Auto merge of #115705 - cjgillot:const-prop-aggregate, r=oli-obk

Read from non-scalar constants and statics in dataflow const-prop

DataflowConstProp is designed to handle scalar values. When MIR features an assignment from a non-scalar constant, we need to manually decompose it into the custom state space.

This PR tweaks interpreter callbacks to allow reusing `eval_mir_constant` without having a stack frame to get a span from.

r? `@oli-obk`
cc `@jachris`

compiler/rustc_const_eval/src/interpret/discriminant.rs

@@ -247,9 +247,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         &self,
         layout: TyAndLayout<'tcx>,
         variant: VariantIdx,
-    ) -> InterpResult<'tcx, Scalar<M::Provenance>> {
+    ) -> InterpResult<'tcx, ImmTy<'tcx, M::Provenance>> {
         let discr_layout = self.layout_of(layout.ty.discriminant_ty(*self.tcx))?;
-        Ok(match layout.ty.discriminant_for_variant(*self.tcx, variant) {
+        let discr_value = match layout.ty.discriminant_for_variant(*self.tcx, variant) {
             Some(discr) => {
                 // This type actually has discriminants.
                 assert_eq!(discr.ty, discr_layout.ty);
@@ -260,6 +260,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 assert_eq!(variant.as_u32(), 0);
                 Scalar::from_uint(variant.as_u32(), discr_layout.size)
             }
-        })
+        };
+        Ok(ImmTy::from_scalar(discr_value, discr_layout))
     }
 }

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -222,7 +222,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let place = self.deref_pointer(&args[0])?;
                 let variant = self.read_discriminant(&place)?;
                 let discr = self.discriminant_for_variant(place.layout, variant)?;
-                self.write_scalar(discr, dest)?;
+                self.write_immediate(*discr, dest)?;
             }
             sym::exact_div => {
                 let l = self.read_immediate(&args[0])?;

compiler/rustc_const_eval/src/interpret/step.rs

@@ -301,7 +301,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let op = self.eval_place_to_op(place, None)?;
                 let variant = self.read_discriminant(&op)?;
                 let discr = self.discriminant_for_variant(op.layout, variant)?;
-                self.write_scalar(discr, &dest)?;
+                self.write_immediate(*discr, &dest)?;
             }
         }
 

compiler/rustc_mir_dataflow/src/value_analysis.rs

@@ -532,7 +532,7 @@ impl<V: Clone + HasTop + HasBottom> State<V> {
     /// places that are non-overlapping or identical.
     ///
     /// The target place must have been flooded before calling this method.
-    fn insert_place_idx(&mut self, target: PlaceIndex, source: PlaceIndex, map: &Map) {
+    pub fn insert_place_idx(&mut self, target: PlaceIndex, source: PlaceIndex, map: &Map) {
         let StateData::Reachable(values) = &mut self.0 else { return };
 
         // If both places are tracked, we copy the value to the target.
@@ -928,6 +928,31 @@ impl Map {
             f(v)
         }
     }
+
+    /// Invoke a function on each value in the given place and all descendants.
+    pub fn for_each_projection_value<O>(
+        &self,
+        root: PlaceIndex,
+        value: O,
+        project: &mut impl FnMut(TrackElem, &O) -> Option<O>,
+        f: &mut impl FnMut(PlaceIndex, &O),
+    ) {
+        // Fast path is there is nothing to do.
+        if self.inner_values[root].is_empty() {
+            return;
+        }
+
+        if self.places[root].value_index.is_some() {
+            f(root, &value)
+        }
+
+        for child in self.children(root) {
+            let elem = self.places[child].proj_elem.unwrap();
+            if let Some(value) = project(elem, &value) {
+                self.for_each_projection_value(child, value, project, f);
+            }
+        }
+    }
 }
 
 /// This is the information tracked for every [`PlaceIndex`] and is stored by [`Map`].

compiler/rustc_mir_transform/src/const_prop.rs

@@ -32,32 +32,30 @@ const MAX_ALLOC_LIMIT: u64 = 1024;
 
 /// Macro for machine-specific `InterpError` without allocation.
 /// (These will never be shown to the user, but they help diagnose ICEs.)
-macro_rules! throw_machine_stop_str {
+pub(crate) macro throw_machine_stop_str($($tt:tt)*) {{
-    ($($tt:tt)*) => {{
+    // We make a new local type for it. The type itself does not carry any information,
-        // We make a new local type for it. The type itself does not carry any information,
+    // but its vtable (for the `MachineStopType` trait) does.
-        // but its vtable (for the `MachineStopType` trait) does.
+    #[derive(Debug)]
-        #[derive(Debug)]
+    struct Zst;
-        struct Zst;
+    // Printing this type shows the desired string.
-        // Printing this type shows the desired string.
+    impl std::fmt::Display for Zst {
-        impl std::fmt::Display for Zst {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            write!(f, $($tt)*)
-                write!(f, $($tt)*)
+        }
-            }
+    }
+
+    impl rustc_middle::mir::interpret::MachineStopType for Zst {
+        fn diagnostic_message(&self) -> rustc_errors::DiagnosticMessage {
+            self.to_string().into()
         }
 
