Constant MAX_LEAF 

pub const MAX_LEAF: CpuidValue<0x0, 0x0, EAX, u32>;

Leaf/Function 0x0, EAX

[intel/vol2]: Table 3-8. Information Returned by CPUID Instruction. [amd/vol3]: E.3.1, CPUID Fn0000_0000_EAX Largest Standard Function Number.