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Enforce safety of downcast_ref at compile time. (#1326)
* Enforce safety of `downcast_ref` at compile time.
The safety of `downcast_ref` requires that `__private_get_type_id__` not
be overriden by callers, since the returned `TypeId` is used to check if
the cast is safe. However, all trait methods in Rust are public, so
users can override `__private_get_type_id__` despite it being
`#[doc(hidden)]`.
This commit makes `__private_get_type_id__` return a type with a private
constructor, ensuring that the only possible implementation is the
default implementation. A more detailed explanation is provided in the
comments added to the file.
Note that the standard library was affected by this type of issue with
the `Error::type_id` function: see https://blog.rust-lang.org/2019/05/14/Rust-1.34.2.html#whats-in-1.34.2-stable
Co-authored-by: Yuki Okushi <[email protected]>
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