What does `impl … for` mean?

Question

I see the following pattern coming up a lot in Rust codebases, but I can't find an explanation why this is used.

For what end does one use the impl... for construction?

Pseudocode:

impl Handler {
    pub fn method1() -> () {
    }
}

impl Dummy for Handler {
    pub fn method2() -> () {
    }
}

Answer 1

impl Struct... adds some methods to Struct . These methods aren't available to other types or traits.

impl Trait for Struct.. implements the trait Trait for the struct Struct . This results in the methods of the trait being available for Struct .

So, even though these two syntaxes look similar, they do 2 completely different things. impl Struct... adds new (not previously defined) methods to the type, while the other adds previously defined methods (from the trait) to the type.