ir: Put function attribute detection under an opt-in flag.

Cargo.toml

@@ -14,7 +14,7 @@ readme = "README.md"
 repository = "https://github.com/rust-lang/rust-bindgen"
 documentation = "https://docs.rs/bindgen"
 homepage = "https://rust-lang.github.io/rust-bindgen/"
-version = "0.44.0"
+version = "0.45.0"
 build = "build.rs"
 
 include = [

src/ir/function.rs

@@ -394,7 +394,9 @@ impl FunctionSig {
             }
         };
 
-        let must_use = cursor.has_simple_attr("warn_unused_result");
+        let must_use =
+            ctx.options().enable_function_attribute_detection &&
+            cursor.has_simple_attr("warn_unused_result");
         let is_method = cursor.kind() == CXCursor_CXXMethod;
         let is_constructor = cursor.kind() == CXCursor_Constructor;
         let is_destructor = cursor.kind() == CXCursor_Destructor;

src/lib.rs

@@ -418,6 +418,9 @@ impl Builder {
         if self.options.enable_cxx_namespaces {
             output_vector.push("--enable-cxx-namespaces".into());
         }
+        if self.options.enable_function_attribute_detection {
+            output_vector.push("--enable-function-attribute-detection".into());
+        }
         if self.options.disable_name_namespacing {
             output_vector.push("--disable-name-namespacing".into());
         }
@@ -1057,6 +1060,18 @@ impl Builder {
         self
     }
 
+    /// Enable detecting must_use attributes on C functions.
+    ///
+    /// This is quite slow in some cases (see #1465), so it's disabled by
+    /// default.
+    ///
+    /// Note that for this to do something meaningful for now at least, the rust
+    /// target version has to have support for `#[must_use]`.
+    pub fn enable_function_attribute_detection(mut self) -> Self {
+        self.options.enable_function_attribute_detection = true;
+        self
+    }
+
     /// Disable name auto-namespacing.
     ///
     /// By default, bindgen mangles names like `foo::bar::Baz` to look like
@@ -1391,6 +1406,10 @@ struct BindgenOptions {
     /// generated bindings.
     enable_cxx_namespaces: bool,
 
+    /// True if we should try to find unexposed attributes in functions, in
+    /// order to be able to generate #[must_use] attributes in Rust.
+    enable_function_attribute_detection: bool,
+
     /// True if we should avoid mangling names with namespaces.
     disable_name_namespacing: bool,
 
@@ -1618,6 +1637,7 @@ impl Default for BindgenOptions {
             derive_partialeq: false,
             derive_eq: false,
             enable_cxx_namespaces: false,
+            enable_function_attribute_detection: false,
             disable_name_namespacing: false,
             use_core: false,
             ctypes_prefix: None,

src/options.rs

@@ -315,6 +315,10 @@ where
                 .takes_value(true)
                 .multiple(true)
                 .number_of_values(1),
+            Arg::with_name("enable-function-attribute-detection")
+                .long("enable-function-attribute-detection")
+                .help("Enables detecting unexposed attributes in functions (slow).
+                       Used to generate #[must_use] annotations."),
         ]) // .args()
         .get_matches_from(args);
 
@@ -484,6 +488,10 @@ where
         builder = builder.enable_cxx_namespaces();
     }
 
+    if matches.is_present("enable-function-attribute-detection") {
+        builder = builder.enable_function_attribute_detection();
+    }
+
     if matches.is_present("disable-name-namespacing") {
         builder = builder.disable_name_namespacing();
     }

tests/expectations/tests/attribute_warn_unused_result_no_attribute_detection.rs

@@ -0,0 +1,41 @@
+/* automatically generated by rust-bindgen */
+
+#![allow(
+    dead_code,
+    non_snake_case,
+    non_camel_case_types,
+    non_upper_case_globals
+)]
+
+#[repr(C)]
+#[derive(Debug, Default, Copy, Clone)]
+pub struct Foo {
+    pub _address: u8,
+}
+#[test]
+fn bindgen_test_layout_Foo() {
+    assert_eq!(
+        ::std::mem::size_of::<Foo>(),
+        1usize,
+        concat!("Size of: ", stringify!(Foo))
+    );
+    assert_eq!(
+        ::std::mem::align_of::<Foo>(),
+        1usize,
+        concat!("Alignment of ", stringify!(Foo))
+    );
+}
+extern "C" {
+    #[link_name = "\u{1}_ZN3Foo3fooEi"]
+    pub fn Foo_foo(this: *mut Foo, arg1: ::std::os::raw::c_int) -> ::std::os::raw::c_int;
+}
+impl Foo {
+    #[inline]
+    pub unsafe fn foo(&mut self, arg1: ::std::os::raw::c_int) -> ::std::os::raw::c_int {
+        Foo_foo(self, arg1)
+    }
+}
+extern "C" {
+    #[link_name = "\u{1}_Z3fooi"]
+    pub fn foo(arg1: ::std::os::raw::c_int) -> ::std::os::raw::c_int;
+}

tests/headers/attribute_warn_unused_result.hpp

@@ -1,4 +1,4 @@
-// bindgen-flags: --rust-target 1.27
+// bindgen-flags: --rust-target 1.27 --enable-function-attribute-detection
 
 class Foo {
 public:

tests/headers/attribute_warn_unused_result_no_attribute_detection.hpp

@@ -0,0 +1,10 @@
+// bindgen-flags: --rust-target 1.27
+
+class Foo {
+public:
+    __attribute__((warn_unused_result))
+    int foo(int);
+};
+
+__attribute__((warn_unused_result))
+int foo(int);