Rollup merge of #123182 - jhpratt:fix-decodable-derive, r=davidtwco

Avoid expanding to unstable internal method

Fixes #123156

Rather than expanding to `std::rt::begin_panic`, the expansion is now to `unreachable!()`. The resulting behavior is identical. A test that previously triggered the same error as #123156 has been added to ensure it does not regress.

r? compiler

Author: jhpratt