Do not consider uninhabited 1-ZST "trivial" for the purpose of repr(transparent)?

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Currently, repr(transparent) ignores all 1-ZST, including uninhabited types. Together with the ABI promise we are making for repr(transparent), that means we cannot in any way exploit the uninhabitedness of a type for the purpose of the function call ABI -- which has led to https://github.com/rust-lang/rust/issues/135802; we had to actually pessimize the ABI for functions returning things like (i32, !) to account for this. (Cc @zachs18)

@rust-lang/lang is that a deliberate feature of repr(transparent)? It seems more like an accident to me.

However, it is an accident that is non-trivial to fix, since it turns out hyper contains a type like this:

#[repr(transparent)]
struct Neutered<B> {
    _inner: B,
    impossible: Impossible,
}

enum Impossible {}

Under a more reasonable set of rules for repr(transparent) that does not just ignore uninhabited 1-ZST, this type would be rejected. @seanmonstar @nox is there any particular reason why this type is repr(transparent)? It is still uninhabited, making it insta-UB to create an element, even by transmutation from B.

A crater run has been queued in https://github.com/rust-lang/rust/pull/147589.

Start by reviewing the repr(transparent) ABI promise and the interaction with uninhabited 1-ZST types described in this issue. Inspect hyper's src/proto/h2/mod.rs Neutered type, then read issue #135802 and the crater run in PR #147589. Done means establishing whether the behavior is deliberate and determining compatible rules or changes for affected code.