Fixes rust-lang#70316

BTreeMap/BTreeSet: implement drain_filter

Provide an implementation of drain_filter for BTreeMap and BTreeSet. Should be optimal when the predicate picks only elements in leaf nodes with at least MIN_LEN remaining elements, which is a common case, at least when draining only a fraction of the map/set, and also when the predicate picks elements stored in internal nodes where the right subtree can easily let go of a replacement element.

The first commit adds benchmarks with an external, naive implementation. to compare how much this claimed optimality-in-some-cases is actually worth.

add `unused_braces` lint

Add the lint `unused_braces` which is warn by default.

`unused_parens` is also extended and now checks anon consts.

closes rust-lang#68387

r? @varkor

parse_and_disallow_postfix_after_cast: account for `ExprKind::Err`.

Fixes rust-lang#70552.

r? @estebank
cc @daboross

Add missing -lmsvcrt on mingw after -lpthread

Fixes rust-lang#70316

Update books.

## reference

2 commits in e2f11fe..89dd146
2020-03-10 06:59:24 +0100 to 2020-03-31 09:42:10 -0700
- Clarify rule for end-of-line backslashes in strings (rust-lang/reference#784)
- Fix and clarify section on re-export (rust-lang/reference#780)

## book

1 commits in 6fb3705e5230311b096d47f7e2c91f9ce24393d0..c8841f2841a2d26124319ddadd1b6a245f9a1856
2020-02-12 13:48:57 -0500 to 2020-03-22 09:07:01 -0500
- Fix broken front_of_house.rs test on nightly. (rust-lang/book#2289)

## rust-by-example

7 commits in cb369ae95ca36b841960182d26f6d5d9b2e3cc18..edd2a7e687358712608896730c083cb76c7b401a
2020-03-14 12:13:22 -0500 to 2020-03-30 09:37:29 -0300
- feat: add doc attributes section to documentation (rust-lang/rust-by-example#1323)
- feat: add playpen topic (rust-lang/rust-by-example#1324)
- feat: add sentence about rustdoc & std library (rust-lang/rust-by-example#1322)
- fix: make spelling/style of rustdoc consistent (rust-lang/rust-by-example#1321)
- Rewrite section on 'static lifetime. (rust-lang/rust-by-example#1320)
- Remove deprecated `std::error::Error` functions and other minor changes (rust-lang/rust-by-example#1319)
- Remove mdbook version flags from README (rust-lang/rust-by-example#1318)

## nomicon

1 commits in 9f797e65e6bcc79419975b17aff8e21c9adc039f..411197b0e77590c967e37e8f6ec681abd359afe8
2020-03-04 03:45:13 +0100 to 2020-03-31 20:46:09 +0200
- Fix negative_impls feature. (rust-lang/nomicon#205)

expand vec![] to Vec::new()

The current expansion of `vec![]` calls `into_vec` on a boxed slice, which results in longer IR, and even after optimization, some unwinding artifacts are still present in the IR. This PR uses `Vec::new()` for `vec![]`.

This also allows `vec![]` to be used in const expressions.