Add `VariantIdx` type and use instead of `usize`

Use read_unaligned instead of read in transmute_copy

Closes: #55044

This change could result in performance regression on non-x86 platforms. (but it also can fix some of UB which lurks in existing programs) An alternative would be to update `transmute_copy` documentation with alignment requirements.

Minor standard library constification

This PR makes some bits of the standard library into `const fn`s.
I've tried to be as aggressive as I possibly could in the constification.
The list is rather small due to how restrictive `const fn` is at the moment.

r? @oli-obk cc @rust-lang/libs

Stable public APIs affected:
+ [x] `Cell::as_ptr`
+ [x] `UnsafeCell::get`
+ [x] `char::is_ascii`
+ [x] `iter::empty`
+ [x] `ManuallyDrop::{new, into_inner}`
+ [x] `RangeInclusive::{start, end}`
+ [x] `NonNull::as_ptr`
+ [x] `{[T], str}::as_ptr`
+ [x] `Duration::{as_secs, subsec_millis, subsec_micros, subsec_nanos}`
+ [x] `CStr::as_ptr`
+ [x] `Ipv4Addr::is_unspecified`
+ [x] `Ipv6Addr::new`
+ [x] `Ipv6Addr::octets`

Unstable public APIs affected:
+ [x] `Duration::{as_millis, as_micros, as_nanos, as_float_secs}`
+ [x] `Wrapping::{count_ones, count_zeros, trailing_zeros, rotate_left, rotate_right, swap_bytes, reverse_bits, from_be, from_le, to_be, to_le, leading_zeros, is_positive, is_negative, leading_zeros}`
+ [x] `core::convert::identity`

--------------------------

## Removed from list in first pass:

Stable public APIs affected:
+ [ ] `BTree{Map, Set}::{len, is_empty}`
+ [ ] `VecDeque::is_empty`
+ [ ] `String::{is_empty, len}`
+ [ ] `FromUtf8Error::utf8_error`
+ [ ] `Vec<T>::{is_empty, len}`
+ [ ] `Layout::size`
+ [ ] `DecodeUtf16Error::unpaired_surrogate`
+ [ ] `core::fmt::{fill, width, precision, sign_plus, sign_minus, alternate, sign_aware_zero_pad}`
+ [ ] `panic::Location::{file, line, column}`
+ [ ] `{ChunksExact, RChunksExact}::remainder`
+ [ ] `Utf8Error::valid_up_to`
+ [ ] `VacantEntry::key`
+ [ ] `NulError::nul_position`
+ [ ] `IntoStringError::utf8_error`
+ [ ] `IntoInnerError::error`
+ [ ] `io::Chain::get_ref`
+ [ ] `io::Take::{limit, get_ref}`
+ [ ] `SocketAddrV6::{flowinfo, scope_id}`
+ [ ] `PrefixComponent::{kind, as_os_str}`
+ [ ] `Path::{ancestors, display}`
+ [ ] `WaitTimeoutResult::timed_out`
+ [ ] `Receiver::{iter, try_iter}`
+ [ ] `thread::JoinHandle::thread`
+ [ ] `SystemTimeError::duration`

Unstable public APIs affected:
+ [ ] `core::fmt::Arguments::new_v1`
+ [ ] `core::fmt::Arguments::new_v1_formatted`
+ [ ] `Pin::{get_ref, into_ref}`
+ [ ] `Utf8Lossy::chunks`
+ [ ] `LocalWaker::as_waker`
+ [ ] `panic::PanicInfo::{internal_constructor, message, location}`
+ [ ] `panic::Location::{internal_constructor }`

## Removed from list in 2nd pass:

Stable public APIs affected:
+ [ ] `LinkedList::{new, iter, is_empty, len}`
+ [ ] `mem::forget`
+ [ ] `Cursor::{new, get_ref, position}`
+ [ ] `io::{empty, repeat, sink}`
+ [ ] `PoisonError::new`
+ [ ] `thread::Builder::new`
+ [ ] `process::Stdio::{piped, inherit, null}`

Unstable public APIs affected:
+ [ ] `io::Initializer::{zeroing, should_initialize}`

Avoid the Box in `TyCtxt::associated_items`.

This reduces instruction counts on `packed_simd` by 2%.

r? @nikomatsakis

This reduces instruction counts on packed_simd by 2%.

Fix emission of niche-filling discriminant values

Bug #55606 points out a regression introduced by #54004; namely that
an assertion can erroneously fire when a niche-filling discriminant
value is emitted.

This fixes the bug by removing the assertion, and furthermore by
arranging for the discriminant value to be masked according to the
size of the niche.  This makes handling the discriminant a bit simpler
for debuggers.

