None of my examples with ripgrep used memory maps, so I don't know why you're bringing that up. My shim for transcoding doesn't assume the presence of a caller provided buffer, but it could and the code would be simpler but make more assumptions.
This conversation is going in circles and there is too much certainty in your comments for my taste. Our lack of shared experience is preventing us from communicating productively, and in particular, it's pretty hard for me to grok everything you're saying. I personally don't have any experience with C#'s pipeline concept, so I can't really keep up. I suspect the reverse is true as well.
Usually the thing that helps move discussions like this forward is code, but building a prototype of your ideas in Rust is probably a lot of work. So I don't know how to continue. Sorry.