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);
    }
}