does_not_work.rs

struct X<T>(T);
impl<T> X<T> {
    pub fn new(t: T) -> Self {
        Self(t)
    }
    pub fn map<U>(&self, mut f: impl FnMut(&T) -> U + 'static) -> X<U> {
        X(f(&self.0))
    }
}

pub fn main() {
    let x = X::new(42);
    let y = x.map(|x| (*x, ""));
    let z = y.map(|(a, _b)| *a);
}

works.rs

struct X<T>(T);
impl<T> X<T> {
    pub fn new(t: T) -> Self {
        Self(t)
    }
    pub fn map<U>(self, mut f: impl FnMut(T) -> U + 'static) -> X<U> {
        X(f(self.0))
    }
}

pub fn main() {
    let x = X::new(42);
    let y = x.map(|x| (x, ""));
    let z = y.map(|(a, _b)| a);
}