-        impl rustc_middle::mir::interpret::MachineStopType for Zst {
+        fn add_args(
-            fn diagnostic_message(&self) -> rustc_errors::DiagnosticMessage {
+            self: Box<Self>,
-                self.to_string().into()
+            _: &mut dyn FnMut(std::borrow::Cow<'static, str>, rustc_errors::DiagnosticArgValue<'static>),
-            }
+        ) {}
-
+    }
-            fn add_args(
+    throw_machine_stop!(Zst)
-                self: Box<Self>,
+}}
-                _: &mut dyn FnMut(std::borrow::Cow<'static, str>, rustc_errors::DiagnosticArgValue<'static>),
-            ) {}
-        }
-        throw_machine_stop!(Zst)
-    }};
-}
 
 pub struct ConstProp;
 

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -3,17 +3,19 @@
 //! Currently, this pass only propagates scalar values.
 
 use rustc_const_eval::const_eval::CheckAlignment;
-use rustc_const_eval::interpret::{ConstValue, ImmTy, Immediate, InterpCx, Scalar};
+use rustc_const_eval::interpret::{ImmTy, Immediate, InterpCx, OpTy, Projectable};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::def::DefKind;
+use rustc_middle::mir::interpret::{AllocId, ConstAllocation, ConstValue, InterpResult, Scalar};
 use rustc_middle::mir::visit::{MutVisitor, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::layout::TyAndLayout;
 use rustc_middle::ty::{self, ScalarInt, Ty, TyCtxt};
 use rustc_mir_dataflow::value_analysis::{
-    Map, State, TrackElem, ValueAnalysis, ValueAnalysisWrapper, ValueOrPlace,
+    Map, PlaceIndex, State, TrackElem, ValueAnalysis, ValueAnalysisWrapper, ValueOrPlace,
 };
 use rustc_mir_dataflow::{lattice::FlatSet, Analysis, Results, ResultsVisitor};
+use rustc_span::def_id::DefId;
 use rustc_span::DUMMY_SP;
 use rustc_target::abi::{Align, FieldIdx, VariantIdx};
 
@@ -77,7 +79,7 @@ struct ConstAnalysis<'a, 'tcx> {
 }
 
 impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
-    type Value = FlatSet<ScalarInt>;
+    type Value = FlatSet<Scalar>;
 
     const NAME: &'static str = "ConstAnalysis";
 
@@ -111,6 +113,18 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
         state: &mut State<Self::Value>,
     ) {
         match rvalue {
+            Rvalue::Use(operand) => {
+                state.flood(target.as_ref(), self.map());
+                if let Some(target) = self.map.find(target.as_ref()) {
+                    self.assign_operand(state, target, operand);
+                }
+            }
+            Rvalue::CopyForDeref(rhs) => {
+                state.flood(target.as_ref(), self.map());
+                if let Some(target) = self.map.find(target.as_ref()) {
+                    self.assign_operand(state, target, &Operand::Copy(*rhs));
+                }
+            }
             Rvalue::Aggregate(kind, operands) => {
                 // If we assign `target = Enum::Variant#0(operand)`,
                 // we must make sure that all `target as Variant#i` are `Top`.
@@ -138,8 +152,7 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                             variant_target_idx,
                             TrackElem::Field(FieldIdx::from_usize(field_index)),
                         ) {
-                            let result = self.handle_operand(operand, state);
+                            self.assign_operand(state, field, operand);
-                            state.insert_idx(field, result, self.map());
                         }
                     }
                 }
@@ -176,7 +189,7 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                     if let Some(overflow_target) = overflow_target {
                         let overflow = match overflow {
                             FlatSet::Top => FlatSet::Top,
-                            FlatSet::Elem(overflow) => FlatSet::Elem(overflow.into()),
+                            FlatSet::Elem(overflow) => FlatSet::Elem(Scalar::from_bool(overflow)),
                             FlatSet::Bottom => FlatSet::Bottom,
                         };
                         // We have flooded `target` earlier.
@@ -198,7 +211,7 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                     && let ty::Array(_, len) = operand_ty.ty.kind()
                     && let Some(len) = ConstantKind::Ty(*len).eval(self.tcx, self.param_env).try_to_scalar_int()
                 {
-                    state.insert_value_idx(target_len, FlatSet::Elem(len), self.map());
+                    state.insert_value_idx(target_len, FlatSet::Elem(len.into()), self.map());
                 }
             }
             _ => self.super_assign(target, rvalue, state),
@@ -216,7 +229,7 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                 if let ty::Array(_, len) = place_ty.ty.kind() {
                     ConstantKind::Ty(*len)
                         .eval(self.tcx, self.param_env)
-                        .try_to_scalar_int()
+                        .try_to_scalar()
                         .map_or(FlatSet::Top, FlatSet::Elem)
                 } else if let [ProjectionElem::Deref] = place.projection[..] {
                     state.get_len(place.local.into(), self.map())
@@ -257,9 +270,10 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                 val
             }
             Rvalue::UnaryOp(op, operand) => match self.eval_operand(operand, state) {
-                FlatSet::Elem(value) => {
+                FlatSet::Elem(value) => self
-                    self.ecx.unary_op(*op, &value).map_or(FlatSet::Top, |val| self.wrap_immty(val))
+                    .ecx
-                }
+                    .unary_op(*op, &value)
+                    .map_or(FlatSet::Top, |val| self.wrap_immediate(*val)),
                 FlatSet::Bottom => FlatSet::Bottom,
                 FlatSet::Top => FlatSet::Top,
             },
@@ -275,7 +289,7 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                         .bytes(),
                     _ => return ValueOrPlace::Value(FlatSet::Top),
                 };
-                ScalarInt::try_from_target_usize(val, self.tcx).map_or(FlatSet::Top, FlatSet::Elem)
+                FlatSet::Elem(Scalar::from_target_usize(val, &self.tcx))
             }
             Rvalue::Discriminant(place) => state.get_discr(place.as_ref(), self.map()),
             _ => return self.super_rvalue(rvalue, state),
@@ -291,7 +305,7 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
         constant
             .literal
             .eval(self.tcx, self.param_env)
-            .try_to_scalar_int()
+            .try_to_scalar()
             .map_or(FlatSet::Top, FlatSet::Elem)
     }
 
