Traits override native functions?

So... not sure exactly what I'm asking here... but... isn't this kinda funky?

Result of the below is "nope - 42 is nothing!"

trait Foo {
    fn is_some(&self) -> bool;
    fn unwrap(self) -> u32;

impl Foo for Option<u32> {
    fn is_some(&self) -> bool {
    fn unwrap(self) -> u32 {

fn handle_foo<T: Foo>(foo:T) {
    match foo.is_some() {
        true => println!("yep - it's something!"),
        false => println!("nope - {} is nothing!", foo.unwrap()),

fn main() {

And it gets weirder: Rust Playground

Is there a good writeup somewhere about building intuition for which functions a trait sees, esp if it has conflicts with the native functions on the struct?

If the concrete type is known (or inferred), i.e. Option<i32> then methods on that type will be preferred. If not, like in a generic function, you can only use methods from trait.

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If I am doing something like this, I think it would be a good idea to differentiate by provide a prefix/suffix to make it clearer which version is being called, yeah?

Yes, and you can also use the UFCS to show that you are calling the trait method.

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If you want a lower-level view, the Guide to Rustc Development has this section:

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Note that the trait's versions of unwrap and is_some will only be called if your Foo trait is in scope. If this were in a different module, you would need to import Foo to get this behavior, otherwise the intrinsic implementations will be used.


Good point!

Seems like all the more reason to add prefixes/suffixes - since that wouldn't be caught at compile-time...

Rust doesn't have classes with inheritance, so overriding a function is not possible.

handle_foo is generic over T, which must implement Foo, so Foo::is_some is called instead of Option::is_some. But if you write Some(42u32).is_some(), then Option::is_some is called.

If you also had a trait Bar : Foo with the same methods and implementations, and changed the signature to fn handle_bar<T: Bar>(foo: T), then Rust would complain:

error[E0034]: multiple applicable items in scope

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