The test case is from Jonathan Turner.

Closes #55606

Make `MatcherPos::stack` a `SmallVec`.

This avoids some allocations.

This seems like a trivial change, but the compiler rejects it:
```
   Compiling syntax v0.0.0 (/home/njn/moz/rust1/src/libsyntax)
error[E0597]: `initial` does not live long enough=========>           ] 89/110: syntax
   --> libsyntax/ext/tt/macro_parser.rs:647:57
    |
647 |     let mut cur_items = smallvec![MatcherPosHandle::Ref(&mut initial)];
    |                                                         ^^^^^^^^^^^^ borrowed value does not live long enough
...
762 | }
    | -
    | |
    | `initial` dropped here while still borrowed
    | borrow later used here, when `initial` is dropped

error: aborting due to previous error
```
This is either a compiler bug, or there's some subtle thing I don't understand. The lifetimes sure seem straightforward: `initial` is declared, and then `cur_items` is declared immediately afterward, and it uses a reference to `initial`. The error message makes it sound like the compiler is dropping the variables in the wrong order.

r? @nikomatsakis, any idea what the problem is?

This avoids some allocations.

Remove support for building against LLVM 4

With emscripten removed in #55626, we no longer need to support building against LLVM 4.

Remove the `alloc_system` crate

In what's hopefully one of the final nails in the coffin of the "old allocator story of yore" this PR deletes the `alloc_system` crate and all traces of it from the compiler. The compiler no longer needs to inject allocator crates anywhere and the `alloc_system` crate has no real reason to exist outside the standard library.

The unstable `alloc_system` crate is folded directly into the standard library where its stable interface, the `System` type, remains the same. All unstable traces of `alloc_system` are removed, however.

This commit deletes the `alloc_system` crate from the standard
distribution. This unstable crate is no longer needed in the modern
stable global allocator world, but rather its functionality is folded
directly into the standard library. The standard library was already the
only stable location to access this crate, and as a result this should
not affect any stable code.

This commit cleans up allocator injection logic found in the compiler
around selecting the global allocator. It turns out that now that
jemalloc is gone the compiler never actually injects anything! This
means that basically everything around loading crates here and there can
be easily pruned.

This also removes the `exe_allocation_crate` option from custom target
specs as it's no longer used by the compiler anywhere.

NLL Diagnostic Review 3: Unions not reinitialized after assignment into field

Fixes #55651, #55652.

This PR makes two changes:

First, it updates the dataflow builder to add an init for the place
containing a union if there is an assignment into the field of
that union.

Second, it stops a "use of uninitialized" error occuring when there is an
assignment into the field of an uninitialized union that was previously
initialized. Making this assignment would re-initialize the union, as
tested in `src/test/ui/borrowck/borrowck-union-move-assign.nll.stderr`.
The check for previous initialization ensures that we do not start
supporting partial initialization yet (cc #21232, #54499, #54986).

This PR also fixes #55652 which was marked as requiring investigation
as the changes in this PR add an error that was previously missing
(and mentioned in the review comments) and confirms that the error
that was present is correct and a result of earlier partial
initialization changes in NLL.

r? @pnkfelix (due to earlier work with partial initialization)
cc @nikomatsakis

Miri engine refactoring

r? @RalfJung

split out the "just moves stuff around" part of https://github.com/rust-lang/rust/pull/55293

Rollup of 17 pull requests

Successful merges:

 - #55630 (resolve: Filter away macro prelude in modules with `#[no_implicit_prelude]` on 2018 edition)
 - #55687 (Take supertraits into account when calculating associated types)
 - #55745 (Convert `outlives_components`' return value to a `SmallVec` outparam.)
 - #55764 (Fix Rc/Arc allocation layout)
 - #55792 (Prevent ICE in const-prop array oob check)
 - #55799 (Removed unneeded instance of `// revisions` from a lint test)
 - #55800 (Fix ICE in `return_type_impl_trait`)
 - #55801 (NLL: Update box insensitivity test)
 - #55802 (Don't inline virtual calls (take 2))
 - #55816 (Use `SmallVec` to avoid allocations in `from_decimal_string`.)
 - #55819 (Typecheck patterns of all match arms first, so we get types for bindings)
 - #55822 (ICE with #![feature(nll)] and elided lifetimes)
 - #55828 (Add missing `rustc_promotable` attribute to unsigned `min_value` and `max_value`)
 - #55839 (Fix docstring spelling mistakes)
 - #55844 (Fix documentation typos.)
 - #55845 (Set BINARYEN_TRAP_MODE=clamp)
 - #55856 (rustdoc: refactor: move all static-file include!s into a single module)

rustdoc: refactor: move all static-file include!s into a single module

This is a smaller refactor that creates a new module `rustdoc::html::static_files`, which contains a bunch of `static` variables with all the files in `html/static` that we use. The idea behind moving them all here was to remove the duplicate `include_bytes!()` that are used by the theme-checker code. It also continues to centralize more operations in rustdoc.

