Add tracking of packed repr, use it to highlight unsafe refs

Taking a reference to a misaligned field on a packed struct is an
unsafe operation. Highlight that behavior. Currently, the misaligned
part isn't tracked, so this highlight is a bit too aggressive.

crates/ra_hir/src/code_model.rs

@@ -4,6 +4,7 @@ use std::{iter, sync::Arc};
 use arrayvec::ArrayVec;
 use either::Either;
 use hir_def::{
+    adt::ReprKind,
     adt::StructKind,
     adt::VariantData,
     builtin_type::BuiltinType,
@@ -431,6 +432,10 @@ impl Struct {
         Type::from_def(db, self.id.lookup(db.upcast()).container.module(db.upcast()).krate, self.id)
     }
 
+    pub fn is_packed(self, db: &dyn HirDatabase) -> bool {
+        matches!(db.struct_data(self.id).repr, Some(ReprKind::Packed))
+    }
+
     fn variant_data(self, db: &dyn HirDatabase) -> Arc<VariantData> {
         db.struct_data(self.id).variant_data.clone()
     }
@@ -1253,6 +1258,19 @@ impl Type {
         )
     }
 
+    pub fn is_packed(&self, db: &dyn HirDatabase) -> bool {
+        let adt_id = match self.ty.value {
+            Ty::Apply(ApplicationTy { ctor: TypeCtor::Adt(adt_id), .. }) => adt_id,
+            _ => return false,
+        };
+
+        let adt = adt_id.into();
+        match adt {
+            Adt::Struct(s) => s.is_packed(db),
+            _ => false,
+        }
+    }
+
     pub fn is_raw_ptr(&self) -> bool {
         matches!(&self.ty.value, Ty::Apply(ApplicationTy { ctor: TypeCtor::RawPtr(..), .. }))
     }