A direct comparison has the same effect. This also avoids the need for a
type test within `compare_const_vals`.

Because they're always equal.

It's never executed when running the entire test suite. I think it's
because of the early return at the top of the function if `a.ty() != ty`
succeeds.

Also, the `try_to_bits` always succeeds, so use `unwrap`.

This is a performance win for `unicode-normalization`.

The commit also removes the closure, which isn't necessary. And
reformulates the comparison into a form I find easier to read.

This is a performance win for `unicode-normalization`.

Also, I find the new formulation easier to read.

[RFC 2011] Minimal initial implementation

Tracking issue: #44838
Third step of #96496

Implementation has ~290 LOC with the bare minimum to be in a functional state. Currently only searches for binary operations to mimic what `assert_eq!` and `assert_ne!` already do.

r? `@oli-obk`

STD support for the Nintendo 3DS

Rustc already supports compiling for the Nintendo 3DS using the `armv6k-nintendo-3ds` target (Tier 3). Until now though, only `core` and `alloc` were supported. This PR adds standard library support for the Nintendo 3DS. A notable exclusion is `std::thread` support, which will come in a follow-up PR as it requires more complicated changes.

This has been a joint effort by `@Meziu,` `@ian-h-chamberlain,` myself, and prior work by `@rust3ds` members.

### Background

The Nintendo 3DS (Horizon OS) is a mostly-UNIX looking system, with the caveat that it does not come with a full libc implementation out of the box. On the homebrew side (I'm not under NDA), the libc interface is partially implemented by the [devkitPro](https://devkitpro.org/wiki/devkitPro_pacman) toolchain and a user library like [`libctru`](https://github.com/devkitPro/libctru). This is important because there are [some possible legal barriers](https://github.com/rust-lang/rust/pull/88529#issuecomment-919938396) to linking directly to a library that uses the underlying platform APIs, since they might be considered a trade secret or under NDA.

To get around this, the standard library impl for the 3DS does not directly depend on any platform-level APIs. Instead, it expects standard libc functions to be linked in. The implementation of these libc functions is left to the user. Some functions are provided by the devkitPro toolchain, but in our testing, we used the following to fill in the other functions:
- [`libctru`] - provides more basic APIs, such as `nanosleep`. Linked in by way of [`ctru-sys`](https://github.com/Meziu/ctru-rs/tree/master/ctru-sys).
- [`pthread-3ds`](https://github.com/Meziu/pthread-3ds) - provides pthread APIs for `std::thread`. Implemented using [`libctru`].
- [`linker-fix-3ds`](https://github.com/Meziu/rust-linker-fix-3ds) - fulfills some other missing libc APIs. Implemented using [`libctru`].

For more details, see the `src/doc/rustc/src/platform-support/armv6k-nintendo-3ds.md` file added in this PR.

### Notes
We've already upstreamed changes to the [`libc`] crate to support this PR, as well as the upcoming threading PR. These changes have all been released as of 0.2.121, so we bump the crate version in this PR.
Edit: After some rebases, the version bump has already been merged so it doesn't appear in this PR.

A lot of the changes in this PR are straightforward, and follow in the footsteps of the ESP-IDF target: https://github.com/rust-lang/rust/pull/87666.

The 3DS does not support user space process spawning, so these APIs are unimplemented (similar to ESP-IDF).

[`libctru`]: https://github.com/devkitPro/libctru
[`libc`]: https://github.com/rust-lang/libc

Rollup of 3 pull requests

Successful merges:

 - #95118 (Implement stabilization of `#[feature(io_safety)]`.)
 - #98110 (Make `ExprKind::Closure` a struct variant.)
 - #98115 (Remove `rustc_deprecated` diagnostics)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Remove `rustc_deprecated` diagnostics

Follow-up on #95960. The diagnostics will remain until the next bootstrap, at which point people will have had six weeks to adjust.

``@rustbot`` label +A-diagnostics

r? ``@compiler-errors``

Make `ExprKind::Closure` a struct variant.

Simple refactor since we both need it to introduce additional fields in `ExprKind::Closure`.

r? ``@Aaron1011``

Implement stabilization of `#[feature(io_safety)]`.

Implement stabilization of [I/O safety], aka `#[feature(io_safety)]`.

Fixes #87074.

[I/O safety]: https://github.com/rust-lang/rfcs/blob/master/text/3128-io-safety.md

Remove thread-local `IGNORED_ATTRIBUTES`.