@@ -330,13 +344,98 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
         }
     }
 
+    /// The caller must have flooded `place`.
+    fn assign_operand(
+        &self,
+        state: &mut State<FlatSet<Scalar>>,
+        place: PlaceIndex,
+        operand: &Operand<'tcx>,
+    ) {
+        match operand {
+            Operand::Copy(rhs) | Operand::Move(rhs) => {
+                if let Some(rhs) = self.map.find(rhs.as_ref()) {
+                    state.insert_place_idx(place, rhs, &self.map);
+                } else if rhs.projection.first() == Some(&PlaceElem::Deref)
+                    && let FlatSet::Elem(pointer) = state.get(rhs.local.into(), &self.map)
+                    && let rhs_ty = self.local_decls[rhs.local].ty
+                    && let Ok(rhs_layout) = self.tcx.layout_of(self.param_env.and(rhs_ty))
+                {
+                    let op = ImmTy::from_scalar(pointer, rhs_layout).into();
+                    self.assign_constant(state, place, op, &rhs.projection);
+                }
+            }
+            Operand::Constant(box constant) => {
+                if let Ok(constant) = self.ecx.eval_mir_constant(&constant.literal, None, None) {
+                    self.assign_constant(state, place, constant, &[]);
+                }
+            }
+        }
+    }
+
+    /// The caller must have flooded `place`.
+    ///
+    /// Perform: `place = operand.projection`.
+    #[instrument(level = "trace", skip(self, state))]
+    fn assign_constant(
+        &self,
+        state: &mut State<FlatSet<Scalar>>,
+        place: PlaceIndex,
+        mut operand: OpTy<'tcx>,
+        projection: &[PlaceElem<'tcx>],
+    ) -> Option<!> {
+        for &(mut proj_elem) in projection {
+            if let PlaceElem::Index(index) = proj_elem {
+                if let FlatSet::Elem(index) = state.get(index.into(), &self.map)
+                    && let Ok(offset) = index.to_target_usize(&self.tcx)
+                    && let Some(min_length) = offset.checked_add(1)
+                {
+                    proj_elem = PlaceElem::ConstantIndex { offset, min_length, from_end: false };
+                } else {
+                    return None;
+                }
+            }
+            operand = self.ecx.project(&operand, proj_elem).ok()?;
+        }
+
+        self.map.for_each_projection_value(
+            place,
+            operand,
+            &mut |elem, op| match elem {
+                TrackElem::Field(idx) => self.ecx.project_field(op, idx.as_usize()).ok(),
+                TrackElem::Variant(idx) => self.ecx.project_downcast(op, idx).ok(),
+                TrackElem::Discriminant => {
+                    let variant = self.ecx.read_discriminant(op).ok()?;
+                    let discr_value = self.ecx.discriminant_for_variant(op.layout, variant).ok()?;
+                    Some(discr_value.into())
+                }
+                TrackElem::DerefLen => {
+                    let op: OpTy<'_> = self.ecx.deref_pointer(op).ok()?.into();
+                    let len_usize = op.len(&self.ecx).ok()?;
+                    let layout =
+                        self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).unwrap();
+                    Some(ImmTy::from_uint(len_usize, layout).into())
+                }
+            },
+            &mut |place, op| {
+                if let Ok(imm) = self.ecx.read_immediate_raw(op)
+                    && let Some(imm) = imm.right()
+                {
+                    let elem = self.wrap_immediate(*imm);
+                    state.insert_value_idx(place, elem, &self.map);
+                }
+            },
+        );
+
+        None
+    }
+
     fn binary_op(
         &self,
-        state: &mut State<FlatSet<ScalarInt>>,
+        state: &mut State<FlatSet<Scalar>>,
         op: BinOp,
         left: &Operand<'tcx>,
         right: &Operand<'tcx>,
-    ) -> (FlatSet<ScalarInt>, FlatSet<bool>) {
+    ) -> (FlatSet<Scalar>, FlatSet<bool>) {
         let left = self.eval_operand(left, state);
         let right = self.eval_operand(right, state);
 
