In rust-lang/rfcs#3535 (comment) it was noticed that it is possible to match a #[non_exhaustive] struct exhaustively outside the originating crate,
if constants for all posible states are accessible or definable in the other crate.

As a result adding fields such that the struct has new states is a breaking change, as the existing constants won't be able to cover all states anymore.
Which makes #[non_exhaustive] ineffective.

For Example:

// bar/src/lib.rs
#[non_exhaustive]
#[derive(PartialEq, Eq)]
pub struct Bar(bool);
pub const TRUE: Bar = Bar(true);
pub const FALSE: Bar = Bar(false);

// foo/src/main.rs
fn main() {
    match bar::TRUE {
        bar::TRUE => {}
        bar::FALSE => {}
    }
}

Changing the bar crate to the following is then a breaking change:

// bar/src/lib.rs
#[non_exhaustive]
#[derive(PartialEq, Eq)]
pub struct Bar(bool, bool);
pub const TRUE: Bar = Bar(true, true);
pub const FALSE: Bar = Bar(false, false);

This breaks the foo crate, eventhough Bar was marked #[non_exhaustive] to allow expanding.

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rustc +stable --version
rustc 1.74.1 (a28077b28 2023-12-04)
rustc +beta --version
rustc 1.76.0-beta.1 (0e09125c6 2023-12-21)
rustc +nightly --version
rustc 1.77.0-nightly (d6d7a9386 2023-12-22)