rust-notes

let _ = .. is not a binding

When you are writing

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let _ = some_function();
// returning value is dropped here.

The return value is dropped immediately, because you cannot gave a value to _ as well as you cannot read it. _ is not a variable so this statement is not a variable binding.

When using RAII in rust, don’t forget this, use _{ident} to denote the return value. That thing is a variable so it follows correct scoping rules.

impl trait vs. dyn trait

We can write these two functions:

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fn print_this(s: impl ToString) {
    println!("{}", s.to_string());
}

fn print_this_dyn(s: &dyn ToString) {
    println!("{}", s.to_string())
}

impl ToString is static dispatch. The compiler specialize the function in compile-time, like a C++ template function.

dyn ToString is a dynamic dispatch, requiring a trait object which has a virtual function table. Here I use a reference because I didn’t require s binding by Size.