This is for a pre-existing FIXME, but it was easy enough to do.

- Remove download-rustc handling from bootstrap.py
- Allow a custom `pattern` in `builder.unpack()`
- Only download rustc once another part of bootstrap depends on it.

  This is somewhat necessary since the download functions rely on having a full
  `Builder`, which isn't available until after config parsing finishes.

This has no logic changes, just a move.

Rollup of 5 pull requests

Successful merges:

 - #97302 (Do writeback of Closure params before visiting the parent expression)
 - #97328 (rustc: Fix ICE in native library error reporting)
 - #97351 (Output correct type responsible for structural match violation)
 - #97398 (Add regression test for #82830)
 - #97400 (Fix a typo on Struct `Substructure`)

Failed merges:

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

Fix a typo on Struct `Substructure`

Add regression test for #82830

Closes #82830
r? `@compiler-errors`

Output correct type responsible for structural match violation

Previously we included the outermost type that caused a structural match violation in the error message and stated that that type must be annotated with `#[derive(Eq, PartialEq)]` even if it already had that annotation. This PR outputs the correct type in the error message.

Fixes https://github.com/rust-lang/rust/issues/97278

rustc: Fix ICE in native library error reporting

Fixes https://github.com/rust-lang/rust/issues/97299

Do writeback of Closure params before visiting the parent expression

This means that given the expression:

```
let x = |a: Vec<_>| {};
```

We will visit the HIR node for `a` before `x`, and report the ambiguity on the former instead of the latter. This also moves writeback for struct field ids and const blocks before, but the ordering of this and walking the expr doesn't seem to matter.

Extend ptr::null and null_mut to all thin (including extern) types

Fixes https://github.com/rust-lang/rust/issues/93959

This change was accepted in https://rust-lang.github.io/rfcs/2580-ptr-meta.html

Note that this changes the signature of **stable** functions. The change should be backward-compatible, but it is **insta-stable** since it cannot (easily, at all?) be made available only through a `#![feature(…)]` opt-in.

The RFC also proposed the same change for `NonNull::dangling`, which makes sense it terms of its signature but not in terms of its implementation. `dangling` uses `align_of()` as an address. But what `align_of()` should be for extern types or whether it should be allowed at all remains an open question.

This commit depends on https://github.com/rust-lang/rust/pull/93977, which is not yet part of the bootstrap compiler. So `#[cfg]` is used to only apply the change in stage 1+. As far a I know bounds cannot be made conditional with `#[cfg]`, so the entire functions are duplicated. This is unfortunate but temporary.

Since this duplication makes it less obvious in the diff, the new definitions differ in:

* More permissive bounds (`Thin` instead of implied `Sized`)
* Different implementation
* Having `rustc_allow_const_fn_unstable(const_fn_trait_bound)`
* Having `rustc_allow_const_fn_unstable(ptr_metadata)`

Rollup of 5 pull requests

Successful merges:

 - #95953 (Modify MIR building to drop repeat expressions with length zero)
 - #96913 (RFC3239: Implement `cfg(target)` - Part 2)
 - #97233 ([RFC 2011] Library code)
 - #97370 (Minor improvement on else-no-if diagnostic)
 - #97384 (Fix metadata stats.)

Failed merges:

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

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`

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: https://github.com/rust-lang/rust/pull/97298#discussion_r880933431

r? `@estebank`

[RFC 2011] Library code

CC https://github.com/rust-lang/rust/pull/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`

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

This pull-request implements the compact `cfg(target(..))` part of [RFC 3239](https://github.com/rust-lang/rust/issues/96901).

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

cc `@GuillaumeGomez`
r? `@petrochenkov`

Modify MIR building to drop repeat expressions with length zero

Closes #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`

add a deep fast_reject routine

continues the work on #97136.

r? `@nnethercote`

Actually agree with you on the match structure 😆 let's see how that impacted perf 😅

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%)
```

Rollup of 5 pull requests

Successful merges:

 - #93604 (Make llvm-libunwind a per-target option)
 - #97026 (Change orderings of `Debug` for the Atomic types to `Relaxed`.)
 - #97105 (Add tests for lint on type dependent on consts)
 - #97323 (Introduce stricter checks for might_permit_raw_init under a debug flag )
 - #97379 (Add aliases for `current_dir`)

Failed merges:

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

`match_impl` has two call sites. For one of them (within `rematch_impl`)
the fast reject test isn't necessary, because any rejection would
represent a compiler bug.

This commit moves the fast reject test to the other `match_impl` call
site, in `assemble_candidates_from_impls`. This lets us move the fast
reject test outside the `probe` call in that function. This avoids the
taking of useless snapshots when the fast reject test succeeds, which
gives a performance win when compiling the `bitmaps` and `nalgebra`
crates.
Co-authored-by: Nname <n.nethercote@gmail.com>

Add aliases for `current_dir`

Aliases were added for the equivalent C/C++ APIs for POSIX and Windows. Also, I added one for `pwd` which users may be more familiar with, from the command line.

Introduce stricter checks for might_permit_raw_init under a debug flag

This is intended to be a version of the strict checks tried out in #79296, but also checking number validity (under the assumption that `let _ = std::mem::uninitialized::<u32>()` is UB, which seems to be what https://github.com/rust-lang/unsafe-code-guidelines/issues/71 is leaning towards.)

Add tests for lint on type dependent on consts

r? `@lcnr`

Change orderings of `Debug` for the Atomic types to `Relaxed`.

This reduces synchronization between threads when debugging the atomic types.  Reducing the synchronization means that executions with and without the debug calls will be more consistent, making it easier to debug.

We discussed this on the Rust Community Discord with `@ibraheemdev` before.

Make llvm-libunwind a per-target option

Fuchsia doesn't ship libunwind in its SDK, so we must provide it statically.

Make `Lazy*<T>` types in `rustc_metadata` not care about lifetimes until decode

This allows us to remove the `'tcx` lifetime from `CrateRoot`. This is necessary because of #97287, which makes the `'tcx` lifetime on `Ty` invariant instead of covariant, so [this hack](https://github.com/rust-lang/rust/blob/0a437b2ca081bc12425a3318cb66aade9824cbae/compiler/rustc_metadata/src/rmeta/decoder.rs#L89-92) no longer holds under that PR.

Introduces a trait called `ParameterizedOverTcx` which has a generic associated type that allows a type to be parameterized over that lifetime. This means we can decode, for example, `Lazy<Ty<'static>>` into any `Ty<'tcx>` depending on the `TyCtxt<'tcx>` we pass into the decode function.

rustdoc: fix few clippy lints

Fix few clippy lints: second commit - perf ones, first - other ones.

Rollup of 6 pull requests

Successful merges:

 - #93966 (document expectations for Waker::wake)
 - #97266 (Make weird name lints trigger behind cfg_attr)
 - #97355 (Remove unused brush image)
 - #97358 (Update minifier-rs version to 0.1.0)
 - #97363 (Fix a small mistake in `SliceIndex`'s documentation)
 - #97364 (Fix weird indentation in continue_keyword docs)

Failed merges:

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

Fix weird indentation in continue_keyword docs

This format was causing every line in the code examples to have a space at the start.

Fix a small mistake in `SliceIndex`'s documentation

Originally, it said "`get_(mut_)unchecked`," but the method's actual name is `get_unchecked_mut`.

Update minifier-rs version to 0.1.0

It fixes a bug with regex parsing.

r? `@notriddle`

Remove unused brush image

r? `@notriddle`