…ytes

Under Stacked Borrows with raw pointer tagging, the previous code was UB
because the code which creates the the second pointer borrows the array
through a tag in the borrow stacks below the Unique tag that our first
pointer is based on, thus invalidating the first pointer.

This is not definitely a bug and may never be real UB, but I desperately
want people to write code that conforms to SB with raw pointer tagging
so that I can write good diagnostics. The alternative aliasing models
aren't possible to diagnose well due to state space explosion.
Therefore, it would be super cool if the standard library nudged people
towards writing code that is valid with respect to SB with raw pointer
tagging.

…ues, r=petrochenkov

Reduce the cost of loading all built-ins targets

This PR started by measuring the exact slowdown of checking of well known conditional values.
Than this PR implemented some technics to reduce the cost of loading all built-ins targets.

cf. rust-lang#82450 (comment)

…rror, r=tmiasko

Don't emit non-asm contents error for naked function composed of errors

## Motivation

For naked functions an error is emitted when they are composed of anything other than a single asm!() block. However, this error triggers in a couple situations in which it adds no additional information or is actively misleading.

One example is if you do have an asm!() block but simply one with a syntax error:
```rust
#[naked]
unsafe extern "C" fn compiler_errors() {
    asm!(invalid_syntax)
}
```

This results in two errors, one for the syntax error itself and another telling you that you need an asm block in your function:

```rust
error[E0787]: naked functions must contain a single asm block
 --> src/main.rs:6:1
  |
6 | / unsafe extern "C" fn naked_compile_error() {
7 | |     asm!(blah)
8 | | }
  | |_^
```

This issue also comes up when [utilizing `compile_error!()` for improving your diagnostics](https://twitter.com/steveklabnik/status/1509538243020218372), such as raising a compiler error when compiling for an unsupported target.

## Implementation

The rules this PR implements are as follows:

1. If any non-erroneous  non-asm statement is included, an error will still occur
2. If multiple asm statements are included, an error will still occur
3. If 0 or 1 asm statements are present, as well as any non-zero number of erroneous statements, then this error will *not* be raised as it is likely either redundant or incorrect

The rule of thumb is effectively "if an error is present and its correction could change things, don't raise an error".

…play, r=notriddle

Fix rustdoc attribute display

Fixes rust-lang#81482.

r? `@notriddle`

…Dylan-DPC

Fix &mut invalidation in ptr::swap doctest

Under Stacked Borrows with raw pointer tagging, the previous code was UB
because the code which creates the the second pointer borrows the array
through a tag in the borrow stacks below the Unique tag that our first
pointer is based on, thus invalidating the first pointer.

This is not definitely a bug and may never be real UB, but I desperately
want people to write code that conforms to SB with raw pointer tagging
so that I can write good diagnostics. The alternative aliasing models
aren't possible to diagnose well due to state space explosion.
Therefore, it would be super cool if the standard library nudged people
towards writing code that is valid with respect to SB with raw pointer
tagging.

The diagnostics that I want to write are implemented in a branch of Miri and the one for this case is below:
```
error: Undefined Behavior: attempting a read access using <2170> at alloc1068[0x0], but that tag does not exist in the borrow stack for this location
    --> /home/ben/rust/library/core/src/intrinsics.rs:2103:14
     |
2103 |     unsafe { copy_nonoverlapping(src, dst, count) }
     |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |              |
     |              attempting a read access using <2170> at alloc1068[0x0], but that tag does not exist in the borrow stack for this location
     |              this error occurs as part of an access at alloc1068[0x0..0x8]
     |
     = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental
     = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <2170> was created due to a retag at offsets [0x0..0x10]
    --> ../libcore/src/ptr/mod.rs:640:9
     |
8    | let x = array[0..].as_mut_ptr() as *mut [u32; 2]; // this is `array[0..2]`
     |         ^^^^^^^^^^^^^^^^^^^^^^^
help: <2170> was later invalidated due to a retag at offsets [0x0..0x10]
    --> ../libcore/src/ptr/mod.rs:641:9
     |
9    | let y = array[2..].as_mut_ptr() as *mut [u32; 2]; // this is `array[2..4]`
     |         ^^^^^
     = note: inside `std::intrinsics::copy_nonoverlapping::<[u32; 2]>` at /home/ben/rust/library/core/src/intrinsics.rs:2103:14
     = note: inside `std::ptr::swap::<[u32; 2]>` at /home/ben/rust/library/core/src/ptr/mod.rs:685:9
note: inside `main::_doctest_main____libcore_src_ptr_mod_rs_635_0` at ../libcore/src/ptr/mod.rs:12:5
    --> ../libcore/src/ptr/mod.rs:644:5
     |
12   |     ptr::swap(x, y);
     |     ^^^^^^^^^^^^^^^
note: inside `main` at ../libcore/src/ptr/mod.rs:15:3
    --> ../libcore/src/ptr/mod.rs:647:3
     |
15   | } _doctest_main____libcore_src_ptr_mod_rs_635_0() }
     |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error
```

core: document that the align_of* functions return the alignment in bytes