)

Remove another wrong entry from RELEASES.md

The entry is under 1.11.0, but the feature is only available since 1.12.0 (for which an identical entry exists).

![Screenshot_2020-03-22 Support `cfg_attr` on `path` attributes by jseyfried · Pull Request #34546 · rust-lang rust](https://user-images.githubusercontent.com/951129/77238862-85c8f580-6bd4-11ea-8d31-77e2994a4b5a.png)

Remove wrong entry from RELEASES.md

resolves #70247

Return NonZeroU64 from ThreadId::as_u64.

As discussed in #67939, this allows turning Option<ThreadId> into Option<NonZeroU64> which
can then be stored inside an AtomicU64.

more clippy fixes

* remove unused unit values (clippy::unused_unit)
* make some let-if-bindings more idiomatic (clippy::useless_let_if_seq)
* clarify when we pass () to functions (clippy::unit_arg)
* don't redundantly repeat field names (clippy::redundant_field_names)
* remove redundant returns (clippy::needless_return)
* use let instead of match for matches with single bindings (clippy::match_single_binding)
* don't convert results to options just for matching (clippy::if_let_some_result)

fix type of const params in associated types.

fixes #66906
fixes #70167

r? @EddyB

parser: recover on `for<'a> |...| body` closures

When encountering `for` and `<` is 1 token ahead, interpret this as an explicitly quantified generic closure and recover, rather than attempting to parse a `for` loop. This provides both improved diagnostics as well as an insurance policy for the ability to use this as the syntax for generic closures in the future.

As requested by r? @EddyB

parse/lexer: support `StringReader::retokenize` called on external files.

This ~~should theoretically~~ fixes #69933, ~~but I'm not sure what the best way to test it is~~.
**EDIT**: see https://github.com/rust-lang/rust/issues/69933#issuecomment-602019598.

r? @petrochenkov cc @Xanewok @staktrace

Amend Rc/Arc::from_raw() docs regarding unsafety

[This](https://stackoverflow.com/questions/59671647/is-it-safe-to-clone-a-type-erased-arc-via-raw-pointer) question on SO boils down to "is it safe to `::from_raw()` a `Rc<T>`/`Arc<T>` using a dummy `T` even if `T` is never dereferenced via the new `Rc`/`Arc`?". It almost never is.

This PR amends the docs of `from_raw()` regarding this point.

Remove `finished` flag from `MapWhile`

This PR removes  `finished` flag from `MapWhile` as been proposed in https://github.com/rust-lang/rust/pull/66577#discussion_r370958025.

This also resolves open questions of the tracking issue (#68537):
- `MapWhile` can't implement both
  + `DoubleEndedIterator` (discussed in https://github.com/rust-lang/rust/pull/66577#discussion_r370947990 and following comments)
  + `FusedIterator` (this pr removes `finished` flag, so `MapWhile` isn't fused anymore)
- Debug output (this pr removes `finished` flag, so there is no question in including it in debug output)

r? @Mark-Simulacrum

Test gdb pretty printing more and fix overzealous type substitution

Adresses a problem concerning printing BTreeMap / BTreeSet data in gdb: when the key or value type name contains substring "LeafNode", and the map has multiple nodes (e.g. more than 11 elements), printing causes an exception. E.g.

```
rustc -g - <<EOF
    use std::collections::BTreeMap;

    struct MyLeafNode(i8);

    fn main() {
        let m: BTreeMap<i8, MyLeafNode> = (0..12).map(|i| (i, MyLeafNode(i))).collect();
        assert!(!m.is_empty());
    }
EOF
```

```
$ rust-gdb rust_out
(gdb) b 7
(gdb) r
(gdb) p m
$1 = BTreeMap<i8, rust_out::MyLeafNode>(len: 12)Python Exception <class 'gdb.error'> No type named alloc::collections::btree::node::InternalNode<i8, rust_out::MyInternalNode>.:
use std::collections::BTreeMap;
```

The code was written in #56144 by @tromey (and later touched upon by @RalfJung in #57045, but I think that had nothing to do with the issues in this PR).

Update Clippy

Closes #70128

cc rust-lang/rust-clippy#5342

r? @Manishearth

Let align/size_of_of_val intrinsics work on ptrs