…items

… stabilized

I don't know how to test this. I confirmed locally that this gives the
appropriate help with `channel = "beta"`:

```
error[E0554]: `#![feature]` may not be used on the beta release channel
 --> src/lib.rs:2:1
  |
2 | #![feature(min_const_generics)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the attribute
  |
  = help: the feature `min_const_generics` has been stable since 1.51.0 and no longer requires an attribute to enable

error[E0554]: `#![feature]` may not be used on the beta release channel
 --> src/lib.rs:3:1
  |
3 | #![feature(min_const_generics, min_specialization)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: the feature `min_const_generics` has been stable since 1.51.0 and no longer requires an attribute to enable

error[E0554]: `#![feature]` may not be used on the beta release channel
 --> src/lib.rs:4:1
  |
4 | #![feature(box_patterns)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^
```

Signed-off-by: Rustin-Liu <rustin.liu@gmail.com>

resolve conflicts

Co-authored-by: Mara Bos <m-ou.se@m-ou.se>

…, r=eddyb,varkor

Rename AssociatedItems to AssocItems

Signed-off-by: Rustin-Liu <rustin.liu@gmail.com>

Part of rust-lang#60163 (comment)

…und_pair, r=m-ou-se

Implement indexing slices with pairs of core::ops::Bound<usize>

Closes rust-lang#49976.

I am not sure about code duplication between `check_range` and `into_maybe_range`. Should be former implemented in terms of the latter? Also this PR doesn't address code duplication between `impl SliceIndex for Range*`.

Added CharIndices::offset function

The CharIndices iterator has a field internally called front_offset, that I think would be very useful to have access to.

You can already do something like ``char_indices.next().map(|(offset, _)| offset)``, but that is wordy, in addition to not handling the case where the iterator has ended, where you'd want the offset to be equal to the length.

I'm very new to the open source world and the rust repository, so I'm sorry if I missed a step or did something weird.

…, r=pnkfelix

Improve diagnostic for when field is never read

Related to (but does not close) rust-lang#81658

This completes the first step of ``@pnkfelix's`` [mentoring instructions](rust-lang#81658 (comment)) but does not actually improve the diagnostics (yet!). The two tests are heavily reduced versions of code from the original bug report.

I've confirmed that the reduced `field-used-in-ffi` test [fails on nightly](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=f0862c89ddca028c55c20a5ed05e679a) but [passes on stable](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f0862c89ddca028c55c20a5ed05e679a). This confirms that the regression is reproduced correctly. The `drop-only-field` test is a case that ``@pnkfelix`` mentioned in his mentoring instructions. It is not a regression, but will come in handy when we make the diagnostic smarter by looking at whether the field type implements `Drop`.

Per the [rustc-dev-guide](https://rustc-dev-guide.rust-lang.org/tests/adding.html), each test includes a comment summarizing what it is about.

On stable, suggest removing `#![feature]` for features that have been stabilized

I don't know how to test this (https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Run.20tests.20without.20enabling.20nightly.20features.3F). I confirmed locally that this gives the
appropriate help with `channel = "beta"`:

```
error[E0554]: `#![feature]` may not be used on the beta release channel
 --> src/lib.rs:2:1
  |
2 | #![feature(min_const_generics)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the attribute
  |
  = help: the feature `min_const_generics` has been stable since 1.51.0 and no longer requires an attribute to enable

error[E0554]: `#![feature]` may not be used on the beta release channel
 --> src/lib.rs:3:1
  |
3 | #![feature(min_const_generics, min_specialization)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: the feature `min_const_generics` has been stable since 1.51.0 and no longer requires an attribute to enable

error[E0554]: `#![feature]` may not be used on the beta release channel
 --> src/lib.rs:4:1
  |
4 | #![feature(box_patterns)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^
```

Closes rust-lang#83715.

Add a suggestion when using a type alias instead of trait alias

Fixes rust-lang#43913

r? ``@estebank``

implement `TrustedRandomAccess` for `Take` iterator adapter

`TrustedRandomAccess` requires the iterator length to fit within `usize`. `take(n)` only constrains the upper bound of an iterator. So if the inner is `TrustedRandomAccess` (which already implies a finite length) then so can be `Take`.

`@rustbot` label T-libs-impl

Explicitly implement `!Send` and `!Sync` for `sys::{Args, Env}`

Remove the field `_dont_send_or_sync_me: PhantomData<*mut ()>` in favor of an explicit implementation of `!Send` and `!Sync`.

move core::hint::black_box under its own feature gate

The `black_box` function had its own RFC and is tracked separately from the `test` feature at rust-lang#64102. Let's reflect this in the feature gate.

To avoid breaking all the benchmarks, libtest's `test::black_box` is a wrapping definition, not a reexport -- this means it is still under the `test` feature gate.

…es, r=varkor

various const parameter defaults improvements

Actually resolve names in const parameter defaults, fixing `struct Foo<const N: usize = { usize::MAX }>`.

---
Split generic parameter ban rib for types and consts, allowing
```rust
#![feature(const_generics_defaults)]
struct Q;
struct Foo<T = Q, const Q: usize = 3>(T);
```

---
Remove the type/const ordering restriction if `const_generics_defaults` is active, even if `const_generics` is not. allowing us to stabilize and test const param defaults separately.

---
Check well formedness of const parameter defaults, eagerly emitting an error for `struct Foo<const N: usize = { 0 - 1 }>`

---
Do not forbid const parameters in param defaults, allowing `struct Foo<const N: usize, T = [u8; N]>(T)` and `struct Foo<const N: usize, const M: usize = N>`. Note that this should not change anything which is stabilized, as on stable, type parameters must be in front of const parameters, which means that type parameter defaults are only allowed if no const parameters exist.

We still forbid generic parameters inside of const param types.

r? `@varkor` `@petrochenkov`