cleanup op_to_const a bit; rename ConstValue::ByRef → Indirect

Author: RalfJung

compiler/rustc_codegen_cranelift/src/constant.rs

@@ -116,7 +116,7 @@ pub(crate) fn codegen_const_value<'tcx>(
     }
 
     match const_val {
-        ConstValue::ZeroSized => unreachable!(), // we already handles ZST above
+        ConstValue::ZeroSized => unreachable!(), // we already handled ZST above
         ConstValue::Scalar(x) => match x {
             Scalar::Int(int) => {
                 if fx.clif_type(layout.ty).is_some() {
@@ -200,7 +200,7 @@ pub(crate) fn codegen_const_value<'tcx>(
                 CValue::by_val(val, layout)
             }
         },
-        ConstValue::ByRef { alloc_id, offset } => {
+        ConstValue::Indirect { alloc_id, offset } => {
             let alloc = fx.tcx.global_alloc(alloc_id).unwrap_memory();
             // FIXME: avoid creating multiple allocations for the same AllocId?
             CValue::by_ref(

compiler/rustc_codegen_cranelift/src/intrinsics/simd.rs

@@ -172,7 +172,7 @@ pub(super) fn codegen_simd_intrinsic_call<'tcx>(
                     .expect("simd_shuffle idx not const");
 
                 let idx_bytes = match idx_const {
-                    ConstValue::ByRef { alloc_id, offset } => {
+                    ConstValue::Indirect { alloc_id, offset } => {
                         let alloc = fx.tcx.global_alloc(alloc_id).unwrap_memory();
                         let size = Size::from_bytes(
                             4 * ret_lane_count, /* size_of([u32; ret_lane_count]) */

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -116,7 +116,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                 let b_llval = bx.const_usize((end - start) as u64);
                 OperandValue::Pair(a_llval, b_llval)
             }
-            ConstValue::ByRef { alloc_id, offset } => {
+            ConstValue::Indirect { alloc_id, offset } => {
                 let alloc = bx.tcx().global_alloc(alloc_id).unwrap_memory();
                 // FIXME: should we attempt to avoid building the same AllocId multiple times?
                 return Self::from_const_alloc(bx, layout, alloc, offset);

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -112,13 +112,13 @@ pub(super) fn op_to_const<'tcx>(
     ecx: &CompileTimeEvalContext<'_, 'tcx>,
     op: &OpTy<'tcx>,
 ) -> ConstValue<'tcx> {
+    // Handle ZST consistently and early.
+    if op.layout.is_zst() {
+        return ConstValue::ZeroSized;
+    }
+
     // We do not have value optimizations for everything.
     // Only scalars and slices, since they are very common.
-    // Note that further down we turn scalars of uninitialized bits back to `ByRef`. These can result
-    // from scalar unions that are initialized with one of their zero sized variants. We could
-    // instead allow `ConstValue::Scalar` to store `ScalarMaybeUninit`, but that would affect all
-    // the usual cases of extracting e.g. a `usize`, without there being a real use case for the
-    // `Undef` situation.
     let try_as_immediate = match op.layout.abi {
         Abi::Scalar(abi::Scalar::Initialized { .. }) => true,
         Abi::ScalarPair(..) => match op.layout.ty.kind() {
@@ -134,7 +134,7 @@ pub(super) fn op_to_const<'tcx>(
     let immediate = if try_as_immediate {
         Right(ecx.read_immediate(op).expect("normalization works on validated constants"))
     } else {
-        // It is guaranteed that any non-slice scalar pair is actually ByRef here.
+        // It is guaranteed that any non-slice scalar pair is actually `Indirect` here.
         // When we come back from raw const eval, we are always by-ref. The only way our op here is
         // by-val is if we are in destructure_mir_constant, i.e., if this is (a field of) something that we
         // "tried to make immediate" before. We wouldn't do that for non-slice scalar pairs or
@@ -144,28 +144,15 @@ pub(super) fn op_to_const<'tcx>(
 
     debug!(?immediate);
 
