improper_ctypes lint unaffected by lint attributes on the foreign items themselves.

[eddyb]

// The attribute around the whole FFI block works.
#[allow(improper_ctypes)]
extern {
    // But this one on an individual FFI item doesn't.
    #[allow(improper_ctypes)]
    fn exit(_: &String);
}

The problem arises because the lint iterates over the children of ForeignMod (extern {...}):

rust/src/librustc_lint/types.rs

Lines 788 to 792 in 2ddc0cb

	fn check_item(&mut self, cx: &LateContext, it: &hir::Item) {
	let mut vis = ImproperCTypesVisitor { cx: cx };
	if let hir::ItemKind::ForeignMod(ref nmod) = it.node {
	if nmod.abi != Abi::RustIntrinsic && nmod.abi != Abi::PlatformIntrinsic {
	for ni in &nmod.items {

and it does so through check_item, but only check_foreign_item would be affected by attributes on the individual foreign fn / static / type children of the extern {...} block.
check_foreign_item could be used instead, with the small change that the abi has to be obtained from cx.tcx.hir.get_foreign_abi(ni.id), instead of nmod's field (as nmod would be gone).

cc @nikomatsakis @Manishearth

[vpashkov]

I would like to take this on if nobody else is working on that.