Now that it's been removed from the prelude, we need to treat things differently.

Fixes #17967

This commit is an implementation of [RFC 503][rfc] which is a stabilization
story for the prelude. Most of the RFC was directly applied, removing reexports.
Some reexports are kept around, however:

* `range` remains until range syntax has landed to reduce churn.
* `Path` and `GenericPath` remain until path reform lands. This is done to
  prevent many imports of `GenericPath` which will soon be removed.
* All `io` traits remain until I/O reform lands so imports can be rewritten all
  at once to `std::io::prelude::*`.

This is a breaking change because many prelude reexports have been removed, and
the RFC can be consulted for the exact list of removed reexports, as well as to
find the locations of where to import them.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0503-prelude-stabilization.md
[breaking-change]

Closes #20068

various bugs in `trait_id_of_impl`. The end result was that looking up
the "trait_id_of_impl" with a trait's def-id yielded the same trait
again, even though it ought to have yielded None.

rules and cannot be updated until the libraries are synced, nor can
they opt in to the old semantics.

Fix orphan checking (cc #19470). (This is not a complete fix of #19470 because of the backwards compatibility feature gate.)

This is a [breaking-change]. The new rules require that, for an impl of a trait defined
in some other crate, two conditions must hold:

1. Some type must be local.
2. Every type parameter must appear "under" some local type.

Here are some examples that are legal:

```rust
struct MyStruct<T> { ... }

// Here `T` appears "under' `MyStruct`.
impl<T> Clone for MyStruct<T> { }

// Here `T` appears "under' `MyStruct` as well. Note that it also appears
// elsewhere.
impl<T> Iterator<T> for MyStruct<T> { }
```

Here is an illegal example:

```rust
// Here `U` does not appear "under" `MyStruct` or any other local type.
// We call `U` "uncovered".
impl<T,U> Iterator<U> for MyStruct<T> { }
```

There are a couple of ways to rewrite this last example so that it is
legal:

1. In some cases, the uncovered type parameter (here, `U`) should be converted
   into an associated type. This is however a non-local change that requires access
   to the original trait. Also, associated types are not fully baked.
2. Add `U` as a type parameter of `MyStruct`:
   ```rust
   struct MyStruct<T,U> { ... }
   impl<T,U> Iterator<U> for MyStruct<T,U> { }
   ```
3. Create a newtype wrapper for `U`
   ```rust
   impl<T,U> Iterator<Wrapper<U>> for MyStruct<T,U> { }
   ```

Because associated types are not fully baked, which in the case of the
`Hash` trait makes adhering to this rule impossible, you can
temporarily disable this rule in your crate by using
`#![feature(old_orphan_check)]`. Note that the `old_orphan_check`
feature will be removed before 1.0 is released.

This commit performs a second pass over the `std::boxed` module, taking the
following actions:

* `boxed` is now stable
* `Box` is now stable
* `BoxAny` is removed in favor of a direct `impl Box<Any>`
* `Box::downcast` remains unstable while the name of the `downcast` family of
  methods is determined.

This is a breaking change due to the removal of the `BoxAny` trait (note that
the `downcast` method still exists), and existing consumers of `BoxAny` simply
need to remove the import in their modules.

[breaking-change]

This commit is an implementation of [RFC 526][rfc] which is a change to alter
the definition of the old `fmt::FormatWriter`. The new trait, renamed to
`Writer`, now only exposes one method `write_str` in order to guarantee that all
implementations of the formatting traits can only produce valid Unicode.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0526-fmt-text-writer.md

One of the primary improvements of this patch is the performance of the
`.to_string()` method by avoiding an almost-always redundant UTF-8 check. This
is a breaking change due to the renaming of the trait as well as the loss of the
`write` method, but migration paths should be relatively easy:

* All usage of `write` should move to `write_str`. If truly binary data was
  being written in an implementation of `Show`, then it will need to use a
  different trait or an altogether different code path.

* All usage of `write!` should continue to work as-is with no modifications.

* All usage of `Show` where implementations just delegate to another should
  continue to work as-is.

[breaking-change]

Closes #20352

This pass performs a second pass of stabilization through the `std::sync`
module, avoiding modules/types that are being handled in other PRs (e.g.
mutexes, rwlocks, condvars, and channels).

The following items are now stable

* `sync::atomic`
* `sync::atomic::ATOMIC_BOOL_INIT` (was `INIT_ATOMIC_BOOL`)
* `sync::atomic::ATOMIC_INT_INIT` (was `INIT_ATOMIC_INT`)
* `sync::atomic::ATOMIC_UINT_INIT` (was `INIT_ATOMIC_UINT`)
* `sync::Once`
* `sync::ONCE_INIT`
* `sync::Once::call_once` (was `doit`)
  * C == `pthread_once(..)`
  * Boost == `call_once(..)`
  * Windows == `InitOnceExecuteOnce`
* `sync::Barrier`
* `sync::Barrier::new`
* `sync::Barrier::wait` (now returns a `bool`)
* `sync::Semaphore::new`
* `sync::Semaphore::acquire`
* `sync::Semaphore::release`

The following items remain unstable

* `sync::SemaphoreGuard`
* `sync::Semaphore::access` - it's unclear how this relates to the poisoning
                              story of mutexes.
* `sync::TaskPool` - the semantics of a failing task and whether a thread is
                     re-attached to a thread pool are somewhat unclear, and the
                     utility of this type in `sync` is question with respect to
                     the jobs of other primitives. This type will likely become
                     stable or move out of the standard library over time.
* `sync::Future` - futures as-is have yet to be deeply re-evaluated with the
                   recent core changes to Rust's synchronization story, and will
                   likely become stable in the future but are unstable until
                   that time comes.

[breaking-change]

the only things that trait selection needs.

Doesn't yet converge on a fixed point, but generally works. A better algorithm
will come with the implementation of default type parameter fallback.

If inference fails to determine an exact integral or floating point type, it
will set the type to i32 or f64, respectively.

Closes #16968

Closes #19999

[breaking-change]

Use [_; n] instead.

Closes #17732
Closes #18819
Closes #19479
Closes #19631
Closes #19632
Closes #19850
Closes #19883
Closes #20005
Closes #20009
Closes #20389

Uses the same approach as https://github.com/rust-lang/rust/pull/17286 (and
subsequent changes making it more correct), where the visitor will skip any
pieces of the AST that are from "foreign code", where the spans don't line up,
indicating that that piece of code is due to a macro expansion.

If this breaks your code, read the error message to determine which feature
gate you should add to your crate, and bask in the knowledge that your code
won't mysteriously break should you try to use the 1.0 release.

Closes #18102

[breaking-change]

Reimplement a minor optimization in String::from_utf8_lossy that avoids having to loop the slice from the begining.