Move FnWithoutBody error to ast_lowering.

[m-ou-se]

This moves the "missing function body" error from ast_passes to ast_lowering. This doesn't change much, except that it allows for builtin macro attributes to accept functions without a body.

We need this for externally implementable items:

// in core:
#[externally_implementable]
fn panic_handler();

Before this change, this would result in an error even before the (builtin) attribute macro gets expanded.

[petrochenkov]

Before this change, this would result in an error even before the (builtin) attribute macro gets expanded.

Are you sure?
ast_passes should also run after all the macro expansion.
This snippet already compiles successfully on stable.

#[cfg(FALSE)]
fn panic_handler();

[m-ou-se]

Hmm, this change might have been a left-over from when my experimental implementation used a builtin attribute rather than a builtin macro. 🤔

Thanks for your quick response. I'll close this for now.