fix(clap_derive): Unwrap syn::TypeGroup when checking field types

Due to macro expansions, a `syn` type may be wrapped in multiple
'layers' of `syn::Type::Group`. However, `clap_derive` currently does
not check for `syn::Type::Group`, which will cause an `Option` (along
with other matched types) to fail to be detected when it results from a
macro expansion.

This commit 'unwraps' outer type groups before checking the
user-provided types against well-known types. Currently, these groups
may not be present due to a rustc bug (rust-lang/rust#43081)

However, once rust-lang/rust#73084 is merged,
these groups will be present in more cases. This commit makes `clap`
compatible with both older and newer versions of rustc.

clap_derive/src/utils/ty.rs

@@ -44,7 +44,10 @@ pub fn sub_type(ty: &syn::Type) -> Option<&syn::Type> {
     subty_if(ty, |_| true)
 }
 
-fn only_last_segment(ty: &syn::Type) -> Option<&PathSegment> {
+fn only_last_segment(mut ty: &syn::Type) -> Option<&PathSegment> {
+    while let syn::Type::Group(syn::TypeGroup { elem, .. }) = ty {
+        ty = elem;
+    }
     match ty {
         Type::Path(TypePath {
             qself: None,

clap_derive/tests/nested.rs

@@ -0,0 +1,33 @@
+// Copyright 2018 Guillaume Pinot (@TeXitoi) <[email protected]>,
+// Kevin Knapp (@kbknapp) <[email protected]>, and
+// Andrew Hobden (@hoverbear) <[email protected]>
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+//
+// This work was derived from Structopt (https://github.com/TeXitoi/structopt)
+// commit#ea76fa1b1b273e65e3b0b1046643715b49bec51f which is licensed under the
+// MIT/Apache 2.0 license.
+
+use clap::Clap;
+
+// Tests that clap_derive properly detects an `Option` field
+// that results from a macro expansion
+#[test]
+fn use_option() {
+    macro_rules! expand_ty {
+        ($name:ident: $ty:ty) => {
+            #[derive(Clap)]
+            struct Outer {
+                #[clap(short, long)]
+                #[allow(dead_code)]
+                $name: $ty,
+            }
+        };
+    }
+
+    expand_ty!(my_field: Option<String>);
+}