@@ -345,9 +444,7 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             // Both sides are known, do the actual computation.
             (FlatSet::Elem(left), FlatSet::Elem(right)) => {
                 match self.ecx.overflowing_binary_op(op, &left, &right) {
-                    Ok((Scalar::Int(val), overflow, _)) => {
+                    Ok((val, overflow, _)) => (FlatSet::Elem(val), FlatSet::Elem(overflow)),
-                        (FlatSet::Elem(val), FlatSet::Elem(overflow))
-                    }
                     _ => (FlatSet::Top, FlatSet::Top),
                 }
             }
@@ -359,9 +456,6 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
                 }
 
                 let arg_scalar = const_arg.to_scalar();
-                let Ok(arg_scalar) = arg_scalar.try_to_int() else {
-                    return (FlatSet::Top, FlatSet::Top);
-                };
                 let Ok(arg_value) = arg_scalar.to_bits(layout.size) else {
                     return (FlatSet::Top, FlatSet::Top);
                 };
@@ -387,7 +481,7 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
     fn eval_operand(
         &self,
         op: &Operand<'tcx>,
-        state: &mut State<FlatSet<ScalarInt>>,
+        state: &mut State<FlatSet<Scalar>>,
     ) -> FlatSet<ImmTy<'tcx>> {
         let value = match self.handle_operand(op, state) {
             ValueOrPlace::Value(value) => value,
@@ -397,35 +491,30 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             FlatSet::Top => FlatSet::Top,
             FlatSet::Elem(scalar) => {
                 let ty = op.ty(self.local_decls, self.tcx);
-                self.tcx
+                self.tcx.layout_of(self.param_env.and(ty)).map_or(FlatSet::Top, |layout| {
-                    .layout_of(self.param_env.and(ty))
+                    FlatSet::Elem(ImmTy::from_scalar(scalar.into(), layout))
-                    .map(|layout| FlatSet::Elem(ImmTy::from_scalar(scalar.into(), layout)))
+                })
-                    .unwrap_or(FlatSet::Top)
             }
             FlatSet::Bottom => FlatSet::Bottom,
         }
     }
 
-    fn eval_discriminant(&self, enum_ty: Ty<'tcx>, variant_index: VariantIdx) -> Option<ScalarInt> {
+    fn eval_discriminant(&self, enum_ty: Ty<'tcx>, variant_index: VariantIdx) -> Option<Scalar> {
         if !enum_ty.is_enum() {
             return None;
         }
-        let discr = enum_ty.discriminant_for_variant(self.tcx, variant_index)?;
+        let enum_ty_layout = self.tcx.layout_of(self.param_env.and(enum_ty)).ok()?;
-        let discr_layout = self.tcx.layout_of(self.param_env.and(discr.ty)).ok()?;
+        let discr_value = self.ecx.discriminant_for_variant(enum_ty_layout, variant_index).ok()?;
-        let discr_value = ScalarInt::try_from_uint(discr.val, discr_layout.size)?;
+        Some(discr_value.to_scalar())
-        Some(discr_value)
     }
 
-    fn wrap_immediate(&self, imm: Immediate) -> FlatSet<ScalarInt> {
+    fn wrap_immediate(&self, imm: Immediate) -> FlatSet<Scalar> {
         match imm {
-            Immediate::Scalar(Scalar::Int(scalar)) => FlatSet::Elem(scalar),
+            Immediate::Scalar(scalar) => FlatSet::Elem(scalar),
+            Immediate::Uninit => FlatSet::Bottom,
             _ => FlatSet::Top,
         }
     }
-
-    fn wrap_immty(&self, val: ImmTy<'tcx>) -> FlatSet<ScalarInt> {
-        self.wrap_immediate(*val)
-    }
 }
 
 struct CollectAndPatch<'tcx, 'locals> {
@@ -464,7 +553,7 @@ impl<'mir, 'tcx>
     ResultsVisitor<'mir, 'tcx, Results<'tcx, ValueAnalysisWrapper<ConstAnalysis<'_, 'tcx>>>>
     for CollectAndPatch<'tcx, '_>
 {
-    type FlowState = State<FlatSet<ScalarInt>>;
+    type FlowState = State<FlatSet<Scalar>>;
 
