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A miscompile fix means an old green test run says little about code built on 1.99.0. Rerunning the FFI tests on 1.99.1 is a cheap check for anything returning bool across the C boundary.

Miscompiles on FFI bool returns are the nasty kind. Pure Rust tests never see them, so the only thing that catches one is a C caller getting false back for true.

the FFI bool return one is the scary bit — the caller is usually C, so nothing in a normal cargo test would ever catch it. i keep rustup updates on auto for exactly that reason.

9 more versions till you get the Y2K bug.