Update lang/rust to version 1.43.0.

Pkgsrc changes:
* Bump rust bootstrap version to 1.42.0, except for Darwin/i686 where the
  bootstrap is not (yet?) available.

Upstream changes:

Version 1.43.0 (2020-04-23)
==========================

Language
--------
- [Fixed using binary operations with `&{number}` (e.g. `&1.0`) not having
  the type inferred correctly.][68129]
- [Attributes such as `#[cfg()]` can now be used on `if` expressions.][69201]

**Syntax only changes**
- [Allow `type Foo: Ord` syntactically.][69361]
- [Fuse associated and extern items up to defaultness.][69194]
- [Syntactically allow `self` in all `fn` contexts.][68764]
- [Merge `fn` syntax + cleanup item parsing.][68728]
- [`item` macro fragments can be interpolated into `trait`s, `impl`s,
  and `extern` blocks.][69366]
  For example, you may now write:
  ```rust
  macro_rules! mac_trait {
      ($i:item) => {
          trait T { $i }
      }
  }
  mac_trait! {
      fn foo() {}
  }
  ```
These are still rejected *semantically*, so you will likely receive an error but
these changes can be seen and parsed by macros and
conditional compilation.

Compiler
--------
- [You can now pass multiple lint flags to rustc to override the previous
  flags.][67885] For example; `rustc -D unused -A unused-variables` denies
  everything in the `unused` lint group except `unused-variables` which
  is explicitly allowed. However, passing `rustc -A unused-variables -D unused` denies
  everything in the `unused` lint group **including** `unused-variables` since
  the allow flag is specified before the deny flag (and therefore overridden).
- [rustc will now prefer your system MinGW libraries over its bundled libraries
  if they are available on `windows-gnu`.][67429]
- [rustc now buffers errors/warnings printed in JSON.][69227]

Libraries
---------
- [`Arc<[T; N]>`, `Box<[T; N]>`, and `Rc<[T; N]>`, now implement
  `TryFrom<Arc<[T]>>`,`TryFrom<Box<[T]>>`, and `TryFrom<Rc<[T]>>`
  respectively.][69538] **Note** These conversions are only available when `N`
  is `0..=32`.
- [You can now use associated constants on floats and integers directly, rather
  than having to import the module.][68952] e.g. You can now write `u32::MAX` or
  `f32::NAN` with no imports.
- [`u8::is_ascii` is now `const`.][68984]
- [`String` now implements `AsMut<str>`.][68742]
- [Added the `primitive` module to `std` and `core`.][67637] This module
  reexports Rust's primitive types. This is mainly useful in macros
  where you want avoid these types being shadowed.
- [Relaxed some of the trait bounds on `HashMap` and `HashSet`.][67642]
- [`string::FromUtf8Error` now implements `Clone + Eq`.][68738]

Stabilized APIs
---------------
- [`Once::is_completed`]
- [`f32::LOG10_2`]
- [`f32::LOG2_10`]
- [`f64::LOG10_2`]
- [`f64::LOG2_10`]
- [`iter::once_with`]

Cargo
-----
- [You can now set config `[profile]`s in your `.cargo/config`, or through
  your environment.][cargo/7823]
- [Cargo will now set `CARGO_BIN_EXE_<name>` pointing to a binary's
  executable path when running integration tests or benchmarks.][cargo/7697]
  `<name>` is the name of your binary as-is e.g. If you wanted the executable
  path for a binary named `my-program`you would use
  `env!("CARGO_BIN_EXE_my-program")`.

Misc
----
- [Certain checks in the `const_err` lint were deemed unrelated to const
  evaluation][69185], and have been moved to the `unconditional_panic` and
  `arithmetic_overflow` lints.

Compatibility Notes
-------------------

- [Having trailing syntax in the `assert!` macro is now a hard error.][69548]
  This has been a warning since 1.36.0.
- [Fixed `Self` not having the correctly inferred type.][69340] This incorrectly
  led to some instances being accepted, and now correctly emits a hard error.

[69340]: https://github.com/rust-lang/rust/pull/69340

Internal Only
-------------
These changes provide no direct user facing benefits, but represent significant
improvements to the internals and overall performance of `rustc` and
related tools.

- [All components are now built with `opt-level=3` instead of `2`.][67878]
- [Improved how rustc generates drop code.][67332]
- [Improved performance from `#[inline]`-ing certain hot functions.][69256]
- [traits: preallocate 2 Vecs of known initial size][69022]
- [Avoid exponential behaviour when relating types][68772]
- [Skip `Drop` terminators for enum variants without drop glue][68943]
- [Improve performance of coherence checks][68966]
- [Deduplicate types in the generator witness][68672]
- [Invert control in struct_lint_level.][68725]

[67332]: https://github.com/rust-lang/rust/pull/67332/
[67429]: https://github.com/rust-lang/rust/pull/67429/
[67637]: https://github.com/rust-lang/rust/pull/67637/
[67642]: https://github.com/rust-lang/rust/pull/67642/
[67878]: https://github.com/rust-lang/rust/pull/67878/
[67885]: https://github.com/rust-lang/rust/pull/67885/
[68129]: https://github.com/rust-lang/rust/pull/68129/
[68672]: https://github.com/rust-lang/rust/pull/68672/
[68725]: https://github.com/rust-lang/rust/pull/68725/
[68728]: https://github.com/rust-lang/rust/pull/68728/
[68738]: https://github.com/rust-lang/rust/pull/68738/
[68742]: https://github.com/rust-lang/rust/pull/68742/
[68764]: https://github.com/rust-lang/rust/pull/68764/
[68772]: https://github.com/rust-lang/rust/pull/68772/
[68943]: https://github.com/rust-lang/rust/pull/68943/
[68952]: https://github.com/rust-lang/rust/pull/68952/
[68966]: https://github.com/rust-lang/rust/pull/68966/
[68984]: https://github.com/rust-lang/rust/pull/68984/
[69022]: https://github.com/rust-lang/rust/pull/69022/
[69185]: https://github.com/rust-lang/rust/pull/69185/
[69194]: https://github.com/rust-lang/rust/pull/69194/
[69201]: https://github.com/rust-lang/rust/pull/69201/
[69227]: https://github.com/rust-lang/rust/pull/69227/
[69548]: https://github.com/rust-lang/rust/pull/69548/
[69256]: https://github.com/rust-lang/rust/pull/69256/
[69361]: https://github.com/rust-lang/rust/pull/69361/
[69366]: https://github.com/rust-lang/rust/pull/69366/
[69538]: https://github.com/rust-lang/rust/pull/69538/
[cargo/7823]: https://github.com/rust-lang/cargo/pull/7823
[cargo/7697]: https://github.com/rust-lang/cargo/pull/7697
[`Once::is_completed`]: https://doc.rust-lang.org/std/sync/struct.Once.html#method.is_completed
[`f32::LOG10_2`]: https://doc.rust-lang.org/std/f32/consts/constant.LOG10_2.html
[`f32::LOG2_10`]: https://doc.rust-lang.org/std/f32/consts/constant.LOG2_10.html
[`f64::LOG10_2`]: https://doc.rust-lang.org/std/f64/consts/constant.LOG10_2.html
[`f64::LOG2_10`]: https://doc.rust-lang.org/std/f64/consts/constant.LOG2_10.html
[`iter::once_with`]: https://doc.rust-lang.org/std/iter/fn.once_with.html

(he)