     fn visit_statement_before_primary_effect(
         &mut self,
@@ -494,14 +583,10 @@ impl<'mir, 'tcx>
                 // Don't overwrite the assignment if it already uses a constant (to keep the span).
             }
             StatementKind::Assign(box (place, _)) => {
-                match state.get(place.as_ref(), &results.analysis.0.map) {
+                if let FlatSet::Elem(Scalar::Int(value)) =
-                    FlatSet::Top => (),
+                    state.get(place.as_ref(), &results.analysis.0.map)
-                    FlatSet::Elem(value) => {
+                {
-                        self.assignments.insert(location, value);
+                    self.assignments.insert(location, value);
-                    }
-                    FlatSet::Bottom => {
-                        // This assignment is either unreachable, or an uninitialized value is assigned.
-                    }
                 }
             }
             _ => (),
@@ -571,7 +656,7 @@ impl<'tcx> MutVisitor<'tcx> for CollectAndPatch<'tcx, '_> {
 }
 
 struct OperandCollector<'tcx, 'map, 'locals, 'a> {
-    state: &'a State<FlatSet<ScalarInt>>,
+    state: &'a State<FlatSet<Scalar>>,
     visitor: &'a mut CollectAndPatch<'tcx, 'locals>,
     map: &'map Map,
 }
@@ -579,7 +664,7 @@ struct OperandCollector<'tcx, 'map, 'locals, 'a> {
 impl<'tcx> Visitor<'tcx> for OperandCollector<'tcx, '_, '_, '_> {
     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
         if let Some(place) = operand.place() {
-            if let FlatSet::Elem(value) = self.state.get(place.as_ref(), self.map) {
+            if let FlatSet::Elem(Scalar::Int(value)) = self.state.get(place.as_ref(), self.map) {
                 self.visitor.before_effect.insert((location, place), value);
             } else if !place.projection.is_empty() {
                 // Try to propagate into `Index` projections.
@@ -590,7 +675,7 @@ impl<'tcx> Visitor<'tcx> for OperandCollector<'tcx, '_, '_, '_> {
 
     fn visit_local(&mut self, local: Local, ctxt: PlaceContext, location: Location) {
         if let PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy | NonMutatingUseContext::Move) = ctxt
-            && let FlatSet::Elem(value) = self.state.get(local.into(), self.map)
+            && let FlatSet::Elem(Scalar::Int(value)) = self.state.get(local.into(), self.map)
         {
             self.visitor.before_effect.insert((location, local.into()), value);
         }
@@ -604,8 +689,11 @@ impl<'mir, 'tcx: 'mir> rustc_const_eval::interpret::Machine<'mir, 'tcx> for Dumm
     type MemoryKind = !;
     const PANIC_ON_ALLOC_FAIL: bool = true;
 
+    #[inline(always)]
     fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> CheckAlignment {
-        unimplemented!()
+        // We do not check for alignment to avoid having to carry an `Align`
+        // in `ConstValue::ByRef`.
+        CheckAlignment::No
     }
 
     fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>, _layout: TyAndLayout<'tcx>) -> bool {
@@ -620,6 +708,27 @@ impl<'mir, 'tcx: 'mir> rustc_const_eval::interpret::Machine<'mir, 'tcx> for Dumm
         unimplemented!()
     }
 
+    fn before_access_global(
+        _tcx: TyCtxt<'tcx>,
+        _machine: &Self,
+        _alloc_id: AllocId,
+        alloc: ConstAllocation<'tcx>,
+        _static_def_id: Option<DefId>,
+        is_write: bool,
+    ) -> InterpResult<'tcx> {
+        if is_write {
+            crate::const_prop::throw_machine_stop_str!("can't write to global");
+        }
+
+        // If the static allocation is mutable, then we can't const prop it as its content
+        // might be different at runtime.
+        if alloc.inner().mutability.is_mut() {
+            crate::const_prop::throw_machine_stop_str!("can't access mutable globals in ConstProp");
+        }
+
+        Ok(())
+    }
+
     fn find_mir_or_eval_fn(
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
         _instance: ty::Instance<'tcx>,
@@ -688,7 +797,8 @@ impl<'mir, 'tcx: 'mir> rustc_const_eval::interpret::Machine<'mir, 'tcx> for Dumm
         _ecx: &'a InterpCx<'mir, 'tcx, Self>,
     ) -> &'a [rustc_const_eval::interpret::Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>]
     {
-        unimplemented!()
+        // Return an empty stack instead of panicking, as `cur_span` uses it to evaluate constants.
+        &[]
     }
 
