Fixes#50595.
This bug currently affects stable. Why I think we can go for hard error:
- It will in stable for at most one cycle and there is no legitimate
reason to abuse it, nor any known uses in the wild.
- It only affects `bin` crates (which have a `main`), so there is
little practical difference between a hard error or a deny lint, both
are a one line fix.
The fix was to just unshadow a variable. Thanks @nikomatsakis for the
mentoring!
r? @nikomatsakis
