rust - Borrowing from Weak<T> -

i feel rc::weak use (sort of) asref trait implementation. i'm trying borrow shared content weak pointer, won't compile:

use std::rc::weak;  struct thing<t>(weak<t>);   impl<t> thing<t> {   fn as_ref(&self) -> option<&t> {     self.0.upgrade().map(|rc| {       rc.as_ref()     })   }    // clarity, without confusing closure   fn unwrapped_as_ref(&self) -> &t {     self.0.upgrade().unwrap().as_ref()   } } 

i understand why: upgraded rc not survive as_ref call. seems me sound. possible magic trick using unsafe compile:

impl<t> thing<t> {   fn unwrapped_as_ref<'a>(&'a self) -> &'a t {     let rc = self.0.upgrade().unwrap();     unsafe {       std::mem::transmute(rc.as_ref())     }   } } 

so:

• are there downsides solution? sound? can think of simpler alternative?
• would make sense implement as_ref(&self) -> option<&t> in standard library?

you can’t borrow weak reference, can’t. it’s weak, not guarantee underlying object exists (that’s why upgrade() returns option). , if lucky , value still alive @ point accessed through weak reference (upgrade() returned some), can freed next moment, upgraded reference goes out of scope.

in order reference underlying value need something keep alive (e.g. strong reference), means you’ll have return along reference.