This patch adds the `core::stream` submodule and implements `core::stream::Stream` in accordance with RFC2996.

Add feedback from @camelid

…res when feature capture_disjoint_fields is enabled

This is a temporary change only, as we wait to resolve dynamic dispatch issues. The `Stream::next` method and corresponding documentation are expected to be fully restored once we have a path to proceed.

Ref: rust-lang/rfcs#2996 (comment)

update docs

Reverts PR #80830
Fixes taiki-e/pin-project#312

We can have an arbitrary number of `None`-delimited group frames pushed
on the stack due to proc-macro invocations, which can legally be exited.
Attempting to account for this would add a lot of complexity for a tiny
performance gain, so let's just use the original strategy.

The `Deref` cycle checks added as part of #80653 were "unbalanced" in the sense
that the main content code path checks for cycles _before_ descending, while the
sidebar checks _after_. Checking _before_ is correct, so this changes the
sidebar path to match the main content path.

Previously, the HTML output format was represented by both
`Some(OutputFormat::Html)` and `None` so there's no need to have an
optional. Instead, `OutputFormat::Html` is explicitly the default and we
no longer have a "tri-state enum".

Replacing with equal number of values does not increase the length of the vec.

Reference: https://stackoverflow.com/a/62559271/3990767

This commit makes cfg(version) treat the nightlies
for version 1.n.0 as 1.n.0, even though that nightly
version might not have all stabilizations and features
of the released 1.n.0. This is done for greater
convenience for people who want to test a newly
stabilized feature on nightly.

For users who wish to pin nightlies, this commit adds
a -Z assume-incomplete-release option that they can
enable if there are any issues due to this change.

parentesized -> parenthesized

Before, it could print this error if no toolchain was configured:

```
error: no default toolchain configured
error: backtrace:
error: stack backtrace:
   0: error_chain::backtrace::imp::InternalBacktrace::new
   1: rustup::config::Cfg::toolchain_for_dir
   2: rustup_init::run_rustup_inner
   3: rustup_init::main
   4: std::rt::lang_start::{{closure}}
   5: main
   6: __libc_start_main
   7: _start
```

Add `core::stream::Stream`

[[Tracking issue: #79024](#79024)]

This patch adds the `core::stream` submodule and implements `core::stream::Stream` in accordance with [RFC2996](rust-lang/rfcs#2996). The RFC hasn't been merged yet, but as requested by the libs team in rust-lang/rfcs#2996 (comment) I'm filing this PR to get the ball rolling.

## Documentatation

The docs in this PR have been adapted from [`std::iter`](https://doc.rust-lang.org/std/iter/index.html), [`async_std::stream`](https://docs.rs/async-std/1.7.0/async_std/stream/index.html), and [`futures::stream::Stream`](https://docs.rs/futures/0.3.8/futures/stream/trait.Stream.html). Once this PR lands my plan is to follow this up with PRs to add helper methods such as `stream::repeat` which can be used to document more of the concepts that are currently missing. That will allow us to cover concepts such as "infinite streams" and "laziness" in more depth.

## Feature gate

The feature gate for `Stream` is `stream_trait`. This matches the `#[lang = "future_trait"]` attribute name. The intention is that only the APIs defined in RFC2996 will use this feature gate, with future additions such as `stream::repeat` using their own feature gates. This is so we can ensure a smooth path towards stabilizing the `Stream` trait without needing to stabilize all the APIs in `core::stream` at once. But also don't start expanding the API until _after_ stabilization, as was the case with `std::future`.

__edit:__ the feature gate has been changed to `async_stream` to match the feature gate proposed in the RFC.

## Conclusion

This PR introduces `core::stream::{Stream, Next}` and re-exports it from `std` as `std::stream::{Stream, Next}`. Landing `Stream` in the stdlib has been a mult-year process; and it's incredibly exciting for this to finally happen!

---

r? ````@KodrAus````
cc/ ````@rust-lang/wg-async-foundations```` ````@rust-lang/libs````

Stabilize raw ref macros

This stabilizes `raw_ref_macros` (#73394), which is possible now that #74355 is fixed.

However, as I already said in #73394 (comment), I am not particularly happy with the current names of the macros. So I propose we also change them, which means I am proposing to stabilize the following in `core::ptr`:
```rust
pub macro const_addr_of($e:expr) {
    &raw const $e
}

pub macro mut_addr_of($e:expr) {
    &raw mut $e
}
```

The macro name change means we need another round of FCP. Cc ````@rust-lang/libs````
Fixes #73394

Stabilize `unsigned_abs`

Resolves #74913.

This PR stabilizes the `i*::unsigned_abs()` method, which returns the absolute value of an integer _as its unsigned equivalent_. This has the advantage that it does not overflow on `i*::MIN`.

I have gone ahead and used this in a couple locations throughout the repository.

…ulacrum

BTreeMap: correct node size test case for choices of B

r? ````````@Mark-Simulacrum````````

…ord-update-syntax-error, r=nikomatsakis

Support FRU pattern with `[feature(capture_disjoint_fields)]`

In case of a functional record update syntax for creating a structure, `ExprUseVisitor` to only detect the precise use of some of the field in the `..x` part of the syntax. However, when we start building MIR, we
1. First, build the place for `x`
2. and then, add precise field projections so that only some parts of `x` end up getting read.

