[AIX] Ignore linting on repr(C) structs with repr(packed) or repr(align(n))

This PR updates the lint added in 9b40bd7 to ignore repr(C) structs that also
have repr(packed) or repr(align(n)).

As these representations can be modifiers on repr(C), it is assumed that users
that add these should know what they are doing, and thus the the lint should not
warn on the respective structs. For example, for the time being, using
repr(packed) and manually padding a repr(C) struct can be done to correctly
align struct members on AIX.

Author: amy-kwan

compiler/rustc_lint/src/types.rs

@@ -1638,6 +1638,9 @@ impl ImproperCTypesDefinitions {
             return true;
         } else if let Adt(adt_def, _) = ty.kind()
             && adt_def.is_struct()
+            && adt_def.repr().c()
+            && !adt_def.repr().packed()
+            && adt_def.repr().align.is_none()
         {
             let struct_variant = adt_def.variant(VariantIdx::ZERO);
             // Within a nested struct, all fields are examined to correctly
@@ -1659,8 +1662,11 @@ impl ImproperCTypesDefinitions {
         item: &'tcx hir::Item<'tcx>,
     ) {
         let adt_def = cx.tcx.adt_def(item.owner_id.to_def_id());
+        // repr(C) structs also with packed or aligned representation
+        // should be ignored.
         if adt_def.repr().c()
             && !adt_def.repr().packed()
+            && adt_def.repr().align.is_none()
             && cx.tcx.sess.target.os == "aix"
             && !adt_def.all_fields().next().is_none()
         {

tests/ui/layout/reprc-power-alignment.rs

@@ -148,5 +148,29 @@ pub struct I {
     d: f32,
     e: f64,
 }
-
+#[repr(C)]
+pub struct J {
+    a: u8,
+    b: I,
+}
+// The lint also ignores diagnosing #[repr(align(n))].
+#[repr(C, align(8))]
+pub struct K {
+    a: u8,
+    b: u8,
+    c: f64,
+    d: f32,
+    e: f64,
+}
+#[repr(C)]
+pub struct L {
+    a: u8,
+    b: K,
+}
+#[repr(C, align(8))]
+pub struct M {
+    a: u8,
+    b: K,
+    c: L,
+}
 fn main() { }