Rollup merge of #40092 - sinkuu:fix_suggestion_index, r=pnkfelix

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.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/librustc_errors/lib.rs b/src/librustc_errors/lib.rs
index d7bd5ed23c2..4c889dad8ca 100644
--- a/src/librustc_errors/lib.rs
+++ b/src/librustc_errors/lib.rs
@@ -90,7 +90,8 @@ impl CodeSuggestion {
                          hi_opt: Option<&Loc>) {
             let (lo, hi_opt) = (lo.col.to_usize(), hi_opt.map(|hi| hi.col.to_usize()));
             if let Some(line) = line_opt {
-                if line.len() > lo {
+                if let Some(lo) = line.char_indices().map(|(i, _)| i).nth(lo) {
+                    let hi_opt = hi_opt.and_then(|hi| line.char_indices().map(|(i, _)| i).nth(hi));
                     buf.push_str(match hi_opt {
                         Some(hi) => &line[lo..hi],
                         None => &line[lo..],