ir: Dig into atomic types.

Fixes rust-lang#2920

Author: emilio

bindgen-tests/tests/expectations/tests/atomic-constant.rs

@@ -0,0 +1,2 @@
+_Atomic(int) a;
+int b;

bindgen-tests/tests/headers/atomic-constant.h

@@ -28,7 +28,7 @@ path = "lib.rs"
 annotate-snippets = { version = "0.9.1", features = ["color"], optional = true }
 bitflags = "2.2.1"
 cexpr = "0.6"
-clang-sys = { version = "1", features = ["clang_6_0"] }
+clang-sys = { version = "1", features = ["clang_11_0"] }
 itertools = { version = ">=0.10,<0.14", default-features = false }
 log = { version = "0.4", optional = true }
 prettyplease = { version = "0.2.7", optional = true, features = ["verbatim"] }

bindgen/Cargo.toml

@@ -1493,6 +1493,15 @@ impl Type {
         }
     }
 
+    /// For atomic types, get the underlying type.
+    pub(crate) fn atomic_value_type(&self) -> Type {
+        unsafe {
+            Type {
+                x: clang_Type_getValueType(self.x),
+            }
+        }
+    }
+
     /// Is this a valid type?
     pub(crate) fn is_valid(&self) -> bool {
         self.kind() != CXType_Invalid

bindgen/clang.rs

@@ -1167,6 +1167,18 @@ impl Type {
                     .expect("Not able to resolve array element?");
                     TypeKind::Array(inner, ty.num_elements().unwrap())
                 }
+                CXType_Atomic => {
+                    // TODO(emilio): Maybe we can preserve the "is atomic" bit somehow and generate
+                    // something more useful... But for now this is better than panicking or
+                    // generating nothing.
+                    return Self::from_clang_ty(
+                        potential_id,
+                        &ty.atomic_value_type(),
+                        location,
+                        parent_id,
+                        ctx,
+                    );
+                }
                 CXType_Elaborated => {
                     return Self::from_clang_ty(
                         potential_id,

bindgen/ir/ty.rs

@@ -320,7 +320,7 @@ impl ClangSubItemParser for Var {
                             matches!(ty.kind(), CXType_Auto | CXType_Unexposed),
                             "Couldn't resolve constant type, and it \
                              wasn't an nondeductible auto type or unexposed \
-                             type!"
+                             type: {:?}", ty
                         );
                         return Err(e);
                     }