Always force repr(align) attributes for stuff with alignment >= 16

To work-around some cases of rust-lang/rust#54341.

Other cases where u128 and u64 are mixed in fields might not behave correctly,
but the field offset assertions would catch them.

Fixes rust-lang#1370

Author: emilio

src/codegen/struct_layout.rs

@@ -287,18 +287,23 @@ impl<'a> StructLayoutTracker<'a> {
     }
 
     pub fn requires_explicit_align(&self, layout: Layout) -> bool {
+        let repr_align = self.ctx.options().rust_features().repr_align;
+
+        // Always force explicit repr(align) for stuff more than 16-byte aligned
+        // to work-around https://github.com/rust-lang/rust/issues/54341.
+        //
+        // Worst-case this just generates redundant alignment attributes.
+        if repr_align && self.max_field_align >= 16 {
+            return true;
+        }
+
         if self.max_field_align >= layout.align {
             return false;
         }
-        // At this point we require explicit alignment, but we may not be able
-        // to generate the right bits, let's double check.
-        if self.ctx.options().rust_features().repr_align {
-            return true;
-        }
 
         // We can only generate up-to a word of alignment unless we support
         // repr(align).
-        layout.align <= self.ctx.target_pointer_size()
+        repr_align || layout.align <= self.ctx.target_pointer_size()
     }
 
     fn padding_bytes(&self, layout: Layout) -> usize {

tests/expectations/tests/i128.rs

@@ -8,6 +8,7 @@
 )]
 
 #[repr(C)]
+#[repr(align(16))]
 #[derive(Debug, Default, Copy, Clone)]
 pub struct foo {
     pub my_signed: i128,

tests/expectations/tests/long_double.rs

@@ -8,6 +8,7 @@
 )]
 
 #[repr(C)]
+#[repr(align(16))]
 #[derive(Debug, Default, Copy, Clone)]
 pub struct foo {
     pub bar: u128,