Cannot derive Debug for struct named builder

[alanhdu]

If I try to compile:

#[derive(Debug)]
struct builder;

I get a very cryptic error message:

error[E0530]: let bindings cannot shadow unit structs

error: aborting due to previous error

This is with both Rust 1.17 and the current nightly.

Playpen link.

(Originally discovered via diesel-rs/diesel#934)

[nagisa]

This seems like a general hygiene issue, but am not sure if the built-in derives are using the new procedural macros. If they aren’t, porting may suffice. If they are, there’s a bug in our procedural macros.

cc @jseyfried?

[alanhdu]

Closed by #48934