Unitialized memory

Under what conditions is it valid to use any of these?

• let x: T = std::mem::uninitialized(); on the stack
• Box::new(std::mem::uninitialized())
• The part of Vec between .len() and .capacity()
• The memory pointed to by alloc::heap::allocate() without first writing to it
• (Possibly many other similar cases…)

Let’s assume !Drop (or that we use std::mem::forget and are being very careful about panic-safety), and types (like u32) for which all bit patterns are valid.

Reading uninitialized memory is Bad and should be avoided, but what’s the worst that could happen? Undefined values might fine in many cases. Or is it Undefined Behavior of the “the optimizer is allowed to eat your lunch and elide half your program” kind?

As a concrete example, consider reallocate which when copying reads from the source pointer. That data is not necessarily entirely initialized: Vec::with_capacity(10).reserve(100)

[eddyb]

@SimonSapin Instead of XOR one could also use x != x or x == x.

[RalfJung]

The story of mem::uninitialized has progressed quite a lot in the last years (and that function has been deprecated and declared to be basically impossible to use correctly), so I will close this.