When `capture_disjoint_fields` is enabled, and FRU is used within a closure `x` won't be completely captured, and therefore the first step will fail. This PR updates `mir_build` to create a place builder in the first step and then create place from the builder only after applying the field projection.

Closes rust-lang/project-rfc-2229#32
r? `````@nikomatsakis`````

Slight simplification of chars().count()

Slight simplification: No need to call len(), we can just count the number of non continuation bytes.

I can't see any reason not to do this, can you?

cfg(version): treat nightlies as complete

This PR makes cfg(version) treat the nightlies
for version 1.n.0 as 1.n.0, even though that nightly
version might not have all stabilizations and features
of the released 1.n.0. This is done for greater
convenience for people who want to test a newly
stabilized feature on nightly, or in other words,
give newly stabilized features as many eyeballs
as possible.

For users who wish to pin nightlies, this commit adds
a -Z assume-incomplete-release option that they can
enable if they run into any issues due to this change.
Implements the suggestion in #64796 (comment)

…enkov

Clone entire `TokenCursor` when collecting tokens

Reverts PR #80830
Fixes taiki-e/pin-project#312

We can have an arbitrary number of `None`-delimited group frames pushed
on the stack due to proc-macro invocations, which can legally be exited.
Attempting to account for this would add a lot of complexity for a tiny
performance gain, so let's just use the original strategy.

…omez

Balance sidebar `Deref` cycle check with main content

The `Deref` cycle checks added as part of #80653 were "unbalanced" in the sense
that the main content code path checks for cycles _before_ descending, while the
sidebar checks _after_. Checking _before_ is correct, so this changes the
sidebar path to match the main content path.

Fixes #81395

r? ````@GuillaumeGomez````

…illaumeGomez

rustdoc: Remove unnecessary optional

Previously, the HTML output format was represented by both
`Some(OutputFormat::Html)` and `None` so there's no need to have an
optional. Instead, `OutputFormat::Html` is explicitly the default and we
no longer have a "tri-state enum".

r? ````@GuillaumeGomez````

Updated Vec::splice documentation

Replacing with equal number of values does not increase the length of the vec.

Reference: https://stackoverflow.com/a/62559271/3990767

update rustfmt to v1.4.34

Short summary: Various formatting fixes (several const generic related) and introduction of `imports_granularity` config option

Long summary copied from changelog:

#### Changed
- `merge_imports` configuration has been deprecated in favor of the new `imports_granularity` option. Any existing usage of `merge_imports` will be automatically mapped to the corresponding value on `imports_granularity` with a warning message printed to encourage users to update their config files.

#### Added
- New `imports_granularity` option has been added which succeeds `merge_imports`. This new option supports several additional variants which allow users to merge imports at different levels (crate or module), and even flatten imports to have a single use statement per item. ([PR rust-lang/rustfmt#4634](rust-lang/rustfmt#4634), [PR rust-lang/rustfmt#4639](rust-lang/rustfmt#4639))

See the section on the configuration site for more information
https://rust-lang.github.io/rustfmt/?version=v1.4.33&search=#imports_granularity

#### Fixed
- Fix erroneous removal of `const` keyword on const trait impl ([rust-lang/rustfmt#4084](rust-lang/rustfmt#4084))
- Fix incorrect span usage wit const generics in supertraits ([rust-lang/rustfmt#4204](rust-lang/rustfmt#4204))
- Use correct span for const generic params ([rust-lang/rustfmt#4263](rust-lang/rustfmt#4263))
- Correct span on const generics to include type bounds ([rust-lang/rustfmt#4310](rust-lang/rustfmt#4310))
- Idempotence issue on blocks containing only empty statements ([rust-lang/rustfmt#4627](rust-lang/rustfmt#4627) and [#3868](rust-lang/rustfmt#3868))
- Fix issue with semicolon placement on required functions that have a trailing comment that ends in a line-style comment before the semicolon ([rust-lang/rustfmt#4646](rust-lang/rustfmt#4646))
- Avoid shared interned cfg_if symbol since rustfmt can re-initialize the rustc_ast globals on multiple inputs ([rust-lang/rustfmt#4656](rust-lang/rustfmt#4656))
- Don't insert trailing comma on (base-less) rest in struct literals within macros ([rust-lang/rustfmt#4675](rust-lang/rustfmt#4675))

…dani

Add missing variants in match binding

cc ````@bugadani```` ````@CraftSpider````
r? ````@camelid````

Fix typo in pat.rs

parentesized -> parenthesized

Don't print error output from rustup when detecting default build triple

Before, it could print this error if no toolchain was configured:

```
error: no default toolchain configured
error: backtrace:
error: stack backtrace:
   0: error_chain::backtrace::imp::InternalBacktrace::new
   1: rustup::config::Cfg::toolchain_for_dir
   2: rustup_init::run_rustup_inner
   3: rustup_init::main
   4: std::rt::lang_start::{{closure}}
   5: main
   6: __libc_start_main
   7: _start
```