`Reader::push` function does not abstract its unsafe interior away properly

[tbu-]

It assumes that the implementor of the Reader trait does not touch the bytes passed to it in its read function.

[tbu-]

See also #21076.

[alexcrichton]

old_io is going to be deprecated and std::io has this problem fixed, so I'm going to close this.

[tbu-]

That's good to hear!