We currently provide wrong suggestions and unhelpful errors on closure
bodies with braces missing. For example, given the following code:

```
fn main() {
    let _x = Box::new(|x|x+1;);
}
```

the current output is like this:

```
error: expected expression, found `)`
 --> ./main.rs:2:30
  |
2 |     let _x = Box::new(|x|x+1;);
  |                              ^ expected expression

error: closure bodies that contain statements must be surrounded by braces
 --> ./main.rs:2:25
  |
2 |     let _x = Box::new(|x|x+1;);
  |                         ^
3 | }
  | ^
  |

...

help: try adding braces
  |
2 ~     let _x = Box::new(|x| {x+1;);
3 ~ }}

...

error: expected `;`, found `}`
 --> ./main.rs:2:32
  |
2 |     let _x = Box::new(|x|x+1;);
  |                                ^ help: add `;` here
3 | }
  | - unexpected token

error: aborting due to 3 previous errors
```

This commit allows outputting correct suggestions and errors. The above
code would output like this:

```
error: closure bodies that contain statements must be surrounded by braces
 --> ./main.rs:2:25
  |
2 |     let _x = Box::new(|x|x+1;);
  |                         ^    ^
  |
note: statement found outside of a block
 --> ./main.rs:2:29
  |
2 |     let _x = Box::new(|x|x+1;);
  |                          ---^ this `;` turns the preceding closure into a statement
  |                          |
  |                          this expression is a statement because of the trailing semicolon
note: the closure body may be incorrectly delimited
 --> ./main.rs:2:23
  |
2 |     let _x = Box::new(|x|x+1;);
  |                       ^^^^^^ - ...but likely you meant the closure to end here
  |                       |
  |                       this is the parsed closure...
help: try adding braces
  |
2 |     let _x = Box::new(|x| {x+1;});
  |                           +    +

error: aborting due to previous error
```

…o, r=thomcc

implement const iterator using `rustc_do_not_const_check`

Previous experiment: rust-lang#102225.

Explanation: rather than making all default methods work under `const` all at once, this uses `rustc_do_not_const_check` as a workaround to "trick" the compiler to not run any checks on those other default methods. Any const implementations are only required to implement the `next` method. Any actual calls to the trait methods other than `next` will either error in compile time (at CTFE runs), or run the methods correctly if they do not have any non-const operations. This is extremely easy to maintain, remove, or improve.

Rebuild BinaryHeap on unwind from retain

This closes the hole identified in rust-lang#71503 (comment) which had made it possible for the caller to end up with a heap in invalid state. As of rust-lang#105851, heaps in invalid state are not supposed to exist.

Restore behavior when primary bundle is missing

Fixes rust-lang#106755 by restoring some of the behavior prior to rust-lang#106427

Still, I have no idea how this debug assertion can even hit while using `en-US` as primary  bundle.

r? `@davidtwco`

Make query keys `Copy`

This regressed compiler performance locally, so I'm curious what perf will say about it.

<table><tr><td rowspan="2">Benchmark</td><td colspan="1"><b>Before</b></th><td colspan="2"><b>After</b></th></tr><tr><td align="right">Time</td><td align="right">Time</td><td align="right">%</th></tr><tr><td>🟣 <b>clap</b>:check</td><td align="right">1.7566s</td><td align="right">1.7657s</td><td align="right"> 0.52%</td></tr><tr><td>🟣 <b>hyper</b>:check</td><td align="right">0.2572s</td><td align="right">0.2578s</td><td align="right"> 0.20%</td></tr><tr><td>🟣 <b>regex</b>:check</td><td align="right">0.9863s</td><td align="right">0.9900s</td><td align="right"> 0.37%</td></tr><tr><td>🟣 <b>syn</b>:check</td><td align="right">1.6018s</td><td align="right">1.6073s</td><td align="right"> 0.34%</td></tr><tr><td>🟣 <b>syntex_syntax</b>:check</td><td align="right">6.2493s</td><td align="right">6.2920s</td><td align="right"> 0.68%</td></tr><tr><td>Total</td><td align="right">10.8512s</td><td align="right">10.9127s</td><td align="right"> 0.57%</td></tr><tr><td>Summary</td><td align="right">1.0000s</td><td align="right">1.0042s</td><td align="right"> 0.42%</td></tr></table>

…rmed-closure, r=davidtwco

parser: provide better suggestions and errors on closures with braces missing

We currently provide wrong suggestions and unhelpful errors on closure bodies with braces missing.

For example, given the following code:

```rust
fn main() {
    let _x = Box::new(|x|x+1;);
}
```

the current output is:

```
error: expected expression, found `)`
 --> ./main.rs:2:30
  |
2 |     let _x = Box::new(|x|x+1;);
  |                              ^ expected expression

error: closure bodies that contain statements must be surrounded by braces
 --> ./main.rs:2:25
  |
2 |     let _x = Box::new(|x|x+1;);
  |                         ^
3 | }
  | ^
  |
note: statement found outside of a block
 --> ./main.rs:2:29
  |
2 |     let _x = Box::new(|x|x+1;);
  |                          ---^ this `;` turns the preceding closure into a statement
  |                          |
  |                          this expression is a statement because of the trailing semicolon
note: the closure body may be incorrectly delimited
 --> ./main.rs:2:23
  |
2 |     let _x = Box::new(|x|x+1;);
  |                       ^^^^^^ this is the parsed closure...
3 | }
  | - ...but likely you meant the closure to end here
help: try adding braces
  |
2 ~     let _x = Box::new(|x| {x+1;);
3 ~ }}
  |

error: expected `;`, found `}`
 --> ./main.rs:2:32
  |
2 |     let _x = Box::new(|x|x+1;);
  |                                ^ help: add `;` here
3 | }
  | - unexpected token

error: aborting due to 3 previous errors
```

We got 3 errors, but all but the second are unnecessary or just wrong.

This commit allows outputting correct suggestions and errors. The above code would output like this:

```
error: closure bodies that contain statements must be surrounded by braces
 --> ./main.rs:2:25
  |
2 |     let _x = Box::new(|x|x+1;);
  |                         ^    ^
  |
note: statement found outside of a block
 --> ./main.rs:2:29
  |
2 |     let _x = Box::new(|x|x+1;);
  |                          ---^ this `;` turns the preceding closure into a statement
  |                          |
  |                          this expression is a statement because of the trailing semicolon
note: the closure body may be incorrectly delimited
 --> ./main.rs:2:23
  |
2 |     let _x = Box::new(|x|x+1;);
  |                       ^^^^^^ - ...but likely you meant the closure to end here
  |                       |
  |                       this is the parsed closure...
help: try adding braces
  |
2 |     let _x = Box::new(|x| {x+1;});
  |                           +    +

error: aborting due to previous error
```

Fixes rust-lang#107959.

r? diagnostics