also start deduplicating the docs that are getting moved to this
section.

It requires no additonal changes to these crates, but will prevent
unnecessary `pub`s in the future.

But explain why.

Documents that `BikeshedIntrinsicFrom` models transmute-via-union,
which is slightly more expressive than the transmute-via-cast
implemented by `transmute_copy`. Additionally, we provide an
implementation of transmute-via-union as a method on the
`BikeshedIntrinsicFrom` trait with additional documentation on
the boundary between trait invariants and caller obligations.

Whether or not transmute-via-union is the right kind of transmute
to model remains up for discussion [1]. Regardless, it seems wise
to document the present behavior.

[1] https://rust-lang.zulipchat.com/#narrow/stream/216762-project-safe-transmute/topic/What.20'kind'.20of.20transmute.20to.20model.3F/near/426331967

* Choose test inputs more thoroughly and systematically.
* Check that `isqrt` and `checked_isqrt` have equivalent results for
  signed types, either equivalent numerically or equivalent as a panic
  and a `None`.
* Check that `isqrt` has numerically-equivalent results for unsigned
  types and their `NonZero` counterparts.
* Reuse `ilog10` benchmarks, plus benchmarks that use a uniform
  distribution.

* Use a lookup table for 8-bit integers and the Karatsuba square root
  algorithm for larger integers.
* Include optimization hints that give the compiler the exact numeric
  range of results.

When looking for multiple versions of the same crate, do not blindly construct the receiver type.

Follow up to rust-lang#128786. Fix rust-lang#129205.

Add `#[warn(unreachable_pub)]` to a bunch of compiler crates

By default `unreachable_pub` identifies things that need not be `pub` and tells you to make them `pub(crate)`. But sometimes those things don't need any kind of visibility. So they way I did these was to remove the visibility entirely for each thing the lint identifies, and then add `pub(crate)` back in everywhere the compiler said it was necessary. (Or occasionally `pub(super)` when context suggested that was appropriate.) Tedious, but results in more `pub` removal.

There are plenty more crates to do but this seems like enough for a first PR.

r? `@compiler-errors`

…=cuviper

deduplicate and clarify rules for converting pointers to references

part of rust-lang#124669

Improved `checked_isqrt` and `isqrt` methods

### Improved tests of `isqrt` and `checked_isqrt` implementations

* Inputs chosen more thoroughly and systematically.
* Checks that `isqrt` and `checked_isqrt` have equivalent results for signed types, either equivalent numerically or equivalent as a panic and a `None`.
* Checks that `isqrt` has numerically-equivalent results for unsigned types and their `NonZero` counterparts.

### Added benchmarks for `isqrt` implementations

### Greatly sped up `checked_isqrt` and `isqrt` methods

* Uses a lookup table for 8-bit integers and then the Karatsuba square root algorithm for larger integers.
* Includes optimization hints that give the compiler the exact numeric range of results.

### Feature tracking issue

`isqrt` is an unstable feature tracked at rust-lang#116226.

<details><summary>Benchmarked improvements</summary>

### Command used to benchmark

    ./x bench library/core -- int_sqrt

### Before

    benchmarks:
        num::int_sqrt::i128::isqrt           439591.65/iter  +/- 6652.70
        num::int_sqrt::i16::isqrt              5302.97/iter   +/- 160.93
        num::int_sqrt::i32::isqrt             62999.11/iter  +/- 2022.05
        num::int_sqrt::i64::isqrt            125248.81/iter  +/- 1674.43
        num::int_sqrt::i8::isqrt                123.56/iter     +/- 1.87
        num::int_sqrt::isize::isqrt          125356.56/iter  +/- 1017.03
        num::int_sqrt::non_zero_u128::isqrt  437443.75/iter  +/- 3535.43
        num::int_sqrt::non_zero_u16::isqrt     8604.58/iter    +/- 94.76
        num::int_sqrt::non_zero_u32::isqrt    62933.33/iter   +/- 517.30
        num::int_sqrt::non_zero_u64::isqrt   125076.38/iter +/- 11340.61
        num::int_sqrt::non_zero_u8::isqrt       221.51/iter     +/- 1.58
        num::int_sqrt::non_zero_usize::isqrt 136005.21/iter  +/- 2020.35
        num::int_sqrt::u128::isqrt           439014.55/iter  +/- 3920.45
        num::int_sqrt::u16::isqrt              8575.08/iter   +/- 148.06
        num::int_sqrt::u32::isqrt             63008.89/iter   +/- 803.67
        num::int_sqrt::u64::isqrt            125088.09/iter   +/- 879.29
        num::int_sqrt::u8::isqrt                230.18/iter     +/- 2.04
        num::int_sqrt::usize::isqrt          125237.51/iter  +/- 4747.83
### After

