* {:.PREC?} already had legitimately useful behavior (recursive formatting of structs using
  fixed precision for floats) and I suspect that changes to the output there would be unwelcome.

  (besides, precision introduces sinister edge cases where a number can be rounded up to one
  of the thresholds)

  Thus, the new behavior of Debug is, "dynamically switch to exponential, but only if there's
  no precision."

* This could not be implemented in terms of float_to_decimal_common without repeating the branch
  on precision, so 'float_to_general_debug' is a new function.  The name is '_debug' instead of
  '_common' because the considerations in the previous bullet make this logic pretty specific
  to Debug.

* 'float_to_decimal_common' is now only used by Display, so I inlined the min_precision argument
  and renamed the function accordingly.

I originally added this bound in an attempt to make the specialization
sound for owning iterators but it was never correct here and the correct
and already implemented solution is is to place it on the IntoIter
implementation.

…tation page

* Rename "type-decl" into "item-decl" to reflect the change of usage

…ans, r=estebank"

This reverts commit 36a1076, reversing
changes made to e1e9319.

Headings in the top-doc docblock still get a border-bottom due to a rule
that covers all h2, h3, and h4. Method docblocks are generally h5, and
so don't get a border-bottom anymore.

This fixes a problem where a sub-sub-heading within a method would have
a line that went all the way across the page, creating a division that
made that sub-sub-heading look much more important than it really is.

std's begin_panic_fmt and core's panic_fmt are duplicates.
Merge them to declutter code and remove a lang item.

Fix typo.

…, r=yaahc

Automatic exponential formatting in Debug

Context: See [this comment from the libs team](rust-lang/rfcs#2729 (comment))

---

Makes `"{:?}"` switch to exponential for floats based on magnitude. The libs team suggested exploring this idea in the discussion thread for RFC rust-lang/rfcs#2729. (**note:** this is **not** an implementation of the RFC; it is an implementation of one of the alternatives)

Thresholds chosen were 1e-4 and 1e16.  Justification described [here](rust-lang/rfcs#2729 (comment)).

**This will require a crater run.**

---

As mentioned in the commit message of 8731d4d, this behavior will not apply when a precision is supplied, because I wanted to preserve the following existing and useful behavior of `{:.PREC?}` (which recursively applies `{:.PREC}` to floats in a struct):

```rust
assert_eq!(
    format!("{:.2?}", [100.0, 0.000004]),
    "[100.00, 0.00]",
)
```

I looked around and am not sure where there are any tests that actually use this in the test suite, though?

All things considered, I'm surprised that this change did not seem to break even a single existing test in `x.py test --stage 2`.  (even when I tried a smaller threshold of 1e6)

…sleywiser

Add support for artifact size profiling

This adds support for profiling artifact file sizes (incremental compilation artifacts and query cache to begin with).

Eventually we want to track this in perf.rlo so we can ensure that file sizes do not change dramatically on each pull request.

This relies on support in measureme: rust-lang/measureme#169. Once that lands we can update this PR to not point to a git dependency.

This was worked on together with `@michaelwoerister.`

r? `@wesleywiser`

…aahc,dtolnay

Alloc features cleanup

This sorts and categorizes the `#![features]` in `alloc` and removes unused ones.

This is part of rust-lang#87766

The following feature attributes were unnecessary and are removed:

```diff
// Library features:
-#![feature(cow_is_borrowed)]
-#![feature(maybe_uninit_uninit_array)]
-#![feature(slice_partition_dedup)]

// Language features:
-#![feature(arbitrary_self_types)]
-#![feature(auto_traits)]
-#![feature(box_patterns)]
-#![feature(decl_macro)]
-#![feature(nll)]
```

remove unnecessary bound on Zip specialization impl

I originally added this bound in an attempt to make the specialization
sound for owning iterators but it was never correct here and the correct
and [already implemented](https://github.com/rust-lang/rust/blob/497ee321af3b8496eaccd7af7b437f18bab81abf/library/alloc/src/vec/into_iter.rs#L220-L232) solution is is to place it on the IntoIter
implementation.

