Is transmute from `&&T` to `&&mut T` sound?

Ddystopia · August 29, 2025, 1:43pm

Is transmute from &&T to &&mut T sound? Or maybe going through the pointer cast? Is there a difference for tree borrows and stacked borrows?

steffahn · August 29, 2025, 2:14pm

github.com/rust-lang/unsafe-code-guidelines

Is transmuting `&’a &’b T` to `&’a &’b mut T` sound or UB?

opened 07:36PM - 13 Jan 21 UTC

RustyYato

Crossposting from https://users.rust-lang.org/t/is-transmuting-a-b-t-to-a-b-mut-…t-sound-or-ub/54070 @steffahn writes > I just came across wondering about this (here 3) and I don’t know where to find an accurate answer. > > The question is whether this is sound > ```rust > pub fn some_casting_function<'a, 'b, T>(x: &'a &'b T) -> &'a &'b mut T { > unsafe {std::mem::transmute(x)} > } > ``` > > I think the only thing I’m uncertain about is whether such a function would be instant UB 1. > > Pros: You can’t really mutate anything through a `&&mut T` that you wouldn’t be able to mutate through a `&&T` as well (e.g. through interior mutability). And a `&&mut T` is not unique. > > Cons: There might be some kind of possibility for misoptimization that I’m missing, or just some kind of language specification saying that it isn’t allowed. Getting creative here: the compiler might assume that for `x: &&mut T and y: &&mut T` holds: `if (x as *const _ != y as *const`), then (`*x as *const T != *y as *const T`). > > Also, there could be API’s that only hand out `&T`, or non-unique `Rc<T>` or whatever (`T: !Clone`) so that you wouldn’t be able to get you hands on a value of type `&&mut T` any other way. And such a library could, potentially, for whatever reason, rely on this... > > Another Pros (counterargument to the second Cons): Libraries like take_mut do, too, allow for getting types, e.g. a `T` by value, from APIs where that might otherwise be impossible, e.g. it only hands out `&mut T` references and `T: !Clone + !Default`, etc.. > > I’d also appreciate e.g. links to previous discussions or existing libraries if you know of any.

system · November 27, 2025, 2:15pm

This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.

Topic		Replies	Views
Is transmuting `&'a &'b T` to `&'a &'b mut T` sound or UB? help	16	1479	April 17, 2021
Safe `&&T` to `&&mut T` conversion help	21	656	December 3, 2025
Mutability transmute with a lifetime help	2	367	December 24, 2020
Is This The Right Way to Transmut &mut [u8] to &mut MyType?	7	672	January 7, 2021
Casting borrows from immutable to mutable - how bad is it? help	17	1812	August 15, 2023

Is transmute from `&&T` to `&&mut T` sound?

Related topics