Calling b.b() in static_foo is direct or via vtable

[zylthinking1]

The code works, but I don't know why rustc knows T is i32.
the i32 is passed from dynamic_bar, which has lost the information.

One explain of this is b.b() is called via vtable, but if there is another call like static_foo(&7), it is absolutely called directly, and there will be two versions of static_foo::<i32>.

trait Trait {
    fn a(&self)
    where
        Self: Sized,
    {
        println!("fuck");
    }

    fn b(&self) {
        println!("fuck");
    }
}

fn static_foo<T: Trait + ?Sized>(b: &T) {
    b.b()
}

fn dynamic_bar(a: &dyn Trait) {
    static_foo(a)
}

fn main() {
    impl Trait for i32 {
        fn b(&self) where Self: Sized {
            println!("fuck2");
        }
    }
    dynamic_bar(&9);
    9.a()
}

[Cerber-Ursi]

@zylthinking1

why rustc knows T is i32.

In static_foo, it doesn't. It just passes a &dyn Trait, containing the vtable which was initially constructed when calling dynamic_bar with &i32.