It required a little bit of messing with optimisation settings and library generation in Rust, but they emit very very similar x86-64 assembly:

https://godbolt.org/z/89W4srz4d

▲

steveklabnik 5 hours ago | parent [-]

Nice, thank you for picking up after my laziness. Surely only a few bytes different in the binary, and much, much smaller of a delta than the source.

▲

aw1621107 3 hours ago | parent [-]

You can further reduce the difference by passing Expr by pointer in the C version. At that point I think the only difference in the assembly is the order in which the cases are handed.

	▲	steveklabnik 2 hours ago \| parent [-]
		Ah yeah, honestly both should probably be passed by pointer anyway. But that makes me wonder about the actual differences here and why... maybe something fun to dig into.