Look for must_use on typdefs in function return

Closes rust-lang#2206

src/codegen/mod.rs

@@ -3965,11 +3965,35 @@ impl CodeGenerator for Function {
 
         let mut attributes = vec![];
 
-        if signature.must_use() &&
-            ctx.options().rust_features().must_use_function
-        {
-            attributes.push(attributes::must_use());
+        if ctx.options().rust_features().must_use_function {
+            let must_use = signature.must_use() || {
+                let ret_ty = signature.return_type();
+
+                let resolved_ret = ret_ty
+                    .into_resolver()
+                    .through_type_refs()
+                    .through_type_aliases()
+                    .resolve(ctx);
+
+                let must_use_resolved_ty =
+                    resolved_ret.annotations().must_use_type() ||
+                        ctx.must_use_type_by_name(resolved_ret);
+
+                let ret = ctx.resolve_item(ret_ty);
+                let must_use_ty = ret.annotations().must_use_type() ||
+                    ctx.must_use_type_by_name(ret);
+
+                // If the return type already has #[must_use], the function does not
+                // need the annotation. This preserves the codegen behavior before
+                // type aliases with #[must_use] were supported.
+                !must_use_resolved_ty && must_use_ty
+            };
+
+            if must_use {
+                attributes.push(attributes::must_use());
+            }
         }
+
         if let Some(comment) = item.comment(ctx) {
             attributes.push(attributes::doc(comment));
         }

src/ir/context.rs

@@ -1921,7 +1921,8 @@ If you encounter an error missing from this list, please file an issue or a PR!"
         let item = Item::new(
             with_id,
             None,
-            None,
+            self.resolve_item_fallible(wrapped_id)
+                .map(|item| item.annotations().clone()),
             parent_id.unwrap_or_else(|| self.current_module.into()),
             ItemKind::Type(ty),
             Some(location),

tests/expectations/tests/func_return_must_use.rs

@@ -0,0 +1,76 @@
+#![allow(
+    dead_code,
+    non_snake_case,
+    non_camel_case_types,
+    non_upper_case_globals
+)]
+
+pub type MustUseInt = ::std::os::raw::c_int;
+extern "C" {
+    #[must_use]
+    pub fn return_int() -> MustUseInt;
+}
+#[repr(C)]
+#[derive(Debug, Copy, Clone)]
+#[must_use]
+pub struct MustUseStruct {
+    _unused: [u8; 0],
+}
+extern "C" {
+    pub fn return_struct() -> MustUseStruct;
+}
+/// <div rustbindgen mustusetype></div>
+pub type AnnotatedInt = ::std::os::raw::c_int;
+extern "C" {
+    #[must_use]
+    pub fn return_annotated_int() -> AnnotatedInt;
+}
+extern "C" {
+    pub fn return_plain_int() -> ::std::os::raw::c_int;
+}
+/// <div rustbindgen mustusetype></div>
+#[repr(C)]
+#[derive(Debug, Default, Copy, Clone)]
+#[must_use]
+pub struct AnnotatedStruct {}
+#[test]
+fn bindgen_test_layout_AnnotatedStruct() {
+    assert_eq!(
+        ::std::mem::size_of::<AnnotatedStruct>(),
+        0usize,
+        concat!("Size of: ", stringify!(AnnotatedStruct))
+    );
+    assert_eq!(
+        ::std::mem::align_of::<AnnotatedStruct>(),
+        1usize,
+        concat!("Alignment of ", stringify!(AnnotatedStruct))
+    );
+}
+extern "C" {
+    pub fn return_annotated_struct() -> AnnotatedStruct;
+}
+#[repr(C)]
+#[derive(Debug, Default, Copy, Clone)]
+pub struct PlainStruct {}
+#[test]
+fn bindgen_test_layout_PlainStruct() {
+    assert_eq!(
+        ::std::mem::size_of::<PlainStruct>(),
+        0usize,
+        concat!("Size of: ", stringify!(PlainStruct))
+    );
+    assert_eq!(
+        ::std::mem::align_of::<PlainStruct>(),
+        1usize,
+        concat!("Alignment of ", stringify!(PlainStruct))
+    );
+}
+/// <div rustbindgen mustusetype></div>
+pub type TypedefPlainStruct = PlainStruct;
+extern "C" {
+    pub fn return_plain_struct() -> PlainStruct;
+}
+extern "C" {
+    #[must_use]
+    pub fn return_typedef_struct() -> TypedefPlainStruct;
+}

tests/headers/func_return_must_use.h

@@ -0,0 +1,36 @@
+// bindgen-flags: --must-use-type 'MustUse.*'
+
+typedef int MustUseInt;
+
+MustUseInt return_int();
+
+struct MustUseStruct;
+
+struct MustUseStruct return_struct();
+
+/**
+ * <div rustbindgen mustusetype></div>
+ */
+typedef int AnnotatedInt;
+
+AnnotatedInt return_annotated_int();
+
+int return_plain_int();
+
+/**
+ * <div rustbindgen mustusetype></div>
+ */
+struct AnnotatedStruct {};
+
+struct AnnotatedStruct return_annotated_struct();
+
+struct PlainStruct {};
+
+/**
+ * <div rustbindgen mustusetype></div>
+ */
+typedef struct PlainStruct TypedefPlainStruct;
+
+struct PlainStruct return_plain_struct();
+
+TypedefPlainStruct return_typedef_struct();