Borrow checker and &mut borrows for generic params

Question

Why can I not borrow &mut x multiple times when using is_mut_t. But it's fine with is_mut_specific ?

#[cfg(test)]
mod test {
    use std::marker::PhantomData;

    #[test]
    fn test_wc() {
        struct Test<T>{ phantom_data: PhantomData<T>}
        impl <T> Test<T> {
            fn new() -> Self {
                Test { phantom_data: PhantomData }
            }
            fn is_mut_t(&mut self, i: T) {}
            fn is_mut_specific(&mut self, i: &mut i32) {}
        }

        let mut x = 5;
        let mut t: Test<&mut i32> = Test::new();
        
        t.is_mut_specific(&mut x);
        t.is_mut_specific(&mut x);
        // t.is_mut_t(&mut x);
        // t.is_mut_t(&mut x);
    }
}

Answer 1

Let's give names for lifetimes (using imaginary syntax):

#[cfg(test)]
mod test {
    use std::marker::PhantomData;

    #[test]
    fn test_wc() {
        struct Test<T> { phantom_data: PhantomData<T> }
        impl<T> Test<T> {
            fn new() -> Self {
                Test { phantom_data: PhantomData }
            }
            fn is_mut_t<'a>(&'a mut self, i: T) {}
            fn is_mut_specific<'b, 'c>(&'b mut self, i: &'c mut i32) {}
        }

        let mut x = 5;
        let mut t: Test<&'d mut i32> = Test::new();
        
        (&'e mut t).is_mut_specific::<'e, 'f>(&'f mut x);
        (&'g mut t).is_mut_specific::<'g, 'h>(&'h mut x);
        // (&'i mut t).is_mut_t::<'i>(&'j mut x);
        // (&'k mut t).is_mut_t::<'k>(&'l mut x);
    }
}

Each occurrence of T is replaced by &'d mut i32. Can you spot the problem?

When we say &mut i32 in the signature, that gets assigned a generic lifetime that can be any.

But when we say T, that must be exactly the same as T. And that includes the same lifetime. But if we borrow x for 'd, we cannot borrow it again in 'd, which means at least as long as t exists.