-    // We know `offset` is relative to the allocation, so we can use `into_parts`.
-    let to_const_value = |mplace: &MPlaceTy<'_>| {
-        debug!("to_const_value(mplace: {:?})", mplace);
-        match mplace.ptr().into_parts() {
-            (Some(alloc_id), offset) => ConstValue::ByRef { alloc_id, offset },
-            (None, offset) => {
-                assert!(mplace.layout.is_zst());
-                assert_eq!(
-                    offset.bytes() % mplace.layout.align.abi.bytes(),
-                    0,
-                    "this MPlaceTy must come from a validated constant, thus we can assume the \
-                alignment is correct",
-                );
-                ConstValue::ZeroSized
-            }
-        }
-    };
     match immediate {
-        Left(ref mplace) => to_const_value(mplace),
+        Left(ref mplace) => {
+            // We know `offset` is relative to the allocation, so we can use `into_parts`.
+            let (alloc_id, offset) = mplace.ptr().into_parts();
+            let alloc_id = alloc_id.expect("cannot have `fake` place fot non-ZST type");
+            ConstValue::Indirect { alloc_id, offset }
+        }
         // see comment on `let try_as_immediate` above
         Right(imm) => match *imm {
-            _ if imm.layout.is_zst() => ConstValue::ZeroSized,
             Immediate::Scalar(x) => ConstValue::Scalar(x),
             Immediate::ScalarPair(a, b) => {
                 debug!("ScalarPair(a: {:?}, b: {:?})", a, b);
@@ -186,7 +173,7 @@ pub(super) fn op_to_const<'tcx>(
                 let len: usize = len.try_into().unwrap();
                 ConstValue::Slice { data, start, end: start + len }
             }
-            Immediate::Uninit => to_const_value(&op.assert_mem_place()),
+            Immediate::Uninit => bug!("`Uninit` is not a valid value for {}", op.layout.ty),
         },
     }
 }

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -756,7 +756,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         };
         let layout = from_known_layout(self.tcx, self.param_env, layout, || self.layout_of(ty))?;
         let op = match val_val {
-            ConstValue::ByRef { alloc_id, offset } => {
+            ConstValue::Indirect { alloc_id, offset } => {
                 // We rely on mutability being set correctly in that allocation to prevent writes
                 // where none should happen.
                 let ptr = self.global_base_pointer(Pointer::new(alloc_id, offset))?;

compiler/rustc_middle/src/mir/interpret/value.rs

@@ -30,19 +30,21 @@ pub struct ConstAlloc<'tcx> {
 #[derive(Copy, Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable, Hash)]
 #[derive(HashStable, Lift)]
 pub enum ConstValue<'tcx> {
-    /// Used only for types with `layout::abi::Scalar` ABI.
+    /// Used for types with `layout::abi::Scalar` ABI.
     ///
     /// Not using the enum `Value` to encode that this must not be `Uninit`.
     Scalar(Scalar),
 
-    /// Only used for ZSTs.
+    /// Only for ZSTs.
     ZeroSized,
 
-    /// Used only for `&[u8]` and `&str`
+    /// Used for `&[u8]` and `&str`.
+    ///
+    /// This is worth the optimization since Rust has literals of that type.
     Slice { data: ConstAllocation<'tcx>, start: usize, end: usize },
 
-    /// A value not represented/representable by `Scalar` or `Slice`
-    ByRef {
+    /// A value not representable by the other variants; needs to be stored in-memory.
+    Indirect {
         /// The backing memory of the value. May contain more memory than needed for just the value
         /// if this points into some other larger ConstValue.
         ///
@@ -62,7 +64,7 @@ impl<'tcx> ConstValue<'tcx> {
     #[inline]
     pub fn try_to_scalar(&self) -> Option<Scalar<AllocId>> {
         match *self {
-            ConstValue::ByRef { .. } | ConstValue::Slice { .. } | ConstValue::ZeroSized => None,
+            ConstValue::Indirect { .. } | ConstValue::Slice { .. } | ConstValue::ZeroSized => None,
             ConstValue::Scalar(val) => Some(val),
         }
     }

compiler/rustc_middle/src/mir/mod.rs

@@ -2914,7 +2914,7 @@ fn pretty_print_const_value<'tcx>(
                     _ => {}
                 }
             }
-            (ConstValue::ByRef { alloc_id, offset }, ty::Array(t, n)) if *t == u8_type => {
+            (ConstValue::Indirect { alloc_id, offset }, ty::Array(t, n)) if *t == u8_type => {
                 let n = n.try_to_target_usize(tcx).unwrap();
                 let alloc = tcx.global_alloc(alloc_id).unwrap_memory();
                 // cast is ok because we already checked for pointer size (32 or 64 bit) above

compiler/rustc_middle/src/mir/pretty.rs

@@ -460,7 +460,7 @@ impl<'tcx> Visitor<'tcx> for ExtraComments<'tcx> {
                 ConstValue::ZeroSized => "<ZST>".to_string(),
                 ConstValue::Scalar(s) => format!("Scalar({s:?})"),
                 ConstValue::Slice { .. } => "Slice(..)".to_string(),
-                ConstValue::ByRef { .. } => "ByRef(..)".to_string(),
+                ConstValue::Indirect { .. } => "ByRef(..)".to_string(),
             };
 
