Better error note on unimplemented Index trait for string

[folex]

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:

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:

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 if that PR will get approved :)

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @cramertj (or someone else) soon.

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[varkor]

r? @estebank

[estebank]

@bors r+ rollup

[bors]

📌 Commit c120199 has been approved by estebank