Set BINARYEN_TRAP_MODE=clamp

This fixes the wasm32-unknown-emscripten test failure mentioned in https://github.com/rust-lang/rust/pull/55626#issuecomment-437084774, by making binaryen operate in clamp rather than trap mode.

The issue is that the current `-Zsaturating-float-casts` implementation uses `fpto[us]i` unconditionally (and selects afterwards), which does not work with trapping implementations of fpto[su]i, which emscripten uses by default.

I've left a FIXME to drop this flag once we have a better solution for saturating casts on the LLVM side.
;

resolve: Filter away macro prelude in modules with `#[no_implicit_prelude]` on 2018 edition

This is a tiny thing.
For historical reasons macro prelude (macros from `#[macro_use] extern crate ...`, including `extern crate std`) is still available in modules with `#[no_implicit_prelude]`.
This PR provides proper isolation and removes those names from scope.

`#[no_implicit_prelude]` modules still have built-in types (`u8`), built-in attributes (`#[inline]`) and built-in macros (`env!("PATH")`) in scope. We can introduce some `#[no_implicit_prelude_at_all]` to remove those as well, but that's a separate issue.

The change is done only on 2018 edition for backward compatibility.
I'm pretty sure this can be done on 2015 as well because `#[no_implicit_prelude]` is rarely used, but I don't want to go through the crater/deprecation process right now, maybe later.

cc https://github.com/rust-lang/rust/issues/53977
r? @ghost

Support for the program data address space option of LLVM's Target Datalayout

This was introduced recently (specifically, for AVR, cc @dylanmckay).

(I came up with this when attempting to run [avr-rust](https://github.com/avr-rust/rust) rebased on the latest [rust-lang](https://github.com/rust-lang/rust) commits. If this requires a different design, some additional discussions, or is not something to pursue right now, I'd be happy to close this PR).

Note that this somewhat overlaps with @DiamondLovesYou's #51576, I think, although the implementation here is significantly simpler: Since the address space applies to _all_ program data, we can just check the pointee's type whenever we create an LLVM pointer type. If it is a function we use the program data address space; if not we use the default address space.

cc @EddyB, who has been reviewing #51576

Ref: https://llvm.org/docs/LangRef.html#data-layout

Fix documentation typos.

Fix docstring spelling mistakes

Add missing `rustc_promotable` attribute to unsigned `min_value` and `max_value`

cc @pnkfelix

fixes #55806

ICE with #![feature(nll)] and elided lifetimes

Fixes #55394.

This commit fixes an ICE and determines the correct return span in cases
with a method implemented on a struct with an an elided lifetime.

r? @pnkfelix

Typecheck patterns of all match arms first, so we get types for bindings

Fix eventually (after backport to beta) the issue #55810

Use `SmallVec` to avoid allocations in `from_decimal_string`.

This reduces the number of allocations in a "check clean" build of
`tuple-stress` by 14%, reducing instruction counts by 0.6%.

Don't inline virtual calls (take 2)

When I fixed the previous mis-optimizations, I didn't realize there were
actually two different places where we mutate `callsites` and both of
them should have the same behavior.

As a result, if a function was inlined and that function contained
virtual function calls, they were incorrectly being inlined. I also
added a test case which covers this.

NLL: Update box insensitivity test

This is just keeping one of our tests honest with respect to NLL, in two ways:

 1. Adds uses of borrows that would otherwise be too short to observe the error that we would have expected to see...
 2. ... I say "would have expected" because all of the errors in this file are part of the reversion of rust-lang/rfcs#130 that is attached to NLL (you can see more discussion of this here https://github.com/rust-lang/rust/issues/43234#issuecomment-411017768 )

Fix ICE in `return_type_impl_trait`

Fix #55796.

Rollup merge of #55799 - pnkfelix:remove-useless-revisions-marker-from-lint-unused-mut-variables, r=davidtwco

Removed unneeded instance of `// revisions` from a lint test

Removed an unneeded instance of `// revisions`; the compare-mode=nll shows the output is identical now.

cc #54528

Prevent ICE in const-prop array oob check

fixes https://github.com/rust-lang/rust/issues/55772
fixes https://github.com/rust-lang/rust/issues/54541

Fix Rc/Arc allocation layout

* Rounds allocation layout up to a multiple of alignment
* Adds a convenience method `Layout::pad_to_align` to perform rounding

Closes #55747

cc #55724