People who've migrated existing C/C++ codebases to Rust, how much problem did you get from APIs not being clear about ownership and storing pointers to things you didn't expect them to and causing memory unsafety and how did you find them?

Ownership is generally not terrible, modern C++ is relatively explicit about it. Hidden caches are the worst though, both lame const_cast/mutable members to cache stuff, and global caches in functions deep in the call graph that assume all callers come from the same thread.

I remember when I asked @davidbaron and @really_bz about "hey y'all are grizzled c++ hackers, what issues did you have with rust" and to my surprise none of their issues had anything to do with ownership/borrowing, because, "this is how we think about stuff anyway, just explicit"