Apply visibility restriction to struct fields

When changing the visibility of a struct field from `pub` to `pub(crate)`, also apply the same change to any struct fields that are also `pub`.

Fixes #7.

example/Cargo.toml

@@ -8,6 +8,7 @@ publish = false
 
 [features]
 unstable-risky-function = []
+unstable-risky-struct = []
 
 [dependencies.stability]
 path = "../"

example/src/lib.rs

@@ -1,3 +1,5 @@
+#![forbid(unreachable_pub)]
+
 //! This is an example library demonstrating various attributes from the
 //! stability crate.
 
@@ -8,3 +10,11 @@
 pub fn risky_function() {
     unimplemented!()
 }
+
+/// This struct does something really risky!
+///
+/// Don't use it yet!
+#[stability::unstable(feature = "risky-struct", issue = "#102")]
+pub struct RiskyStruct {
+    pub x: u8,
+}

src/lib.rs

@@ -38,6 +38,14 @@ mod unstable;
 ///   certain crate feature is enabled. This ensures that internal code within
 ///   the crate can always use the item, but downstream consumers cannot access
 ///   it unless they opt-in to the unstable API.
+///   - Visibility of certain child items of the annotated item will also be
+///     changed to match the new item visibility, such as struct fields. Children
+///     that are not public will not be affected.
+///   - Child items of annotated modules will *not* have their visibility changed,
+///     as it might be desirable to be able to re-export them even if the module
+///     visibility is restricted. You should apply the attribute to each item
+///     within the module with the same feature name if you want to restrict the
+///     module's contents itself and not just the module namespace.
 /// - Appends an "Availability" section to the item's documentation that notes
 ///   that the item is unstable, and indicates the name of the crate feature to
 ///   enable it.
@@ -57,6 +65,8 @@ mod unstable;
 ///   this item's availability. The crate feature will have the string
 ///   `unstable-` prepended to it. If not specified, it will be guarded by a
 ///   catch-all `unstable` feature.
+/// - `issue`: Provide a link or reference to a tracking issue for the unstable
+///   feature. This will be included in the item's documentation.
 ///
 /// # Examples
 ///

src/unstable.rs

@@ -72,9 +72,9 @@ The tracking issue is: `{}`
             }
 
             let mut hidden_item = item.clone();
-            *hidden_item.visibility_mut() = parse_quote! {
+            hidden_item.set_visibility(parse_quote! {
                 pub(crate)
-            };
+            });
 
             TokenStream::from(quote! {
                 #[cfg(feature = #feature_name)]
@@ -97,7 +97,7 @@ pub(crate) trait ItemLike {
 
     fn visibility(&self) -> &Visibility;
 
-    fn visibility_mut(&mut self) -> &mut Visibility;
+    fn set_visibility(&mut self, visibility: Visibility);
 
     fn is_public(&self) -> bool {
         matches!(self.visibility(), Visibility::Public(_))
@@ -120,8 +120,8 @@ macro_rules! impl_has_visibility {
                     &self.vis
                 }
 
-                fn visibility_mut(&mut self) -> &mut Visibility {
-                    &mut self.vis
+                fn set_visibility(&mut self, visibility: Visibility) {
+                    self.vis = visibility;
                 }
             }
         )*
@@ -131,10 +131,34 @@ macro_rules! impl_has_visibility {
 impl_has_visibility!(
     syn::ItemType,
     syn::ItemEnum,
-    syn::ItemStruct,
     syn::ItemFn,
     syn::ItemMod,
     syn::ItemTrait,
     syn::ItemConst,
     syn::ItemStatic,
 );
+
+impl ItemLike for syn::ItemStruct {
+    fn attrs(&self) -> &[syn::Attribute] {
+        &self.attrs
+    }
+
+    fn push_attr(&mut self, attr: syn::Attribute) {
+        self.attrs.push(attr);
+    }
+
+    fn visibility(&self) -> &Visibility {
+        &self.vis
+    }
+
+    fn set_visibility(&mut self, visibility: Visibility) {
+        // Also constrain visibility of all fields to be at most the given
+        // item visibility.
+        self.fields
+            .iter_mut()
+            .filter(|field| matches!(&field.vis, Visibility::Public(_)))
+            .for_each(|field| field.vis = visibility.clone());
+
+        self.vis = visibility;
+    }
+}