    benchmarks:
        num::int_sqrt::i128::isqrt           105184.89/iter +/- 1171.38
        num::int_sqrt::i16::isqrt              1910.26/iter   +/- 78.50
        num::int_sqrt::i32::isqrt             34260.34/iter  +/- 960.84
        num::int_sqrt::i64::isqrt             45939.19/iter +/- 2525.65
        num::int_sqrt::i8::isqrt                 22.87/iter    +/- 0.45
        num::int_sqrt::isize::isqrt           45884.17/iter  +/- 595.49
        num::int_sqrt::non_zero_u128::isqrt  106344.27/iter  +/- 780.99
        num::int_sqrt::non_zero_u16::isqrt     2790.19/iter   +/- 53.43
        num::int_sqrt::non_zero_u32::isqrt    33613.99/iter  +/- 362.96
        num::int_sqrt::non_zero_u64::isqrt    46235.42/iter  +/- 429.69
        num::int_sqrt::non_zero_u8::isqrt        31.78/iter    +/- 0.75
        num::int_sqrt::non_zero_usize::isqrt  46208.75/iter  +/- 375.27
        num::int_sqrt::u128::isqrt           106385.94/iter +/- 1649.95
        num::int_sqrt::u16::isqrt              2747.69/iter   +/- 28.72
        num::int_sqrt::u32::isqrt             33627.09/iter  +/- 475.68
        num::int_sqrt::u64::isqrt             46182.29/iter  +/- 311.16
        num::int_sqrt::u8::isqrt                 33.10/iter    +/- 0.30
        num::int_sqrt::usize::isqrt           46165.00/iter  +/- 388.41

</details>

…er-errors

Document & implement the transmutation modeled by `BikeshedIntrinsicFrom`

Documents that `BikeshedIntrinsicFrom` models transmute-via-union, which is slightly more expressive than the transmute-via-cast implemented by `transmute_copy`. Additionally, we provide an implementation of transmute-via-union as a method on the `BikeshedIntrinsicFrom` trait with additional documentation on the boundary between trait invariants and caller obligations.

Whether or not transmute-via-union is the right kind of transmute to model remains up for discussion [1]. Regardless, it seems wise to document the present behavior.

[1] https://rust-lang.zulipchat.com/#narrow/stream/216762-project-safe-transmute/topic/What.20'kind'.20of.20transmute.20to.20model.3F/near/426331967

Tracking Issue: rust-lang#99571

r? `@compiler-errors`

cc `@scottmcm,` `@Lokathor`

…errors

Do not ICE on non-ADT rcvr type when looking for crate version collision

When looking for multiple versions of the same crate, do not blindly construct the receiver type.

Follow up to rust-lang#128786.
Fixes rust-lang#129205
Fixes rust-lang#129216

…rce-link, r=notriddle

[rustdoc] Generate source link on impl associated types

Currently, impl associated types are generated but don't get a source link. This PR fixes that.

Before:

![image](https://github.com/user-attachments/assets/3a22adb5-8b66-4124-9267-7c26eed1aa5e)

After:

![Screenshot from 2024-08-25 16-31-36](https://github.com/user-attachments/assets/6e9b35e7-4357-4ecf-8c49-1d8294051283)

r? `@notriddle`

Remove a couple of unused feature enables

… r=fmease

Rename `ParenthesizedGenericArgs` to `GenericArgsMode`

A bit easier to digest name. Broken out of a PR implementing return type notation.

…fmease

Remove `ParamMode::ExplicitNamed`

This was introduced as a hack to improve a diagnostics suggestion in rust-lang#61679. It was subsequently broken, but also it was an incomplete hack that I don't believe we need to support, so let's just remove it.