1 files changed, 63 insertions, 0 deletions
diff --git a/compiler/rustc_const_eval/src/util/alignment.rs b/compiler/rustc_const_eval/src/util/alignment.rs
new file mode 100644
index 000000000..4f39dad20
--- /dev/null
+++ b/compiler/rustc_const_eval/src/util/alignment.rs
@@ -0,0 +1,63 @@
+use rustc_middle::mir::*;
+use rustc_middle::ty::{self, TyCtxt};
+use rustc_target::abi::Align;
+
+/// Returns `true` if this place is allowed to be less aligned
+/// than its containing struct (because it is within a packed
+/// struct).
+pub fn is_disaligned<'tcx, L>(
+    tcx: TyCtxt<'tcx>,
+    local_decls: &L,
+    param_env: ty::ParamEnv<'tcx>,
+    place: Place<'tcx>,
+) -> bool
+where
+    L: HasLocalDecls<'tcx>,
+{
+    debug!("is_disaligned({:?})", place);
+    let Some(pack) = is_within_packed(tcx, local_decls, place) else {
+        debug!("is_disaligned({:?}) - not within packed", place);
+        return false;
+    };
+
+    let ty = place.ty(local_decls, tcx).ty;
+    match tcx.layout_of(param_env.and(ty)) {
+        Ok(layout) if layout.align.abi <= pack => {
+            // If the packed alignment is greater or equal to the field alignment, the type won't be
+            // further disaligned.
+            debug!(
+                "is_disaligned({:?}) - align = {}, packed = {}; not disaligned",
+                place,
+                layout.align.abi.bytes(),
+                pack.bytes()
+            );
+            false
+        }
+        _ => {
+            debug!("is_disaligned({:?}) - true", place);
+            true
+        }
+    }
+}
+
+fn is_within_packed<'tcx, L>(
+    tcx: TyCtxt<'tcx>,
+    local_decls: &L,
+    place: Place<'tcx>,
+) -> Option<Align>
+where
+    L: HasLocalDecls<'tcx>,
+{
+    place
+        .iter_projections()
+        .rev()
+        // Stop at `Deref`; standard ABI alignment applies there.
+        .take_while(|(_base, elem)| !matches!(elem, ProjectionElem::Deref))
+        // Consider the packed alignments at play here...
+        .filter_map(|(base, _elem)| {
+            base.ty(local_decls, tcx).ty.ty_adt_def().and_then(|adt| adt.repr().pack)
+        })
+        // ... and compute their minimum.
+        // The overall smallest alignment is what matters.
+        .min()
+}