Auto merge of #106934 - DrMeepster:offset_of, r=WaffleLapkin

Add offset_of! macro (RFC 3308)

Implements https://github.com/rust-lang/rfcs/pull/3308 (tracking issue #106655) by adding the built in macro `core::mem::offset_of`. Two of the future possibilities are also implemented:

* Nested field accesses (without array indexing)
* DST support (for `Sized` fields)

I wrote this a few months ago, before the RFC merged. Now that it's merged, I decided to rebase and finish it.

cc `@thomcc` (RFC author)

compiler/rustc_middle/src/mir/mod.rs

@@ -2041,7 +2041,11 @@ impl<'tcx> Debug for Rvalue<'tcx> {
             }
             UnaryOp(ref op, ref a) => write!(fmt, "{:?}({:?})", op, a),
             Discriminant(ref place) => write!(fmt, "discriminant({:?})", place),
-            NullaryOp(ref op, ref t) => write!(fmt, "{:?}({:?})", op, t),
+            NullaryOp(ref op, ref t) => match op {
+                NullOp::SizeOf => write!(fmt, "SizeOf({:?})", t),
+                NullOp::AlignOf => write!(fmt, "AlignOf({:?})", t),
+                NullOp::OffsetOf(fields) => write!(fmt, "OffsetOf({:?}, {:?})", t, fields),
+            },
             ThreadLocalRef(did) => ty::tls::with(|tcx| {
                 let muta = tcx.static_mutability(did).unwrap().prefix_str();
                 write!(fmt, "&/*tls*/ {}{}", muta, tcx.def_path_str(did))

compiler/rustc_middle/src/mir/syntax.rs

@@ -1115,7 +1115,7 @@ pub enum Rvalue<'tcx> {
     CheckedBinaryOp(BinOp, Box<(Operand<'tcx>, Operand<'tcx>)>),
 
     /// Computes a value as described by the operation.
-    NullaryOp(NullOp, Ty<'tcx>),
+    NullaryOp(NullOp<'tcx>, Ty<'tcx>),
 
     /// Exactly like `BinaryOp`, but less operands.
     ///
@@ -1211,12 +1211,14 @@ pub enum AggregateKind<'tcx> {
     Generator(DefId, SubstsRef<'tcx>, hir::Movability),
 }
 
-#[derive(Copy, Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
-pub enum NullOp {
+#[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub enum NullOp<'tcx> {
     /// Returns the size of a value of that type
     SizeOf,
     /// Returns the minimum alignment of a type
     AlignOf,
+    /// Returns the offset of a field
+    OffsetOf(&'tcx List<FieldIdx>),
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]

compiler/rustc_middle/src/mir/tcx.rs

@@ -188,7 +188,9 @@ impl<'tcx> Rvalue<'tcx> {
             }
             Rvalue::UnaryOp(UnOp::Not | UnOp::Neg, ref operand) => operand.ty(local_decls, tcx),
             Rvalue::Discriminant(ref place) => place.ty(local_decls, tcx).ty.discriminant_ty(tcx),
-            Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf, _) => tcx.types.usize,
+            Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf | NullOp::OffsetOf(..), _) => {
+                tcx.types.usize
+            }
             Rvalue::Aggregate(ref ak, ref ops) => match **ak {
                 AggregateKind::Array(ty) => tcx.mk_array(ty, ops.len() as u64),
                 AggregateKind::Tuple => {

compiler/rustc_middle/src/mir/type_foldable.rs

@@ -16,7 +16,6 @@ TrivialTypeTraversalAndLiftImpls! {
     UserTypeAnnotationIndex,
     BorrowKind,
     CastKind,
-    NullOp,
     hir::Movability,
     BasicBlock,
     SwitchTargets,
@@ -26,6 +25,7 @@ TrivialTypeTraversalAndLiftImpls! {
 
 TrivialTypeTraversalImpls! {
     ConstValue<'tcx>,
+    NullOp<'tcx>,
 }
 
 impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx [InlineAsmTemplatePiece] {

compiler/rustc_middle/src/thir.rs

@@ -20,7 +20,7 @@ use rustc_middle::mir::interpret::AllocId;
 use rustc_middle::mir::{self, BinOp, BorrowKind, FakeReadCause, Mutability, UnOp};
 use rustc_middle::ty::adjustment::PointerCast;
 use rustc_middle::ty::subst::SubstsRef;
-use rustc_middle::ty::{self, AdtDef, FnSig, Ty, UpvarSubsts};
+use rustc_middle::ty::{self, AdtDef, FnSig, List, Ty, UpvarSubsts};
 use rustc_middle::ty::{CanonicalUserType, CanonicalUserTypeAnnotation};
 use rustc_span::def_id::LocalDefId;
 use rustc_span::{sym, Span, Symbol, DUMMY_SP};
@@ -481,6 +481,11 @@ pub enum ExprKind<'tcx> {
     },
     /// Inline assembly, i.e. `asm!()`.
     InlineAsm(Box<InlineAsmExpr<'tcx>>),
+    /// Field offset (`offset_of!`)
+    OffsetOf {
+        container: Ty<'tcx>,
+        fields: &'tcx List<FieldIdx>,
+    },
     /// An expression taking a reference to a thread local.
     ThreadLocalRef(DefId),
     /// A `yield` expression.

