Just by virtue of Rust being relatively short-lived I would guess that your code base is modular enough to live inside reasonable context limits, and written following mostly standard practice.
One of the main files I work on is ~40k lines of code, and one of the main proprietary API headers I consume is ~40k lines of code.
My attempts at getting the models available to Copilot to author functions for me have often failed spectacularly - as in I can't even get it to generate edits at prescribed places in the source code, follow examples from prescribed places. And the hallucination issue is EXTREME when trying to use the big C API I alluded to.
That said Claude Code (which I don't have access to at work) has been pretty impressive (although not what I would call "magical") on personal C++ projects. I don't have Opus, though.
It expands match blocks against highly complex enums from different crates, then tab completes test cases after I write the first one. Sometimes even before that.