Deduplicate panic_fmt

std's begin_panic_fmt and core's panic_fmt are duplicates. Merge them to declutter code and remove a lang item.

Make more `From` impls `const` (libcore)

Adding `const` to `From` implementations in the core. `rustc_const_unstable` attribute is not added to unstable implementations.

Tracking issue: rust-lang#88674

<details>
<summary>Done</summary><div>

- `T` from `T`
- `T` from `!`
- `Option<T>` from `T`
- `Option<&T>` from `&Option<T>`
- `Option<&mut T>` from `&mut Option<T>`
- `Cell<T>` from `T`
- `RefCell<T>` from `T`
- `UnsafeCell<T>` from `T`
- `OnceCell<T>` from `T`
- `Poll<T>` from `T`
- `u32` from `char`
- `u64` from `char`
- `u128` from `char`
- `char` from `u8`
- `AtomicBool` from `bool`
- `AtomicPtr<T>` from `*mut T`
- `AtomicI(bits)` from `i(bits)`
- `AtomicU(bits)` from `u(bits)`
- `i(bits)` from `NonZeroI(bits)`
- `u(bits)` from `NonZeroU(bits)`
- `NonNull<T>` from `Unique<T>`
- `NonNull<T>` from `&T`
- `NonNull<T>` from `&mut T`
- `Unique<T>` from `&mut T`
- `Infallible` from `!`
- `TryIntError` from `!`
- `TryIntError` from `Infallible`
- `TryFromSliceError` from `Infallible`
- `FromResidual for Option<T>`
</div></details>

<details>
<summary>Remaining</summary><dev>

- `NonZero` from `NonZero`
These can't be made const at this time because these use Into::into.
https://github.com/rust-lang/rust/blob/master/library/core/src/convert/num.rs#L393

- `std`, `alloc`
There may still be many implementations that can be made `const`.
</div></details>

… r=jsha

Fix rustdoc UI for very long type names

Fixes rust-lang#89972.

While working on it, I also discovered that when the item name is too long, it also breaks the flow of the page.

To make things right, I also renamed the `type-decl` CSS class into `item-decl` (because this PR also generates it for more than type declarations).

So here are the before/after screenshots:

![Screenshot from 2021-10-18 16-58-03](https://user-images.githubusercontent.com/3050060/137757247-637fcf04-4406-49c6-8a8a-18c2074aacd9.png)
![Screenshot from 2021-10-18 16-58-26](https://user-images.githubusercontent.com/3050060/137757252-17935e63-53b3-449f-a535-7be91ff0e257.png)

![Screenshot from 2021-10-18 16-58-07](https://user-images.githubusercontent.com/3050060/137757278-8b12e348-2980-4fc4-8853-bef99d58981f.png)
![Screenshot from 2021-10-18 16-58-28](https://user-images.githubusercontent.com/3050060/137757282-534a0e1b-3016-49ba-b3ac-e45bdb9035cb.png)

r? ``@jsha``

Revert rust-lang#86011 to fix an incorrect bound check

This reverts commit 36a1076, reversing
changes made to e1e9319.

Fixes rust-lang#89935
r? ``@estebank``

Remove border-bottom from most docblocks.

Headings in the top-doc docblock still get a border-bottom due to a rule
that covers all h2, h3, and h4. Method docblocks are generally h5, and
so don't get a border-bottom anymore.

This fixes a problem where a sub-sub-heading within a method would have
a line that went all the way across the page, creating a division that
made that sub-sub-heading look much more important than it really is.

Fixes rust-lang#90033

Demo at https://jacob.hoffman-andrews.com/rust/less-rule/std/string/struct.String.html

r? ``@GuillaumeGomez``

…lacrum

Update RELEASES.md

Fix typo.