Fixes rust-lang#73836

Make the following methods unstable const under the `const_pin` feature:
- `new`
- `new_unchecked`
- `into_inner`
- `into_inner_unchecked`
- `get_ref`
- `into_ref`

Also adds tests for these methods in a const context.

Tracking issue: rust-lang#76654

`const_mut_refs` doesn't actually work in a `const` or `static`

Make the following methods unstable const under the `const_pin` feature:
- `into_ref`
- `get_mut`
- `get_unchecked_mut`

…d reduce nesting (clippy::single_match)

Co-authored-by: David Tolnay <[email protected]>

See
https://internals.rust-lang.org/t/need-for-controlling-drop-order-of-fields/12914/21
for the discussion.

TL;DR: ManuallyDrop is unsafe and footguny, but you can just ask the
compiler to do all the work for you by re-ordering declarations.

The case shouldn't be necessary and implicitly truncating BytePos is not
desirable.

If the right-hand iterator has exactly one more element than the
left-hand iterator, then both iterators will be fully consumed, but
the extra element will never be compared.

Reduce duplicate in liballoc reserve error handling

Not sure if it affects compilation time.

Don't use `zip` to compare iterators during pretty-print hack

If the right-hand iterator has exactly one more element than the
left-hand iterator, then both iterators will be fully consumed, but
the extra element will never be compared.

Split out from rust-lang#76130

Don't recommend ManuallyDrop to customize drop order

See
https://internals.rust-lang.org/t/need-for-controlling-drop-order-of-fields/12914/21
for the discussion.

TL;DR: ManuallyDrop is unsafe and footguny, but you can just ask the compiler to do all the work for you by re-ordering declarations.

Specifically, the original example from the docs is much better written as

```rust
struct Peach;
struct Banana;
struct Melon;
struct FruitBox {
    melon: Melon,
    // XXX: mind the relative drop order of the fields below
    peach: Peach,
    banana: Banana,
}
```

…73836, r=dtolnay

Implementation of Write for some immutable ref structs

Fixes  rust-lang#73836

…6, r=jyn514

Add explanation for E0756

r? @pickfire

do not ICE on bound variables, return `TooGeneric` instead

fixes rust-lang#73260, fixes rust-lang#74634, fixes rust-lang#76595

r? @nikomatsakis

Make some methods of `Pin` unstable const

Make the following methods unstable const under the `const_pin` feature:
- `new`
- `new_unchecked`
- `into_inner`
- `into_inner_unchecked`
- `get_ref`
- `into_ref`
- `get_mut`
- `get_unchecked_mut`

Of these, `into_inner` and `into_inner_unchecked` require the unstable `const_precise_live_drops`.

Also adds tests for these methods in a const context.

Tracking issue: rust-lang#76654

r? @ecstatic-morse

Only get ImplKind::Impl once

With this, the code panics in one place instead of two.

…d-api, r=oli-obk

Use const-checking to forbid use of unstable features in const-stable functions

First step towards rust-lang#76618.

Currently this code isn't ever hit because `qualify_min_const_fn` runs first and catches pretty much everything. One exception is `const_precise_live_drops`, which does not use the newly added code since it runs as part of a separate pass.

Also contains some unrelated refactoring, which is split into separate commits.

r? @oli-obk

… r=Dylan-DPC

use if let instead of single match arm expressions

use if let instead of single match arm expressions to compact code and reduce nesting (clippy::single_match)

extend `Ty` and `TyCtxt` lints to self types

blocked on rust-lang#76891

r? @ecstatic-morse cc @Aaron1011

Reduce boilerplate for BytePos and CharPos

Reduces boilerplate code for BytePos and CharPos by using a macro to implement shared traits.

…vidtwco

lint missing docs for extern items

fixes rust-lang#76991