How does rust pass or bind to a mutable reference

[quinedot]

I don't know of a succinct set of rules and it's not that well documented. However, from the reference:

• Implicit borrows
• Coercions

And as for the example:

    // Reborrows can happen as a form of coercion at coercion sites
    // A binding with an type annotation is a coercion site:
    let sss: &mut String;
    sss = mut_ref_s; //reborrow from mut_ref_s
    // Function parameters annotated to be `&mut _` reborrow too
    dummy(mut_ref_s); // moving newly reborrowed tmp from mut_ref_s
    // Bindings without explicit types do not coerce or reborrow
    let ss = mut_ref_s; // moving mut_ref_s;