suppress internal error

[rchaser53]

related: #4068

I think #4068 code is invalid, but rustfmt should not cause an internal error. So I send this PR.

[calebcartwright]

I think this should still be unreachable!, since the other variants would be Closure or an Fn with None for the block.

Suggested change

	_ => return,
	_ => unreachable!(),

I think the actual issue is down in visit_item where I forgot to handle the potential None condition for the block while scrambling to update the rustc-ap crates to v642 to fix the broken nightly toolstate 😬

rustfmt/rustfmt-core/rustfmt-lib/src/visitor.rs

Lines 505 to 525 in 9124dd8

	ast::ItemKind::Fn(ref fn_signature, ref generics, ref body) => {
	let inner_attrs = inner_attributes(&item.attrs);
	let fn_ctxt = match fn_signature.header.ext {
	ast::Extern::None => visit::FnCtxt::Free,
	_ => visit::FnCtxt::Foreign,
	};
	self.visit_fn(
	visit::FnKind::Fn(
	fn_ctxt,
	item.ident,
	&fn_signature,
	&item.vis,
	body.as_deref(),
	),
	generics,
	&fn_signature.decl,
	item.span,
	ast::Defaultness::Final,
	Some(&inner_attrs),
	)
	}

Should be:

                ast::ItemKind::Fn(ref fn_signature, ref generics, Some(ref body)) => {
                    let inner_attrs = inner_attributes(&item.attrs);
                    let fn_ctxt = match fn_signature.header.ext {
                        ast::Extern::None => visit::FnCtxt::Free,
                        _ => visit::FnCtxt::Foreign,
                    };
                    self.visit_fn(
                        visit::FnKind::Fn(
                            fn_ctxt,
                            item.ident,
                            &fn_signature,
                            &item.vis,
                            Some(body),
                        ),
                        generics,
                        &fn_signature.decl,
                        item.span,
                        ast::Defaultness::Final,
                        Some(&inner_attrs),
                    )
                }
                ast::ItemKind::Fn(ref fn_signature, ref generics, None) => {
                    let indent = self.block_indent;
                    let rewrite = self.rewrite_required_fn(
                        indent,
                        item.ident,
                        &fn_signature,
                        generics,
                        item.span,
                    );
                    self.push_rewrite(item.span, rewrite);
                }

[rchaser53]

Thank you for the review. I fixed it.

[topecongiro]

Thank you!