Implicit coercion rules in Rust

main.rs

// A normal function that accepts a `&str`

fn a(_: &str) {}

// A normal function that accepts a *trait* that is implemented by `&str`

trait StringNeeded {}
impl<'a> StringNeeded for &'a str {}
fn b<X: StringNeeded>(_: X) {}

// A struct that stores a `&str`

#[allow(dead_code)]
struct C<'a> {
    x: &'a str,
}

fn main() {
    let x = "Hello".to_owned();
    
    a(&x); // `&String` is auto-coerced to `&str`
    
    //b(&x); // It errors; auto-coercion does not happen
    b(&*x); // You should manually coerce `&String` to `&str`
    
    C {
        x: &x,
    }; // Auto-coercion happens
}