     fn stack_mut<'a>(

compiler/rustc_mir_transform/src/lib.rs

@@ -2,6 +2,7 @@
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
 #![feature(box_patterns)]
+#![feature(decl_macro)]
 #![feature(is_sorted)]
 #![feature(let_chains)]
 #![feature(map_try_insert)]

tests/mir-opt/dataflow-const-prop/enum.constant.DataflowConstProp.32bit.diff

@@ -0,0 +1,63 @@
+- // MIR for `constant` before DataflowConstProp
++ // MIR for `constant` after DataflowConstProp
+  
+  fn constant() -> () {
+      let mut _0: ();
+      let _1: E;
+      let mut _3: isize;
+      scope 1 {
+          debug e => _1;
+          let _2: i32;
+          let _4: i32;
+          let _5: i32;
+          scope 2 {
+              debug x => _2;
+          }
+          scope 3 {
+              debug x => _4;
+          }
+          scope 4 {
+              debug x => _5;
+          }
+      }
+  
+      bb0: {
+          StorageLive(_1);
+          _1 = const _;
+          StorageLive(_2);
+-         _3 = discriminant(_1);
+-         switchInt(move _3) -> [0: bb3, 1: bb1, otherwise: bb2];
++         _3 = const 0_isize;
++         switchInt(const 0_isize) -> [0: bb3, 1: bb1, otherwise: bb2];
+      }
+  
+      bb1: {
+          StorageLive(_5);
+          _5 = ((_1 as V2).0: i32);
+          _2 = _5;
+          StorageDead(_5);
+          goto -> bb4;
+      }
+  
+      bb2: {
+          unreachable;
+      }
+  
+      bb3: {
+          StorageLive(_4);
+-         _4 = ((_1 as V1).0: i32);
+-         _2 = _4;
++         _4 = const 0_i32;
++         _2 = const 0_i32;
+          StorageDead(_4);
+          goto -> bb4;
+      }
+  
+      bb4: {
+          _0 = const ();
+          StorageDead(_2);
+          StorageDead(_1);
+          return;
+      }
+  }
+  

tests/mir-opt/dataflow-const-prop/enum.constant.DataflowConstProp.64bit.diff

@@ -0,0 +1,63 @@
+- // MIR for `constant` before DataflowConstProp
++ // MIR for `constant` after DataflowConstProp
+  
+  fn constant() -> () {
+      let mut _0: ();
+      let _1: E;
+      let mut _3: isize;
+      scope 1 {
+          debug e => _1;
+          let _2: i32;
+          let _4: i32;
+          let _5: i32;
+          scope 2 {
+              debug x => _2;
+          }
+          scope 3 {
+              debug x => _4;
+          }
+          scope 4 {
+              debug x => _5;
+          }
+      }
+  
+      bb0: {
+          StorageLive(_1);
+          _1 = const _;
+          StorageLive(_2);
+-         _3 = discriminant(_1);
+-         switchInt(move _3) -> [0: bb3, 1: bb1, otherwise: bb2];
++         _3 = const 0_isize;
++         switchInt(const 0_isize) -> [0: bb3, 1: bb1, otherwise: bb2];
+      }
+  
+      bb1: {
+          StorageLive(_5);
+          _5 = ((_1 as V2).0: i32);
+          _2 = _5;
+          StorageDead(_5);
+          goto -> bb4;
+      }
+  
+      bb2: {
+          unreachable;
+      }
+  
+      bb3: {
+          StorageLive(_4);
+-         _4 = ((_1 as V1).0: i32);
+-         _2 = _4;
++         _4 = const 0_i32;
++         _2 = const 0_i32;
+          StorageDead(_4);
+          goto -> bb4;
+      }
+  
+      bb4: {
+          _0 = const ();
+          StorageDead(_2);
+          StorageDead(_1);
+          return;
+      }
+  }
+  

tests/mir-opt/dataflow-const-prop/enum.multiple.DataflowConstProp.64bit.diff

@@ -0,0 +1,82 @@
+- // MIR for `multiple` before DataflowConstProp
++ // MIR for `multiple` after DataflowConstProp
+  
+  fn multiple(_1: bool, _2: u8) -> () {
+      debug x => _1;
+      debug i => _2;
+      let mut _0: ();
+      let _3: std::option::Option<u8>;
+      let mut _4: bool;
+      let mut _5: u8;
+      let mut _7: isize;
+      scope 1 {
+          debug e => _3;
+          let _6: u8;
+          let _8: u8;
+          scope 2 {
+              debug x => _6;
+              let _9: u8;
+              scope 4 {
+                  debug y => _9;
+              }
+          }
+          scope 3 {
+              debug i => _8;
+          }
+      }
+  
+      bb0: {
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = _1;
+          switchInt(move _4) -> [0: bb2, otherwise: bb1];
+      }
+  
+      bb1: {
+          StorageLive(_5);
+          _5 = _2;
+          _3 = Option::<u8>::Some(move _5);
+          StorageDead(_5);
+          goto -> bb3;
+      }
+  
+      bb2: {
+          _3 = Option::<u8>::None;
+          goto -> bb3;
+      }
+  
+      bb3: {
+          StorageDead(_4);
+          StorageLive(_6);
+          _7 = discriminant(_3);
+          switchInt(move _7) -> [0: bb4, 1: bb6, otherwise: bb5];
+      }
+  
+      bb4: {
+          _6 = const 0_u8;
+          goto -> bb7;
+      }
+  
+      bb5: {
+          unreachable;
+      }
+  
+      bb6: {
+          StorageLive(_8);
+          _8 = ((_3 as Some).0: u8);
+          _6 = _8;
+          StorageDead(_8);
+          goto -> bb7;
+      }
+  
+      bb7: {
+          StorageLive(_9);
+          _9 = _6;
+          _0 = const ();
+          StorageDead(_9);
+          StorageDead(_6);
+          StorageDead(_3);
+          return;
+      }
+  }
+  

