Sometimes a type definition is simply not translatable to Rust, for example it uses C++'s SFINAE for which Rust has no equivalent. In these cases, it is best to treat all occurrences of the type as an opaque blob of bytes with a size and alignment. bindgen will attempt to detect such cases and do this automatically, but other times it needs some explicit help from you.
--opaque-type <type>/// <div rustbindgen opaque></div> class Foo { // ... };