cc rust-lang#40373

Keeps MIR cleaner, `StorageLive(_: !)` makes no sense anyway.

…elix

Fix suggestion span error with a line containing multibyte characters

This PR fixes broken suggestions caused by multibyte characters.

e.g. for this code, rustc provides a broken suggestion ([playground](https://is.gd/DWGLu7)):

```rust
fn main() {
    let tup = (1,);
    println!("☃{}", tup[0]);
}
```

```
error: cannot index a value of type `({integer},)`
 --> <anon>:3:21
  |
3 |     println!("☃{}", tup[0]);
  |                     ^^^^^^
  |
help: to access tuple elements, use tuple indexing syntax as shown
  |     println!("☃{}"tup.00]);

error: aborting due to previous error
```

`CodeSuggestion::splice_lines` is misusing `Loc.col` (`CharPos`) as a byte offset when slicing source.

Better docs of rusty parts of typeck

Clean up rustdoc css

r? @rust-lang/docs

Papercut

r? @steveklabnik

Handle extern functions and statics in save-analysis

r? @eddyb

Improve the LLVM IR we generate for trivial functions, especially #[naked] ones.

These two small changes fix edef1c/libfringe#68:
* Don't emit ZST allocas, such as when returning `()`
* Don't emit a branch from LLVM's entry block to MIR's `START_BLOCK` unless needed
  * That is, if a loop branches back to it, although I'm not sure that's even valid MIR

Give spans to individual path segments in AST

And use these spans in path resolution diagnostics.

The spans are spans of identifiers in segments, not whole segments. I'm not sure what spans are more useful in general, but identifier spans are a better fit for resolve errors.

HIR still doesn't have spans.

Fixes rust-lang#38927 (comment) rust-lang#38890 (comment)

r? @nrc @eddyb

Do not bother creating StorageLive for TyNever

Keeps MIR cleaner, `StorageLive(_: !)` makes no sense anyway.

r? @eddyb

Fix UB in repr(packed) tests

r? @arielb1

cc rust-lang#37609 and rust-lang#27060

Box docs: no allocation is done for ZSTs.

Updated to add a small bit saying that ZSTs don't actually allocate on `Box::new`.

emit !align attributes on stores of operand pairs

This avoids another case of missing-align UB. cc rust-lang#40373

r? @eddyb

Implement placement-in protocol for `VecDeque`

CC rust-lang#30172

r? @nagisa