tests/mir-opt/dataflow-const-prop/enum.mutate_discriminant.DataflowConstProp.64bit.diff

@@ -0,0 +1,26 @@
+- // MIR for `mutate_discriminant` before DataflowConstProp
++ // MIR for `mutate_discriminant` after DataflowConstProp
+  
+  fn mutate_discriminant() -> u8 {
+      let mut _0: u8;
+      let mut _1: std::option::Option<NonZeroUsize>;
+      let mut _2: isize;
+  
+      bb0: {
+          discriminant(_1) = 1;
+          (((_1 as variant#1).0: NonZeroUsize).0: usize) = const 0_usize;
+          _2 = discriminant(_1);
+          switchInt(_2) -> [0: bb1, otherwise: bb2];
+      }
+  
+      bb1: {
+          _0 = const 1_u8;
+          return;
+      }
+  
+      bb2: {
+          _0 = const 2_u8;
+          unreachable;
+      }
+  }
+  

tests/mir-opt/dataflow-const-prop/enum.rs

@@ -1,9 +1,11 @@
 // unit-test: DataflowConstProp
+// EMIT_MIR_FOR_EACH_BIT_WIDTH
 
 #![feature(custom_mir, core_intrinsics, rustc_attrs)]
 
 use std::intrinsics::mir::*;
 
+#[derive(Copy, Clone)]
 enum E {
     V1(i32),
     V2(i32)
@@ -15,6 +17,24 @@ fn simple() {
     let x = match e { E::V1(x) => x, E::V2(x) => x };
 }
 
+// EMIT_MIR enum.constant.DataflowConstProp.diff
+fn constant() {
+    const C: E = E::V1(0);
+    let e = C;
+    let x = match e { E::V1(x) => x, E::V2(x) => x };
+}
+
+// EMIT_MIR enum.statics.DataflowConstProp.diff
+fn statics() {
+    static C: E = E::V1(0);
+    let e = C;
+    let x = match e { E::V1(x) => x, E::V2(x) => x };
+
+    static RC: &E = &E::V2(4);
+    let e = RC;
+    let x = match e { E::V1(x) => x, E::V2(x) => x };
+}
+
 #[rustc_layout_scalar_valid_range_start(1)]
 #[rustc_nonnull_optimization_guaranteed]
 struct NonZeroUsize(usize);
@@ -63,6 +83,8 @@ fn multiple(x: bool, i: u8) {
 
 fn main() {
     simple();
+    constant();
+    statics();
     mutate_discriminant();
     multiple(false, 5);
 }

tests/mir-opt/dataflow-const-prop/enum.simple.DataflowConstProp.64bit.diff

@@ -0,0 +1,63 @@
+- // MIR for `simple` before DataflowConstProp
++ // MIR for `simple` after DataflowConstProp
+  
+  fn simple() -> () {
+      let mut _0: ();
+      let _1: E;
+      let mut _3: isize;
+      scope 1 {
+          debug e => _1;
+          let _2: i32;
+          let _4: i32;
+          let _5: i32;
+          scope 2 {
+              debug x => _2;
+          }
+          scope 3 {
+              debug x => _4;
+          }
+          scope 4 {
+              debug x => _5;
+          }
+      }
+  
+      bb0: {
+          StorageLive(_1);
+          _1 = E::V1(const 0_i32);
+          StorageLive(_2);
+-         _3 = discriminant(_1);
+-         switchInt(move _3) -> [0: bb3, 1: bb1, otherwise: bb2];
++         _3 = const 0_isize;
++         switchInt(const 0_isize) -> [0: bb3, 1: bb1, otherwise: bb2];
+      }
+  
+      bb1: {
+          StorageLive(_5);
+          _5 = ((_1 as V2).0: i32);
+          _2 = _5;
+          StorageDead(_5);
+          goto -> bb4;
+      }
+  
+      bb2: {
+          unreachable;
+      }
+  
+      bb3: {
+          StorageLive(_4);
+-         _4 = ((_1 as V1).0: i32);
+-         _2 = _4;
++         _4 = const 0_i32;
++         _2 = const 0_i32;
+          StorageDead(_4);
+          goto -> bb4;
+      }
+  
+      bb4: {
+          _0 = const ();
+          StorageDead(_2);
+          StorageDead(_1);
+          return;
+      }
+  }
+  

