Draft implementation of the unsafe-fields RFC.

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

compiler/rustc_ast/src/ast.rs

@@ -3028,6 +3028,7 @@ pub struct FieldDef {
     pub id: NodeId,
     pub span: Span,
     pub vis: Visibility,
+    pub safety: Safety,
     pub ident: Option<Ident>,
 
     pub ty: P<Ty>,

compiler/rustc_ast/src/mut_visit.rs

@@ -1044,10 +1044,11 @@ pub fn walk_flat_map_field_def<T: MutVisitor>(
     visitor: &mut T,
     mut fd: FieldDef,
 ) -> SmallVec<[FieldDef; 1]> {
-    let FieldDef { span, ident, vis, id, ty, attrs, is_placeholder: _ } = &mut fd;
+    let FieldDef { span, ident, vis, id, ty, attrs, is_placeholder: _, safety } = &mut fd;
     visitor.visit_id(id);
     visit_attrs(visitor, attrs);
     visitor.visit_vis(vis);
+    visit_safety(visitor, safety);
     visit_opt(ident, |ident| visitor.visit_ident(ident));
     visitor.visit_ty(ty);
     visitor.visit_span(span);

compiler/rustc_ast/src/visit.rs

@@ -929,7 +929,7 @@ pub fn walk_struct_def<'a, V: Visitor<'a>>(
 }
 
 pub fn walk_field_def<'a, V: Visitor<'a>>(visitor: &mut V, field: &'a FieldDef) -> V::Result {
-    let FieldDef { attrs, id: _, span: _, vis, ident, ty, is_placeholder: _ } = field;
+    let FieldDef { attrs, id: _, span: _, vis, ident, ty, is_placeholder: _, safety: _ } = field;
     walk_list!(visitor, visit_attribute, attrs);
     try_visit!(visitor.visit_vis(vis));
     visit_opt!(visitor, visit_ident, ident);

compiler/rustc_ast_lowering/src/item.rs

@@ -724,6 +724,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             },
             vis_span: self.lower_span(f.vis.span),
             ty,
+            safety: self.lower_safety(f.safety, hir::Safety::Safe),
         }
     }
 

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -547,6 +547,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(global_registration, "global registration is experimental");
     gate_all!(return_type_notation, "return type notation is experimental");
     gate_all!(pin_ergonomics, "pinned reference syntax is experimental");
+    gate_all!(unsafe_fields, "`unsafe` fields are experimental");
 
     if !visitor.features.never_patterns() {
         if let Some(spans) = spans.get(&sym::never_patterns) {

compiler/rustc_expand/src/placeholders.rs

@@ -2,7 +2,7 @@ use rustc_ast::mut_visit::*;
 use rustc_ast::ptr::P;
 use rustc_ast::token::Delimiter;
 use rustc_ast::visit::AssocCtxt;
-use rustc_ast::{self as ast};
+use rustc_ast::{self as ast, Safety};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_span::DUMMY_SP;
 use rustc_span::symbol::Ident;
@@ -173,6 +173,7 @@ pub(crate) fn placeholder(
             ty: ty(),
             vis,
             is_placeholder: true,
+            safety: Safety::Default,
         }]),
         AstFragmentKind::Variants => AstFragment::Variants(smallvec![ast::Variant {
             attrs: Default::default(),

compiler/rustc_feature/src/unstable.rs

@@ -628,6 +628,8 @@ declare_features! (
     /// Allows creation of instances of a struct by moving fields that have
     /// not changed from prior instances of the same struct (RFC #2528)
     (unstable, type_changing_struct_update, "1.58.0", Some(86555)),
+    /// Allows declaring fields `unsafe`.
+    (unstable, unsafe_fields, "CURRENT_RUSTC_VERSION", Some(1234567)), // FIXME: proper tracking issue.
     /// Allows const generic parameters to be defined with types that
     /// are not `Sized`, e.g. `fn foo<const N: [u8]>() {`.
     (incomplete, unsized_const_params, "1.82.0", Some(95174)),

compiler/rustc_hir/src/hir.rs

@@ -3192,6 +3192,7 @@ pub struct FieldDef<'hir> {
     pub hir_id: HirId,
     pub def_id: LocalDefId,
     pub ty: &'hir Ty<'hir>,
+    pub safety: Safety,
 }
 
 impl FieldDef<'_> {

compiler/rustc_hir_analysis/src/collect.rs

@@ -1031,6 +1031,7 @@ fn lower_variant(
             did: f.def_id.to_def_id(),
             name: f.ident.name,
             vis: tcx.visibility(f.def_id),
+            safety: f.safety,
         })
         .collect();
     let recovered = match def {

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

@@ -1597,6 +1597,11 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 f.did.index
             }));
 
+            for field in &variant.fields {
+                // FIXME: this probably blows up rmeta size.
+                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

@@ -413,6 +413,7 @@ 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 )

compiler/rustc_middle/src/ty/mod.rs

@@ -1365,6 +1365,7 @@ pub struct FieldDef {
     pub did: DefId,
     pub name: Symbol,
     pub vis: Visibility<DefId>,
+    pub safety: hir::Safety,
 }
 
 impl PartialEq for FieldDef {
@@ -1377,15 +1378,16 @@ impl PartialEq for FieldDef {
         // of `FieldDef` changes, a compile-error will be produced, reminding
         // us to revisit this assumption.
 
