如何在Rust中指定特征中的“where”注释？

Question

I'm trying to re-implement Vec::retain() with print statements so that I can figure out how it works, but I'm stuck on this extended type annotation where F: FnMut(&T) -> bool . I understand why it's there, but I can't figure out how to annotate it in the trait declaration so it stops throwing errors (and lets me fix the other ones in the code):

trait TestVec {
    fn retain_with_prints<F>(&mut self, mut f: F);
}

impl<T> TestVec for Vec<T> {
    fn retain_with_prints<F>(&mut self, mut f: F)
        where F: FnMut(&T) -> bool
    {
        let len = self.len();
        let mut del = 0;
        {
            let v = &mut **self;

            for i in 0..len {
                println!("on position: {}", &i);
                if !f(&v[i]) {
                    del += 1;
                    println!("incremented del to: {}", del);
                } else if del > 0 {
                    println!("swapping {} for {}", v[i - del], v[i]);
                    v.swap(i - del, i);
                }
            }
        }
        if del > 0 {
            println!("removing last {} elements of vector", del);
            self.truncate(len - del);
        }
    }
}

fn main() {
    let v = vec![0,1,2,3,4,5];
    v.retain_with_prints(|item| { item % 2 == 0 });
}

Errors:

• As is: error: the requirement `for<'r> F: std::ops::FnMut<(&'r T,)>` appears on the impl method but not on the corresponding trait method [E0276]
• Adding where clause to trait: error: type name `T` is undefined or not in scope [E0412]

The compiler doesn't seem to like it if I try to specify trait<T> either, and I can't seem to get the right thing to come up in search results.

How do I specify this?

Answer 1

You need to parameterize the trait:

trait TestVec<T> {
    fn retain_with_prints<F>(&mut self, mut f: F)
        where F: FnMut(&T) -> bool;
}

And also link the types at implementation time.

impl<T> TestVec<T> for Vec<T>

---

Beyond that, you will need to require that your T implements Display and make your variable mutable:

impl<T> TestVec<T> for Vec<T>
    where T: std::fmt::Display
{

let mut v = vec![0,1,2,3,4,5];