This commit:
- Counts some things that weren't being counted previously, and adds
  an assertion that ensure everything is counted.
- Reorders things so the `eprintln`s order matches the code order.
- Adds percentages, and makes clear that the zero bytes count is orthogonal to
  the other measurements.

Example of the new output:
```
55463779 metadata bytes, of which 18054531 bytes (32.6%) are zero
             preamble:       30 bytes ( 0.0%)
                  dep:        0 bytes ( 0.0%)
          lib feature:    17458 bytes ( 0.0%)
            lang item:      337 bytes ( 0.0%)
      diagnostic item:     1788 bytes ( 0.0%)
           native lib:        0 bytes ( 0.0%)
      foreign modules:     5113 bytes ( 0.0%)
       def-path table:   720180 bytes ( 1.3%)
               traits:      359 bytes ( 0.0%)
                impls:    64624 bytes ( 0.1%)
     incoherent_impls:      130 bytes ( 0.0%)
                  mir: 16137354 bytes (29.1%)
                 item: 23773099 bytes (42.9%)
interpret_alloc_index:      599 bytes ( 0.0%)
      proc-macro-data:        0 bytes ( 0.0%)
               tables: 10081135 bytes (18.2%)
 debugger visualizers:        0 bytes ( 0.0%)
     exported symbols:     5666 bytes ( 0.0%)
              hygiene:  1539390 bytes ( 2.8%)
      def-path hashes:  2752564 bytes ( 5.0%)
           source_map:   363540 bytes ( 0.7%)
                final:      413 bytes ( 0.0%)
```

Modify MIR building to drop repeat expressions with length zero

Closes rust-lang#74836 .

Previously, when a user wrote `[foo; 0]` we used to simply leak `foo`. The goal is to fix that. This PR changes MIR building to make `[foo; 0]` equivalent to `{ drop(foo); [] }` in all cases. Of course, this is a breaking change (see below). A crater run did not indicate any regressions though, and given that the previous behavior was almost definitely not what any user wanted, it seems unlikely that anyone was relying on this.

Note that const generics are in general unaffected by this. Inserting the extra `drop` is only meaningful/necessary when `foo` is of a non-`Copy` type, and array repeat expressions with const generic repetition count must always be `Copy`.

Besides the obvious change to behavior associated with the additional drop, there are three categories of examples where this also changes observable behavior. In all of these cases, the new behavior is consistent with what you would get by replacing `[foo; 0]` with `{ drop(foo); [] }`. As such, none of these give the user new powers to express more things.

**No longer allowed in const (breaking)**:

```rust
const _: [String; 0] = [String::new(); 0];
```

This compiles on stable today. Because we now introduce the drop of `String`, this no longer compiles as `String` may not be dropped in a const context.

**Reduced dataflow (non-breaking)**:

```rust
let mut x: i32 = 0;
let r = &x;
let a = [r; 0];
x = 5;
let _b = a;
```

Borrowck rejects this code on stable because it believes there is dataflow between `a` and `r`, and so the lifetime of `r` has to extend to the last statement. This change removes the dataflow and the above code is allowed to compile.

**More const promotion (non-breaking)**:

```rust
let _v: &'static [String; 0] = &[String::new(); 0];
```

This does not compile today because `String` having drop glue keeps it from being const promoted (despite that drop glue never being executed). After this change, this is allowed to compile.

### Alternatives

A previous attempt at this tried to reduce breakage by various tricks. This is still a possibility, but given that crater showed no regressions it seems unclear why we would want to introduce this complexity.

Disallowing `[foo; 0]` completely is also an option, but obviously this is more of a breaking change. I do not know how often this is actually used though.

r? `@oli-obk`

RFC3239: Implement `cfg(target)` - Part 2

This pull-request implements the compact `cfg(target(..))` part of [RFC 3239](rust-lang#96901).

I recommend reviewing this PR on a per commit basics, because of some moving parts.

cc `@GuillaumeGomez`
r? `@petrochenkov`

[RFC 2011] Library code

CC rust-lang#96496

Based on https://github.com/dtolnay/case-studies/tree/master/autoref-specialization.

Basically creates two traits with the same method name. One trait is generic over any `T` and the other is specialized to any `T: Printable`.

The compiler will then call the corresponding trait method through auto reference.

```rust
fn main() {
    let mut a = Capture::new();
    let mut b = Capture::new();

    (&Wrapper(&1i32)).try_capture(&mut a); // `try_capture` from `TryCapturePrintable`
    (&Wrapper(&vec![1i32])).try_capture(&mut b); // `try_capture` from `TryCaptureGeneric`

    assert_eq!(format!("{:?}", a), "1");
    assert_eq!(format!("{:?}", b), "N/A");
}
```

r? `@scottmcm`

…an-DPC

Minor improvement on else-no-if diagnostic

Don't suggest wrapping in block since it's highly likely to be a missing `if` after `else`. Also rework message a bit (open to further suggestions).

cc: rust-lang#97298 (comment)

r? `@estebank`

…jorn3

Fix metadata stats.

This commit:
- Counts some things that weren't being counted previously, and adds
  an assertion that ensure everything is counted.
- Reorders things so the `eprintln`s order matches the code order.
- Adds percentages, and makes clear that the zero bytes count is orthogonal to
  the other measurements.

Example of the new output:
```
55463779 metadata bytes, of which 18054531 bytes (32.6%) are zero
             preamble:       30 bytes ( 0.0%)
                  dep:        0 bytes ( 0.0%)
          lib feature:    17458 bytes ( 0.0%)
            lang item:      337 bytes ( 0.0%)
      diagnostic item:     1788 bytes ( 0.0%)
           native lib:        0 bytes ( 0.0%)
      foreign modules:     5113 bytes ( 0.0%)
       def-path table:   720180 bytes ( 1.3%)
               traits:      359 bytes ( 0.0%)
                impls:    64624 bytes ( 0.1%)
     incoherent_impls:      130 bytes ( 0.0%)
                  mir: 16137354 bytes (29.1%)
                 item: 23773099 bytes (42.9%)
interpret_alloc_index:      599 bytes ( 0.0%)
      proc-macro-data:        0 bytes ( 0.0%)
               tables: 10081135 bytes (18.2%)
 debugger visualizers:        0 bytes ( 0.0%)
     exported symbols:     5666 bytes ( 0.0%)
              hygiene:  1539390 bytes ( 2.8%)
      def-path hashes:  2752564 bytes ( 5.0%)
           source_map:   363540 bytes ( 0.7%)
                final:      413 bytes ( 0.0%)
```
r? `@bjorn3`