…ls from "normal" ADT fields.

Fixing confusion between mod and remainder

Like many programming languages, rust too confuses remainder and modulus.  The `%` operator and the associated `Rem` trait is (as the trait name suggests) the remainder, but since most people are linguistically more familiar with the modulus the documentation sometimes claims otherwise.  This PR tries to fix this problem in rustc.

improve panic message for slice windows and chunks

before:
```text
thread 'main' panicked at 'size is zero', /rustc/1e225413a21fa69570bd3fefea9eb05e33f8b917/library/core/src/slice/mod.rs:809:44
```
```text
thread 'main' panicked at 'assertion failed: `(left != right)`
  left: `0`,
 right: `0`: chunks cannot have a size of zero', /rustc/1e225413a21fa69570bd3fefea9eb05e33f8b917/library/core/src/slice/mod.rs:843:9
```

after:
```text
thread 'main' panicked at 'chunk size must be non-zero', src/main.rs:4:22
```

fixes rust-lang#107437

…tlarsan68

Small bootstrap improvements

- i/o-less check for `xz` availability
- cache result of NixOS detection
- load correct `bootstrap` module even when a package of that name is installed
- no `-W semicolon_in_expressions_from_macros` – it is warn-by-default
- one type per variable (making dynamic typing less confusing)
- integrate python-side `--help` flag into the argument parser (makes `-hv` work as a short form of `--help --verbose`)

I even checked that it works with Python 2.

…nd-2, r=estebank

Make the "extra if in let...else block" hint a suggestion

Changes the hint to a suggestion, suggested in rust-lang#107213.

r? ```@estebank```

…ojection-generator-tweak, r=michaelwoerister

Do not depend on Generator trait when deducing closure signature

1. Do not depend on `Generator` trait when deducing closure signature.
2. Compare the name of the `Generator::Return` associated item, rather than its order in the trait. Seems more stable this way.

…te-in-print-type-sizes, r=compiler-errors

Extend `-Z print-type-sizes` to distinguish generator upvars+locals from "normal" fields.

For example, for this code:

```rust
async fn wait() {}

async fn test(arg: [u8; 8192]) {
    wait().await;
    drop(arg);
}

async fn test_ideal(_rg: [u8; 8192]) {
    wait().await;
    // drop(arg);
}

fn main() {
    let gen_t = test([0; 8192]);
    let gen_i = test_ideal([0; 8192]);
    println!("expect {}, got: {}",
             std::mem::size_of_val(&gen_i),
             std::mem::size_of_val(&gen_t));
}
```

the `-Z print-type-sizes` output used to start with:

```
print-type-size type: `[async fn body@issue-62958-a.rs:3:32: 6:2]`: 16386 bytes, alignment: 1 bytes
print-type-size     discriminant: 1 bytes
print-type-size     variant `Suspend0`: 16385 bytes
print-type-size         field `.arg`: 8192 bytes, offset: 0 bytes, alignment: 1 bytes
print-type-size         field `.arg`: 8192 bytes
print-type-size         field `.__awaitee`: 1 bytes
...
print-type-size type: `std::mem::ManuallyDrop<[u8; 8192]>`: 8192 bytes, alignment: 1 bytes
print-type-size     field `.value`: 8192 bytes
...
```

but with this change, it now instead prints:

```
print-type-size type: `[async fn body@issue-62958-a.rs:3:32: 6:2]`: 16386 bytes, alignment: 1 bytes
print-type-size     discriminant: 1 bytes
print-type-size     variant `Suspend0`: 16385 bytes
print-type-size         upvar `.arg`: 8192 bytes, offset: 0 bytes, alignment: 1 bytes
print-type-size         local `.arg`: 8192 bytes
print-type-size         local `.__awaitee`: 1 bytes
...
print-type-size type: `std::mem::ManuallyDrop<[u8; 8192]>`: 8192 bytes, alignment: 1 bytes
print-type-size     field `.value`: 8192 bytes
```

(spawned off of investigation of rust-lang#62958 )