rustdoc: Incorrect synthetic auto trait impl if constituents have applicable non-identity impl of auto trait

[fmease]

Given

pub struct Outer<T>(Inner<T>);
struct Inner<T>(T);

// non-identity impl:
impl Unpin for Inner<()> {} // or any other auto trait

rustdoc currently synthesizes:

impl<T> !Unpin for Outer<T>

However, ideally it would be:

impl Unpin for Outer<()>

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Counterexample:

pub struct Outer<T>(Inner<T, i32>, Inner<T, u32>);
struct Inner<T, D>(T, D);
impl Unpin for Inner<(), i32> {}
impl Unpin for Inner<bool, u32> {}

Here, the synthesized impl<T> !Unpin for Outer<T> is obviously correct (modulo #146571).

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Side note: This only affects non-identity impls (i.e., args differ from impl'er's def) and not conditional impls (i.e., bounds diff from impl'er's def) because the latter is forbidden (E0367).