Auto merge of #1008 - AndrewGaspar:underscore-identifier, r=fitzgen

Translate _ as __

This change treats _ as a reserved identifier to resolve the bug reported in #631.

I have one concern - if the header has both an `_` and `__` identifier in the global namespace, this will cause a conflict. However, it seems like we already don't handle that case for `keyword_` (e.g. `abstract_`, `alignof_`, etc.) so it doesn't seem like we need a solution specifically for `__` in this change.

Fixes #631.

src/ir/context.rs

@@ -680,7 +680,8 @@ impl BindgenContext {
  	            "where" |
                 "while" |
  	            "yield" |
-                "bool" => true,
+                "bool" |
+                "_" => true,
                 _ => false,
             }
         {

tests/expectations/tests/underscore.rs

@@ -0,0 +1,40 @@
+/* automatically generated by rust-bindgen */
+
+
+#![allow(dead_code, non_snake_case, non_camel_case_types, non_upper_case_globals)]
+
+
+pub const __: ::std::os::raw::c_int = 10;
+#[repr(C)]
+#[derive(Debug, Default, Copy)]
+pub struct ptr_t {
+    pub __: [::std::os::raw::c_uchar; 8usize],
+}
+#[test]
+fn bindgen_test_layout_ptr_t() {
+    assert_eq!(
+        ::std::mem::size_of::<ptr_t>(),
+        8usize,
+        concat!("Size of: ", stringify!(ptr_t))
+    );
+    assert_eq!(
+        ::std::mem::align_of::<ptr_t>(),
+        1usize,
+        concat!("Alignment of ", stringify!(ptr_t))
+    );
+    assert_eq!(
+        unsafe { &(*(0 as *const ptr_t)).__ as *const _ as usize },
+        0usize,
+        concat!(
+            "Alignment of field: ",
+            stringify!(ptr_t),
+            "::",
+            stringify!(__)
+        )
+    );
+}
+impl Clone for ptr_t {
+    fn clone(&self) -> Self {
+        *self
+    }
+}

tests/headers/underscore.hpp

@@ -0,0 +1,3 @@
+const int _ = 10;
+
+typedef struct { unsigned char _[8]; } ptr_t;