Also turns a few `box` into `Box::new`

Remove unused feature gates

I think many of the remaining unstable things can be easily be replaced with stable things. I have kept the `#![feature(nll)]` even though it is only necessary in `libstd`, to make regressions of it harder.

Tweak borrow error on `FnMut` when `Fn` is expected

Fix rust-lang#31701, fix rust-lang#66097.

…Simulacrum

Enable Control Flow Guard in rustbuild

Now that Rust supports Control Flow Guard (rust-lang#68180), add a config.toml option to build the standard library with CFG enabled.

r? @nagisa

…enkov

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.

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 rust-lang#66364.

…twice, r=davidtwco

Don't run coherence twice for future-compat lints

This fixes the regression introduced by rust-lang#65232 (which I mentioned in rust-lang#65232 (comment)).

Old algorithm:
* Run coherence with all future-incompatible checks off, reporting errors on any overlap.
* If there's no overlap (common case), run it *again*, with the future-incompatible checks on. Report warnings for any overlap found.

New algorithm:
* Run coherence with all additional future-incompatible checks *on*, which means that we'll find *all* potentially overlapping impls immediately.
* If this found overlap, run coherence again, with the future-incompatible checks off. If that *still* gives an error, we report it. If not, it ought to be a warning.

This reduces time spent in coherence checking for the nrf52810-pac by roughly 50% relative to current master.

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.

…lan-DPC

Clean up E0307 explanation

r? @Dylan-DPC