Type Alias Bool 

pub type Bool = u8;

Mirrors C Bool, which pounce.h — like Ipopt 3.14’s IpStdCInterface.h — declares as the C99 bool. That is one byte, so this is u8 and not c_int.

It was c_int until gh#624, i.e. four bytes on the Rust side against one on every C caller’s, in both directions:

• a callback returning false sets only AL, and the x86-64 psABI leaves the rest of EAX unspecified — read as an i32, a failed evaluation could come back nonzero, which reads as success. The solver would then accept a point it was told it could not evaluate instead of cutting the step. gcc and clang emit movzbl, which is why this stayed latent rather than exploding;
• an array of them would have been a hard stride bug, 1-byte elements read at 4-byte spacing. That is why IpoptSetNonlinearVariables takes a count plus an index list rather than the Bool mask gh#624 originally proposed.

u8 rather than Rust’s bool on purpose: the two have identical layout, but bool carries a validity invariant (it must hold 0 or 1), and a C caller reaching this boundary with anything else — an older header where Bool was int, a hand-rolled binding, a value that came through a memcpy — would be instant undefined behaviour. u8 accepts whatever arrives and tests it the way C does, which is the same reason the entry points validate rather than trust.