Constructing an Rc/Arc is unsafe even if the wrapped `T`
is never dereferenced.

Future-proof these types in case rustc reorders
the inner fields. As per discussion in PR rust-lang#68099.

…les.

…me_result)

…tch_single_binding)

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

…tely (clippy::let_and_return)

…albertodt

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.

…trochenkov

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

This ~~should theoretically~~ fixes rust-lang#69933, ~~but I'm not sure what the best way to test it is~~.
**EDIT**: see rust-lang#69933 (comment).

r? @petrochenkov cc @Xanewok @staktrace

…trochenkov

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

fix type of const params in associated types.

fixes rust-lang#66906
fixes rust-lang#70167

r? @eddyb

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)

Return NonZeroU64 from ThreadId::as_u64.

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

Remove wrong entry from RELEASES.md

resolves rust-lang#70247

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 rust-lang#34546 · rust-lang rust](https://user-images.githubusercontent.com/951129/77238862-85c8f580-6bd4-11ea-8d31-77e2994a4b5a.png)

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)

proc_macro_harness: Use item header spans for errors

Addresses rust-lang#70233 (comment).