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@ -751,8 +751,17 @@ extern "C" {
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```
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By including a private field and no constructor,
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we create an opaque type that we can’t instantiate outside of this module.
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An empty array is both zero-size and compatible with `#[repr(C)]`.
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we create an opaque type that we can't instantiate outside of this module.
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(A struct with no field could be instantiated by anyone.)
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We also want to use this type in FFI, so we have to add `#[repr(C)]`.
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And to avoid warning around using `()` in FFI, we instead use an empty array,
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which works just as well as an empty type but is FFI-compatible.
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But because our `Foo` and `Bar` types are
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different, we’ll get type safety between the two of them, so we cannot
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accidentally pass a pointer to `Foo` to `bar()`.
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Notice that it is a really bad idea to use an empty enum as FFI type.
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The compiler relies on empty enums being uninhabited, so handling values of type
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`&Empty` is a huge footgun and can lead to buggy program behavior (by triggering
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undefined behavior).
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