Diagnostics: suggest adding lifetime to trait object stored in struct

[meithecatte]

This code doesn't compile:

pub struct Thing {
    chars: Box<dyn Iterator<Item = char>>,
}

impl Thing {
    pub fn new(string: &str) -> Self {
        Thing {
            chars: Box::new(string.chars()),
        }
    }

    fn next_char(&mut self) -> Option<char> {
        self.chars.next()
    }
}

(Playground)

Fixing it involves adding a lifetime bound with dyn Iterator<Item = char> + 'a, but this syntax is not something a beginner would know. It would be nice if rustc suggested it.

[estebank]

Triage: no change