pointers - How is a destructor call `fn drop(&mut self)` call inserted when the owning variable is immutable? -

it understanding when variable type implements drop goes out of scope, call fn drop(&mut self) function inserted, , passed newly-created mutable reference variable going out of scope.

however, how possible in cases variable immutably bound, , illegal borrow mutably? here's example of i'm talking about:

fn main() {     let x = vec![1, 2, 3];   let y = &mut x; } 

which produces following error: cannot borrow immutable local variable x mutable expected.

something similar must happen when x getting dropped, because drop expects mutable reference.

the owner of variable gets decide mutability when variable binding created, it's not intrinsic value itself:

fn main() {       let x = vec![1, 2, 3];     let mut z = x;     let y = &mut z; } 

you can think of dropping happening when last programmer-named variable binding gives ownership of variable. magical drop-fairy takes ownership of now-unneeded variable, , uses mutable binding. drop-fairy can call drop::drop before doing final magic free space taken item itself.

note drop-fairy not real rust concept yet. rfc still in preliminary stage.