pub fn reserve_exact(&mut self, additional: usize) {
        let new_cap = self.len + additional;
        if new_cap <= self.cap {
            return;
        }
        unsafe { self.realloc_exact(new_cap) };
    }
/// Reserve enough capacity to add  exactly `additional` elements without realllocating.
Reserve enough capacity to add  exactly `additional` elements without realllocating.
Reserve enough capacity to add exactly ` additional ` elements without realllocating .
pub fn reserve_exact(&mut self, additional: usize) {
        let new_cap = self.len + additional;
        if new_cap <= self.cap {
            return;
        }
        unsafe { self.realloc_exact(new_cap) };
    }
pub fn reserve_exact ( & mut self , additional : usize ) { let new_cap = self . len + additional ; if new_cap <= self . cap { return ; } unsafe { self . realloc_exact ( new_cap ) } ; }
Reserve enough capacity to add  exactly `additional` elements without realllocating.
Reserve enough capacity to add exactly ` additional ` elements without realllocating .
&mut self
additional
usize