-        let Self { did: lhs_did, name: _, vis: _ } = &self;
+        let Self { did: lhs_did, name: _, vis: _, safety: _ } = &self;
 
-        let Self { did: rhs_did, name: _, vis: _ } = other;
+        let Self { did: rhs_did, name: _, vis: _, safety: _ } = other;
 
         let res = lhs_did == rhs_did;
 
         // Double check that implicit assumption detailed above.
         if cfg!(debug_assertions) && res {
-            let deep = self.name == other.name && self.vis == other.vis;
+            let deep =
+                self.name == other.name && self.vis == other.vis && self.safety == other.safety;
             assert!(deep, "FieldDef for the same def-id has differing data");
         }
 
@@ -1405,7 +1407,7 @@ impl Hash for FieldDef {
         // of `FieldDef` changes, a compile-error will be produced, reminding
         // us to revisit this assumption.
 
-        let Self { did, name: _, vis: _ } = &self;
+        let Self { did, name: _, vis: _, safety: _ } = &self;
 
         did.hash(s)
     }

compiler/rustc_middle/src/ty/parameterized.rs

@@ -86,6 +86,7 @@ trivially_parameterized_over_tcx! {
     rustc_attr::Stability,
     rustc_hir::Constness,
     rustc_hir::Defaultness,
+    rustc_hir::Safety,
     rustc_hir::CoroutineKind,
     rustc_hir::IsAsync,
     rustc_hir::LangItem,

compiler/rustc_mir_build/messages.ftl

@@ -125,6 +125,16 @@ mir_build_initializing_type_with_requires_unsafe_unsafe_op_in_unsafe_fn_allowed
     .note = initializing a layout restricted type's field with a value outside the valid range is undefined behavior
     .label = initializing type with `rustc_layout_scalar_valid_range` attr
 
+mir_build_initializing_type_with_unsafe_field_requires_unsafe =
+    initializing type with an unsafe field is unsafe and requires unsafe block
+    .note = unsafe fields may carry library invariants
+    .label = initialization of struct with unsafe field
+
+mir_build_initializing_type_with_unsafe_field_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
+    initializing type with an unsafe field is unsafe and requires unsafe block
+    .note = unsafe fields may carry library invariants
+    .label = initialization of struct with unsafe field
+
 mir_build_inline_assembly_requires_unsafe =
     use of inline assembly is unsafe and requires unsafe block
     .note = inline assembly is entirely unchecked and can cause undefined behavior
@@ -340,6 +350,16 @@ mir_build_unreachable_pattern = unreachable pattern
     .unreachable_covered_by_many = multiple earlier patterns match some of the same values
     .suggestion = remove the match arm
 
+mir_build_unsafe_field_requires_unsafe =
+    use of unsafe field is unsafe and requires unsafe block
+    .note = unsafe fields may carry library invariants
+    .label = use of unsafe field
+
+mir_build_unsafe_field_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
+    use of unsafe field is unsafe and requires unsafe block
+    .note = unsafe fields may carry library invariants
+    .label = use of unsafe field
+
 mir_build_unsafe_fn_safe_body = an unsafe function restricts its caller, but its body is safe by default
 mir_build_unsafe_not_inherited = items do not inherit unsafety from separate enclosing items
 
@@ -388,6 +408,11 @@ mir_build_unsafe_op_in_unsafe_fn_initializing_type_with_requires_unsafe =
     .note = initializing a layout restricted type's field with a value outside the valid range is undefined behavior
     .label = initializing type with `rustc_layout_scalar_valid_range` attr
 
+mir_build_unsafe_op_in_unsafe_fn_initializing_type_with_unsafe_field_requires_unsafe =
+    initializing type with an unsafe field is unsafe and requires unsafe block
+    .note = unsafe fields may carry library invariants
+    .label = initialization of struct with unsafe field
+
 mir_build_unsafe_op_in_unsafe_fn_inline_assembly_requires_unsafe =
     use of inline assembly is unsafe and requires unsafe block
     .note = inline assembly is entirely unchecked and can cause undefined behavior
@@ -408,6 +433,11 @@ mir_build_unsafe_op_in_unsafe_fn_union_field_requires_unsafe =
     .note = the field may not be properly initialized: using uninitialized data will cause undefined behavior
     .label = access to union field
 
+mir_build_unsafe_op_in_unsafe_fn_unsafe_field_requires_unsafe =
+    use of unsafe field is unsafe and requires unsafe block
+    .note = unsafe fields may carry library invariants
+    .label = use of unsafe field
+
 mir_build_unsized_pattern = cannot use unsized non-slice type `{$non_sm_ty}` in constant patterns
 
 mir_build_unused_unsafe = unnecessary `unsafe` block