I agree Content-Length should be mandatory. It's not even specified. Arguably and unfortunately with the standard as is, parsers shouldn't even use it if it's there, because if there's a boundary before that length, your results will differ from other parsers. (My crate expects it though.)

[Not having Content-Length:] makes writing a streaming MIME parser much harder.

I don't think that's a big problem for Rust application servers where there are nice crates for efficient text searching you can plug in. Maybe more so for folks doing low-dependency and/or embedded stuff, especially in C.

But it's just dumb IMHO when you want to send arbitrary data to have to come up with a random boundary that you hope isn't in the data you're sending. With a strong random number generator you can do this to (un)reasonable statistical confidence, but that shouldn't be necessary at all.