Allow to generate associated constants in the body of C++ structs.

Opt-in since it uses a C++17 feature, but this allow exactly the same usage in
C++ and Rust, which I think is nice.

This also fixes constants of transparent structs in general, since the initial
version of this patch broke a test (associated constants in enums), so I added a
test for that too.

tests/rust/associated_in_body.rs

@@ -0,0 +1,22 @@
+bitflags! {
+    /// Constants shared by multiple CSS Box Alignment properties
+    ///
+    /// These constants match Gecko's `NS_STYLE_ALIGN_*` constants.
+    #[derive(MallocSizeOf, ToComputedValue)]
+    #[repr(C)]
+    pub struct AlignFlags: u8 {
+        /// 'auto'
+        const AUTO = 0;
+        /// 'normal'
+        const NORMAL = 1;
+        /// 'start'
+        const START = 1 << 1;
+        /// 'end'
+        const END = 1 << 2;
+        /// 'flex-start'
+        const FLEX_START = 1 << 3;
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn root(flags: AlignFlags) {}

tests/rust/associated_in_body.toml

@@ -0,0 +1,5 @@
+[struct]
+associated_constants_in_body = true
+
+[export]
+prefix = "Style" # Just ensuring they play well together :)

tests/rust/bitflags.rs

@@ -17,3 +17,6 @@ bitflags! {
         const FLEX_START = 1 << 3;
     }
 }
+
+#[no_mangle]
+pub extern "C" fn root(flags: AlignFlags) {}

tests/rust/const_transparent.rs

@@ -0,0 +1,4 @@
+#[repr(transparent)]
+struct Transparent { field: u8 }
+
+const FOO: Transparent = Transparent { field: 0 };

tests/rust/derive-eq/Cargo.lock

@@ -1,3 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 [[package]]
 name = "derive-eq"
 version = "0.1.0"

tests/rust/expand/Cargo.lock

@@ -1,3 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 [[package]]
 name = "expand"
 version = "0.1.0"

tests/rust/expand_default_features/Cargo.lock

@@ -1,3 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 [[package]]
 name = "expand"
 version = "0.1.0"

tests/rust/expand_features/Cargo.lock

@@ -1,3 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 [[package]]
 name = "expand"
 version = "0.1.0"

tests/rust/expand_no_default_features/Cargo.lock

@@ -1,3 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 [[package]]
 name = "expand"
 version = "0.1.0"

tests/rust/mod_attr/Cargo.lock

@@ -1,3 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 [[package]]
 name = "mod_attr"
 version = "0.1.0"

tests/rust/mod_path/Cargo.lock

@@ -1,3 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 [[package]]
 name = "mod_path"
 version = "0.1.0"

tests/rust/rename-crate/Cargo.lock

@@ -1,3 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 [[package]]
 name = "dependency"
 version = "0.1.0"

tests/rust/workspace/Cargo.lock

@@ -1,3 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 [[package]]
 name = "child"
 version = "0.1.0"