It's an open question whether creating a reference to an uninitialized value is instant UB, or only UB if that reference is misused (e.g. if copy_to_slice reads an uninitialized byte). The specific discussion is whether the language requires "recursive validity for references", which would mean constructing a reference to an invalid value is "language UB" (your program is not well specified and the compiler is allowed to "miscompile" it) rather than "library UB" (your program is well-specified, but functions you call might not expect an uninitialized buffer and trigger language UB). See the discussion here: https://github.com/rust-lang/unsafe-code-guidelines/issues/3...
Currently, the team is leaning in the direction of not requiring recursive validity for references. This would mean your code is not language UB as long as you can assume `set_len` and `copy_to_slice` never read from 'data`. However, it's still considered library UB, as this assumption is not documented or specified anywhere and is not guaranteed -- changes to safe code in your program or in the standard library can turn this into language UB, so by doing something like this you're writing fragile code that gives up a lot of Rust's safety by design.
Currently, the team is leaning in the direction of not requiring recursive validity for references. This would mean your code is not language UB as long as you can assume `set_len` and `copy_to_slice` never read from 'data`. However, it's still considered library UB, as this assumption is not documented or specified anywhere and is not guaranteed -- changes to safe code in your program or in the standard library can turn this into language UB, so by doing something like this you're writing fragile code that gives up a lot of Rust's safety by design.