Implement the unsafe-fields RFC.

Co-Authored-By: Jacob Pratt <jacob@jhpratt.dev>

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -15,6 +15,7 @@ use rustc_data_structures::sync::{Lock, Lrc, OnceLock};
 use rustc_data_structures::unhash::UnhashMap;
 use rustc_expand::base::{SyntaxExtension, SyntaxExtensionKind};
 use rustc_expand::proc_macro::{AttrProcMacro, BangProcMacro, DeriveProcMacro};
+use rustc_hir::Safety;
 use rustc_hir::def::Res;
 use rustc_hir::def_id::{CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc_hir::definitions::{DefPath, DefPathData};
@@ -1101,6 +1102,7 @@ impl<'a> CrateMetadataRef<'a> {
                         did,
                         name: self.item_name(did.index),
                         vis: self.get_visibility(did.index),
+                        safety: self.get_safety(did.index),
                     })
                     .collect(),
                 adt_kind,
@@ -1162,6 +1164,10 @@ impl<'a> CrateMetadataRef<'a> {
             .map_id(|index| self.local_def_id(index))
     }
 
+    fn get_safety(self, id: DefIndex) -> Safety {
+        self.root.tables.safety.get(self, id).unwrap_or_else(|| self.missing("safety", id))
+    }
+
     fn get_trait_item_def_id(self, id: DefIndex) -> Option<DefId> {
         self.root.tables.trait_item_def_id.get(self, id).map(|d| d.decode_from_cdata(self))
     }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1599,6 +1599,10 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 f.did.index
             }));
 
+            for field in &variant.fields {
+                self.tables.safety.set_some(field.did.index, field.safety);
+            }
+
             if let Some((CtorKind::Fn, ctor_def_id)) = variant.ctor {
                 let fn_sig = tcx.fn_sig(ctor_def_id);
                 // FIXME only encode signature for ctor_def_id

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -411,6 +411,7 @@ define_tables! {
     associated_item_or_field_def_ids: Table<DefIndex, LazyArray<DefIndex>>,
     def_kind: Table<DefIndex, DefKind>,
     visibility: Table<DefIndex, LazyValue<ty::Visibility<DefIndex>>>,
+    safety: Table<DefIndex, hir::Safety>,
     def_span: Table<DefIndex, LazyValue<Span>>,
     def_ident_span: Table<DefIndex, LazyValue<Span>>,
     lookup_stability: Table<DefIndex, LazyValue<attr::Stability>>,

compiler/rustc_metadata/src/rmeta/table.rs

@@ -198,6 +198,13 @@ fixed_size_enum! {
     }
 }
 
+fixed_size_enum! {
+    hir::Safety {
+        ( Unsafe )
+        ( Safe   )
+    }
+}
+
 fixed_size_enum! {
     ty::Asyncness {
         ( Yes )