             let fmt_valtree = |valtree: &ty::ValTree<'tcx>| match valtree {
@@ -709,7 +709,11 @@ pub fn write_allocations<'tcx>(
                 // `u8`/`str` slices, shouldn't contain pointers that we want to print.
                 Either::Right(std::iter::empty())
             }
-            ConstValue::ByRef { alloc_id, .. } => Either::Left(std::iter::once(alloc_id)),
+            ConstValue::Indirect { alloc_id, .. } => {
+                // FIXME: we don't actually want to print all of these, since some are printed nicely directly as values inline in MIR.
+                // Really we'd want `pretty_print_const_value` to decide which allocations to print, instead of having a separate visitor.
+                Either::Left(std::iter::once(alloc_id))
+            }
         }
     }
     struct CollectAllocIds(BTreeSet<AllocId>);

compiler/rustc_mir_transform/src/const_prop.rs

@@ -143,7 +143,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
     #[inline(always)]
     fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> CheckAlignment {
         // We do not check for alignment to avoid having to carry an `Align`
-        // in `ConstValue::ByRef`.
+        // in `ConstValue::Indirect`.
         CheckAlignment::No
     }
 
@@ -552,7 +552,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                     .ok()?;
 
                 Some(ConstantKind::Val(
-                    ConstValue::ByRef { alloc_id, offset: Size::ZERO },
+                    ConstValue::Indirect { alloc_id, offset: Size::ZERO },
                     value.layout.ty,
                 ))
             }

compiler/rustc_mir_transform/src/large_enums.rs

@@ -153,7 +153,7 @@ impl EnumSizeOpt {
                         span,
                         user_ty: None,
                         literal: ConstantKind::Val(
-                            interpret::ConstValue::ByRef { alloc_id, offset: Size::ZERO },
+                            interpret::ConstValue::Indirect { alloc_id, offset: Size::ZERO },
                             tmp_ty,
                         ),
                     };

compiler/rustc_monomorphize/src/collector.rs

@@ -1470,7 +1470,7 @@ fn collect_const_value<'tcx>(
 ) {
     match value {
         ConstValue::Scalar(Scalar::Ptr(ptr, _size)) => collect_alloc(tcx, ptr.provenance, output),
-        ConstValue::ByRef { alloc_id, .. } => collect_alloc(tcx, alloc_id, output),
+        ConstValue::Indirect { alloc_id, .. } => collect_alloc(tcx, alloc_id, output),
         ConstValue::Slice { data, start: _, end: _ } => {
             for &id in data.inner().provenance().ptrs().values() {
                 collect_alloc(tcx, id, output);

compiler/rustc_smir/src/rustc_smir/alloc.rs

@@ -72,7 +72,7 @@ pub fn new_allocation<'tcx>(
                 .unwrap();
             allocation.stable(tables)
         }
-        ConstValue::ByRef { alloc_id, offset } => {
+        ConstValue::Indirect { alloc_id, offset } => {
             let alloc = tables.tcx.global_alloc(alloc_id).unwrap_memory();
             let ty_size = tables
                 .tcx

src/tools/clippy/clippy_lints/src/utils/internal_lints/unnecessary_def_path.rs

@@ -232,7 +232,7 @@ fn path_to_matched_type(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> Option<Ve
                 cx.tcx.type_of(def_id).instantiate_identity(),
             ),
             Res::Def(DefKind::Const, def_id) => match cx.tcx.const_eval_poly(def_id).ok()? {
-                ConstValue::ByRef { alloc, offset } if offset.bytes() == 0 => {
+                ConstValue::Indirect { alloc, offset } if offset.bytes() == 0 => {
                     read_mir_alloc_def_path(cx, alloc.inner(), cx.tcx.type_of(def_id).instantiate_identity())
                 },
                 _ => None,

src/tools/clippy/clippy_utils/src/consts.rs

@@ -684,7 +684,7 @@ pub fn miri_to_const<'tcx>(lcx: &LateContext<'tcx>, result: mir::ConstantKind<'t
             },
             _ => None,
         },
-        mir::ConstantKind::Val(ConstValue::ByRef { alloc_id, offset: _ }, _) => {
+        mir::ConstantKind::Val(ConstValue::Indirect { alloc_id, offset: _ }, _) => {
             let alloc = lcx.tcx.global_alloc(alloc_id).unwrap_memory();
             match result.ty().kind() {
                 ty::Adt(adt_def, _) if adt_def.is_struct() => Some(Constant::Adt(result)),