Rollup merge of #129467 - dingxiangfei2009:smart-pointer-relax-pointee, r=compiler-errors

derive(SmartPointer): assume pointee from the single generic and better error messages

Fix #129465

Actually RFC says that `#[pointee]` can be inferred when there is no ambiguity, or there is only one generic type parameter so to say.

cc ```@Darksonn```

r? ```@compiler-errors```

Author: workingjubilee