Better error note on unimplemented Index trait for string
fixes#56740
I've tried to compile suggestion from comments in the issue #56740, but unsure of it. So I'm open to advice :)
Current output will be like this:
```rust
error[E0277]: the type `str` cannot be indexed by `{integer}`
--> $DIR/str-idx.rs:3:17
|
LL | let c: u8 = s[4]; //~ ERROR the type `str` cannot be indexed by `{integer}`
| ^^^^ `str` cannot be indexed by `{integer}`
|
= help: the trait `std::ops::Index<{integer}>` is not implemented for `str`
= note: you can use `.chars().nth()` or `.bytes().nth()`
see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>
error: aborting due to previous error
For more information about this error, try `rustc --explain E0277`.
```
`x.py test src/test/ui` succeeded and I've also tested output manually by compiling the following code:
```rust
fn _f() {
let s = std::string::String::from("hello");
let _c = s[0];
let s = std::string::String::from("hello");
let mut _c = s[0];
let s = "hello";
let _c = s[0];
let s = "hello";
let mut _c = &s[0];
}
```
Not sure if some docs should be changed too. I will also fix error message in the [Book :: Indexing into Strings](db53e2e3cd/src/ch08-02-strings.md (indexing-into-strings)) if that PR will get approved :)
