As @chris kindly pointed out on StackOverflow,
std::mem::size_of() cannot currently be used as a constant expression.
size_of::<T>() appears to be evaluated at runtime, which artificially limits the construct in my opinion.
I went searching for historical background and only managed to dig up a mailing list thread from 2012, which punts the issue as a nice to have.
I’m unsure how to proceed at this point. Would an RFC be the logical next step? Does compile time sizeof jive with Rust philosophy?