Suggest using single quotes when confusing single character strings and chars

[xTibor]

Consider the following example:

fn main() {
    // mismatched types expected `char`, found `&str` rustc(E0308)
    let my_character: char = "s";
}

The compiler could be more friendly in this case, having a diagnostic for suggesting using single quotes when people confuse single character strings with char literals. Something like this:

note: character literals are delimited by single quotes 
--> main.rs:3:4
  |
3 | let my_character: char = 's';
  |                          ---
  |                          |
  |                          use single quotes here 

This could help new Rust devs coming from languages where there are no distinction between these things. (Python, Javascript, etc.)

[chordtoll]

@rustbot claim