compiler/rustc_middle/src/thir/visit.rs

@@ -160,6 +160,7 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
                 }
             }
         }
+        OffsetOf { container: _, fields: _ } => {}
         ThreadLocalRef(_) => {}
         Yield { value } => visitor.visit_expr(&visitor.thir()[value]),
     }

compiler/rustc_middle/src/ty/codec.rs

@@ -19,6 +19,7 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_middle::ty::TyCtxt;
 use rustc_serialize::{Decodable, Encodable};
 use rustc_span::Span;
+use rustc_target::abi::FieldIdx;
 pub use rustc_type_ir::{TyDecoder, TyEncoder};
 use std::hash::Hash;
 use std::intrinsics;
@@ -401,6 +402,15 @@ impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for ty::List<ty
     }
 }
 
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for ty::List<FieldIdx> {
+    fn decode(decoder: &mut D) -> &'tcx Self {
+        let len = decoder.read_usize();
+        decoder
+            .interner()
+            .mk_fields_from_iter((0..len).map::<FieldIdx, _>(|_| Decodable::decode(decoder)))
+    }
+}
+
 impl_decodable_via_ref! {
     &'tcx ty::TypeckResults<'tcx>,
     &'tcx ty::List<Ty<'tcx>>,
@@ -412,6 +422,7 @@ impl_decodable_via_ref! {
     &'tcx mir::coverage::CodeRegion,
     &'tcx ty::List<ty::BoundVariableKind>,
     &'tcx ty::List<ty::Predicate<'tcx>>,
+    &'tcx ty::List<FieldIdx>,
 }
 
 #[macro_export]

compiler/rustc_middle/src/ty/context.rs

@@ -155,6 +155,7 @@ pub struct CtxtInterners<'tcx> {
     layout: InternedSet<'tcx, LayoutS>,
     adt_def: InternedSet<'tcx, AdtDefData>,
     external_constraints: InternedSet<'tcx, ExternalConstraintsData<'tcx>>,
+    fields: InternedSet<'tcx, List<FieldIdx>>,
 }
 
 impl<'tcx> CtxtInterners<'tcx> {
@@ -178,6 +179,7 @@ impl<'tcx> CtxtInterners<'tcx> {
             layout: Default::default(),
             adt_def: Default::default(),
             external_constraints: Default::default(),
+            fields: Default::default(),
         }
     }
 
@@ -1571,6 +1573,7 @@ slice_interners!(
     projs: pub mk_projs(ProjectionKind),
     place_elems: pub mk_place_elems(PlaceElem<'tcx>),
     bound_variable_kinds: pub mk_bound_variable_kinds(ty::BoundVariableKind),
+    fields: pub mk_fields(FieldIdx),
 );
 
 impl<'tcx> TyCtxt<'tcx> {
@@ -2239,6 +2242,14 @@ impl<'tcx> TyCtxt<'tcx> {
         T::collect_and_apply(iter, |xs| self.mk_place_elems(xs))
     }
 
+    pub fn mk_fields_from_iter<I, T>(self, iter: I) -> T::Output
+    where
+        I: Iterator<Item = T>,
+        T: CollectAndApply<FieldIdx, &'tcx List<FieldIdx>>,
+    {
+        T::collect_and_apply(iter, |xs| self.mk_fields(xs))
+    }
+
     pub fn mk_substs_trait(
         self,
         self_ty: Ty<'tcx>,

compiler/rustc_middle/src/ty/typeck_results.rs

@@ -208,6 +208,9 @@ pub struct TypeckResults<'tcx> {
     /// Contains the data for evaluating the effect of feature `capture_disjoint_fields`
     /// on closure size.
     pub closure_size_eval: FxHashMap<LocalDefId, ClosureSizeProfileData<'tcx>>,
+
+    /// Container types and field indices of `offset_of!` expressions
+    offset_of_data: ItemLocalMap<(Ty<'tcx>, Vec<FieldIdx>)>,
 }
 
 /// Whenever a value may be live across a generator yield, the type of that value winds up in the
@@ -280,6 +283,7 @@ impl<'tcx> TypeckResults<'tcx> {
             generator_interior_predicates: Default::default(),
             treat_byte_string_as_slice: Default::default(),
             closure_size_eval: Default::default(),
+            offset_of_data: Default::default(),
         }
     }
 
@@ -530,6 +534,14 @@ impl<'tcx> TypeckResults<'tcx> {
     pub fn coercion_casts(&self) -> &ItemLocalSet {
         &self.coercion_casts
     }
+
+    pub fn offset_of_data(&self) -> LocalTableInContext<'_, (Ty<'tcx>, Vec<FieldIdx>)> {
+        LocalTableInContext { hir_owner: self.hir_owner, data: &self.offset_of_data }
+    }
+
+    pub fn offset_of_data_mut(&mut self) -> LocalTableInContextMut<'_, (Ty<'tcx>, Vec<FieldIdx>)> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.offset_of_data }
+    }
 }
 
 /// Validate that the given HirId (respectively its `local_id` part) can be