It's just a copy of the read-only global `ich::IGNORED_ATTRIBUTES`, and
can be removed without any effect.

r? `@michaelwoerister`

Rollup of 7 pull requests

Successful merges:

 - #97822 (Filter out intrinsics if we have other import candidates to suggest)
 - #98026 (Move some tests to more reasonable directories)
 - #98067 (compiler: remove unused deps)
 - #98078 (Use unchecked mul to compute slice sizes)
 - #98083 (Rename rustc_serialize::opaque::Encoder as MemEncoder.)
 - #98087 (Suggest adding a `#[macro_export]` to a private macro)
 - #98113 (Fix misspelling of "constraint" as "contraint")

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Fix misspelling of "constraint" as "contraint"

I misspelled a function name a while back

Suggest adding a `#[macro_export]` to a private macro

fixes #97628

Rename rustc_serialize::opaque::Encoder as MemEncoder.

This avoids the name clash with `rustc_serialize::Encoder` (a trait),
and allows lots qualifiers to be removed and imports to be simplified
(e.g. fewer `as` imports).

(This was previously merged as commit 5 in #94732 and then was reverted
in #97905 because of a perf regression caused by commit 4 in #94732.)

r? ```@bjorn3```

Use unchecked mul to compute slice sizes

This allows LLVM to realize that `slice.len() > 0` iff `slice.len() * size_of::<T>() > 0`, allowing a branch on the latter to be folded into the former when dropping vecs and boxed slices, in some cases.

Fixes (partially) #96497

compiler: remove unused deps

Removed unused dependencies in compiler crates and moves few `libc` under `target.cfg(unix)` .

Move some tests to more reasonable directories

r? ```@petrochenkov```

Filter out intrinsics if we have other import candidates to suggest

Fixes #97618

Also open to just sorting these candidates to be last. Pretty easy to modify the code to do that, too.

Improve parsing errors and suggestions for bad `if` statements

1. Parses `if {}` as `if <err> {}` (block-like conditions that are missing a "then" block), and `if true && {}` as `if true && <err> {}` (unfinished binary operation), which is a more faithful recovery and leads to better typeck errors later on.
1. Points out the span of the condition if we don't see a "then" block after it, to help the user understand what is being parsed as a condition (and by elimination, what isn't).
1. Allow `if cond token else { }` to be fixed properly to `if cond { token } else { }`.
1. Fudge with the error messages a bit. This is somewhat arbitrary and I can revert my rewordings if they're useless.

----

Also this PR addresses a strange parsing regression (1.20 -> 1.21) where we chose to reject this piece of code somewhat arbitrarily, even though we should parse it fine:

```rust
fn main() {
    if { if true { return } else { return }; } {}
}
```

For context, all of these other expressions parse correctly:

```rust
fn main() {
    if { if true { return } else { return } } {}
    if { return; } {}
    if { return } {}
    if { return if true { } else { }; } {}
}
```

The parser used a heuristic to determine if the "the parsed `if` condition makes sense as a condition" that did like a one-expr-deep reachability analysis. This should not be handled by the parser though.

Implement stabilization of [I/O safety], aka `#[feature(io_safety)]`.

Fixes #87074.

[I/O safety]: https://github.com/rust-lang/rfcs/blob/master/text/3128-io-safety.md

Introduce `-Zvirtual-function-elimination` codegen flag

Fixes #68262

This PR adds a codegen flag `-Zvirtual-function-elimination` to enable the VFE optimization in LLVM. To make this work, additonal  information has to be added to vtables ([`!vcall_visibility` metadata](https://llvm.org/docs/TypeMetadata.html#vcall-visibility-metadata) and a `typeid` of the trait). Furthermore, instead of just `load`ing functions, the [`llvm.type.checked.load` intrinsic](https://llvm.org/docs/LangRef.html#llvm-type-checked-load-intrinsic) has to be used to map functions to vtables.

For technical details of the changes, see the commit messages.

I also tested this flag on https://github.com/tock/tock on different boards to verify that this fixes the issue https://github.com/tock/tock/issues/2594. This flag is able to improve the size of the resulting binary by about 8k-9k bytes by removing the unused debug print functions.

[Rendered documentation update](https://github.com/flip1995/rust/blob/pk-vfe/src/doc/rustc/src/codegen-options/index.md#virtual-function-elimination)

...since slice sizes can't signed wrap

see https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html

> The total size len * mem::size_of::<T>() of the slice must be no larger than isize::MAX.

Use valtrees as the type-system representation for constant values

This is not quite ready yet, there are still some problems with pretty printing and symbol mangling and `deref_const` seems to not work correctly in all cases.

Mainly opening now for a perf-run (which should be good to go, despite the still existing problems).

r? `@oli-obk`

cc `@lcnr` `@RalfJung`