Subgroup operations don't appear to violate the Rust safety rules. And at least one other non-uniform operation, is_helper_invocation, isn't unsafe.

It would be more convenient for developers if all subgroup operations were safe except for OpGroupNonUniformBallotFindLSB / OpGroupNonUniformBallotFindMSB (value undefined when 0, though you could probably make the argument that this is also safe, as it only produces garbage and doesn't access memory).

A read through of the SPIR-V and Vulkan specs suggests no cases where other subgroup operations will result in undefined behavior. SPIR-V also makes a fairly strong requirement about program ordering for GroupNonUniform operations:

An invocation will not execute a dynamic instance of this instruction (X') until all invocations in its scope restricted tangle have executed all dynamic instances that are program-ordered before X'.