Union fields have the ParamEnv of the union.

* Handle cycles in `needs_drop` correctly
* Normalize types when computing `needs_drop`
* Move queries from rustc to rustc_ty

* Handle arrays with const-generic lengths
* Use closure for repeated code.

Currently, every iteration of the main loop in `generic_extension`
instantiates a `Parser`, which is expensive because `Parser` is a large
type. Many of those instantiations are only used immutably, particularly
for simple-but-repetitive macros of the sort seen in `html5ever` and PR
68836.

This commit initializes a single "base" parser outside the loop, and
then uses `Cow` to avoid cloning it except for the mutating iterations.
This speeds up `html5ever` runs by up to 15%.

The previous commit wrapped `Parser` within a `Cow` for the hot macro
parsing path. As a result, there's no need for the `Cow` within
`Directory`, because it lies within `Parser`.

This is a small win, because `Failure` is much more common than
`Success`.

Co-Authored-By: varkor <github@varkor.com>

…y ops

Improve `ty.needs_drop`

* Handle cycles in `needs_drop` correctly
* Normalize types when computing `needs_drop`
* Move queries from rustc to rustc_ty
* Avoid query in simple cases

reopens #65918

…nkov

Hasten macro parsing

r? @eddyb

Python script PEP8 style guide space formatting and minor Python source cleanup

This PR includes the following changes in the Python sources based on a flake8 3.7.9 (mccabe: 0.6.1, pycodestyle: 2.5.0, pyflakes: 2.1.1) CPython 3.7.6 on Darwin lint:

- PEP8 style guide spacing updates *without* line length changes
- removal of unused local variable assignments in context managers and exception handling
- removal of unused Python import statements
- removal of unnecessary semicolons

…iser

miri: improve and simplify overflow detection

This simplifies the overflow detection for signed binary operators, and adds overflow detection to unary operators so that const-prop doesn't have to crudely hand-roll that.

It also fixes some bugs in the operator implementation that however, I think, were not observable.

r? @oli-obk @wesleywiser

traits: preallocate 2 Vecs of known initial size

The 2 preallocations are pretty obvious; both vectors will be as big as or larger than the collections they are created from.

In `WfPredicates::normalize` the change from a functional style improves readability and should be perf-friendly, too.

Remove common usage pattern from `AllocRef`

This removes the common usage patterns from `AllocRef`:
- `alloc_one`
- `dealloc_one`
- `alloc_array`
- `realloc_array`
- `dealloc_array`

Actually, they add nothing to `AllocRef` except a [convenience wrapper around `Layout` and other methods in this trait](https://doc.rust-lang.org/1.41.0/src/core/alloc.rs.html#1076-1240) but have a major flaw: The documentation of `AllocRefs` notes, that

> some higher-level allocation methods (`alloc_one`, `alloc_array`) are well-defined on zero-sized types and can optionally support them: it is left up to the implementor whether to return `Err`, or to return `Ok` with some pointer.

With the current API, `GlobalAlloc` does not have those methods, so they cannot be overridden for `liballoc::Global`, which means that even if the global allocator would support zero-sized allocations, `alloc_one`, `alloc_array`, and `realloc_array` for `liballoc::Global` will error, while calling `alloc` with a zeroed-size `Layout` could succeed. Even worse: allocating with `alloc` and deallocating with `dealloc_{one,array}` could end up with not calling `dealloc` at all!

For the full discussion please see rust-lang/wg-allocators#18

r? @Amanieu

Add missing `_zeroed` varants to `AllocRef`

The majority of the allocator wg has decided to add the missing `_zeroed` variants to `AllocRef`:

> these should be added since they can be efficiently implemented with the `mremap` system call on Linux. `mremap` allows you to move/grow/shrink a memory mapping, and any new pages added for growth are guaranteed to be zeroed.
>
> If `AllocRef` does not have these methods then the user will have to manually write zeroes to the added memory since the API makes no guarantees on their contents.

For the full discussion please see rust-lang/wg-allocators#14.

This PR provides default implementations for `realloc_zeroed`, `alloc_excess_zeroed`, `realloc_excess_zeroed`, and `grow_in_place_zeroed`.

r? @Amanieu

Use `dyn Trait` more in tests

Here are some tests using the old trait object type syntax which are not testing the syntax itself.

This has been extracted from #66364.

…trochenkov

ICE in nightly-2020-02-08: handle TerminatorKind::Yield in librustc_mir::transform::promote_consts::Validator method

IR: #69017
regressed commit: f8fd462
Source: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=55e65a869e1f5fef64cc4462b1a5a087

Addresses ICE reported in #69017 by handling `TerminatorKind::Yield` in https://github.com/rust-lang/rust/blob/4d1241f5158ffd66730e094d8f199ed654ed52ae/src/librustc_mir/transform/promote_consts.rs#L465-L468.

<details><summary>Nightly build</summary>
<p>

```
$ cargo +nightly build
Compiling yielder v0.1.0 (/Users/chris/Desktop/tests/rustlang-tests/yielder)
error: internal compiler error: src/librustc_mir/transform/promote_consts.rs:467: _1 = suspend(move _21) -> [resume: bb2, drop: bb3] not promotable
 --> src/main.rs:8:27
  |
8 |         println!("-> {}", yield);
  |                           ^^^^^

thread 'rustc' panicked at 'Box<Any>', <::std::macros::panic macros>:2:4
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.43.0-nightly (71c7e14 2020-02-09) running on x86_64-apple-darwin

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

note: some of the compiler flags provided by cargo are hidden

error: aborting due to previous error

error: could not compile `yielder`.

To learn more, run the command again with --verbose.
```

</p>
</details>

<details><summary>Stage 1 dev build</summary>
<p>

```
$ cargo +stage1 build
Compiling yielder v0.1.0 (/Users/chris/Desktop/tests/rustlang-tests/yielder)
warning: function is never used: `gen`
 --> src/main.rs:6:4
  |
6 | fn gen() -> impl Generator<usize> {
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.53s
```

</p>
</details>

@jonas-schievink @oli-obk

parser: Remove `Parser::prev_token_kind`

Follow-up to #69006.
r? @Centril

Don't rustfmt check the vendor directory.

I need to be able to run `x.py tidy` to do license checks (which requires vendored dependencies).  However, when vendoring is enabled, it wants to rustfmt check the entire vendor directory, which doesn't work.