tests/mir-opt/dataflow-const-prop/enum.statics.DataflowConstProp.32bit.diff

@@ -0,0 +1,126 @@
+- // MIR for `statics` before DataflowConstProp
++ // MIR for `statics` after DataflowConstProp
+  
+  fn statics() -> () {
+      let mut _0: ();
+      let _1: E;
+      let mut _2: &E;
+      let mut _4: isize;
+      let mut _8: &&E;
+      let mut _10: isize;
+      scope 1 {
+          debug e => _1;
+          let _3: i32;
+          let _5: i32;
+          let _6: i32;
+          scope 2 {
+              debug x => _3;
+              let _7: &E;
+              scope 5 {
+                  debug e => _7;
+                  let _9: &i32;
+                  let _11: &i32;
+                  let _12: &i32;
+                  scope 6 {
+                      debug x => _9;
+                  }
+                  scope 7 {
+                      debug x => _11;
+                  }
+                  scope 8 {
+                      debug x => _12;
+                  }
+              }
+          }
+          scope 3 {
+              debug x => _5;
+          }
+          scope 4 {
+              debug x => _6;
+          }
+      }
+  
+      bb0: {
+          StorageLive(_1);
+          StorageLive(_2);
+          _2 = const {alloc1: &E};
+          _1 = (*_2);
+          StorageDead(_2);
+          StorageLive(_3);
+-         _4 = discriminant(_1);
+-         switchInt(move _4) -> [0: bb3, 1: bb1, otherwise: bb2];
++         _4 = const 0_isize;
++         switchInt(const 0_isize) -> [0: bb3, 1: bb1, otherwise: bb2];
+      }
+  
+      bb1: {
+          StorageLive(_6);
+          _6 = ((_1 as V2).0: i32);
+          _3 = _6;
+          StorageDead(_6);
+          goto -> bb4;
+      }
+  
+      bb2: {
+          unreachable;
+      }
+  
+      bb3: {
+          StorageLive(_5);
+-         _5 = ((_1 as V1).0: i32);
+-         _3 = _5;
++         _5 = const 0_i32;
++         _3 = const 0_i32;
+          StorageDead(_5);
+          goto -> bb4;
+      }
+  
+      bb4: {
+          StorageLive(_7);
+          StorageLive(_8);
+          _8 = const {alloc2: &&E};
+          _7 = (*_8);
+          StorageDead(_8);
+          StorageLive(_9);
+          _10 = discriminant((*_7));
+          switchInt(move _10) -> [0: bb6, 1: bb5, otherwise: bb2];
+      }
+  
+      bb5: {
+          StorageLive(_12);
+          _12 = &(((*_7) as V2).0: i32);
+          _9 = &(*_12);
+          StorageDead(_12);
+          goto -> bb7;
+      }
+  
+      bb6: {
+          StorageLive(_11);
+          _11 = &(((*_7) as V1).0: i32);
+          _9 = _11;
+          StorageDead(_11);
+          goto -> bb7;
+      }
+  
+      bb7: {
+          _0 = const ();
+          StorageDead(_9);
+          StorageDead(_7);
+          StorageDead(_3);
+          StorageDead(_1);
+          return;
+      }
+  }
+  
+  alloc2 (static: RC, size: 4, align: 4) {
+      ╾─alloc14─╼                                     │ ╾──╼
+  }
+  
+  alloc14 (size: 8, align: 4) {
+      01 00 00 00 04 00 00 00                         │ ........
+  }
+  
+  alloc1 (static: statics::C, size: 8, align: 4) {
+      00 00 00 00 00 00 00 00                         │ ........
+  }
+  

tests/mir-opt/dataflow-const-prop/enum.statics.DataflowConstProp.64bit.diff

@@ -0,0 +1,126 @@
+- // MIR for `statics` before DataflowConstProp
++ // MIR for `statics` after DataflowConstProp
+  
+  fn statics() -> () {
+      let mut _0: ();
+      let _1: E;
+      let mut _2: &E;
+      let mut _4: isize;
+      let mut _8: &&E;
+      let mut _10: isize;
+      scope 1 {
+          debug e => _1;
+          let _3: i32;
+          let _5: i32;
+          let _6: i32;
+          scope 2 {
+              debug x => _3;
+              let _7: &E;
+              scope 5 {
+                  debug e => _7;
+                  let _9: &i32;
+                  let _11: &i32;
+                  let _12: &i32;
+                  scope 6 {
+                      debug x => _9;
+                  }
+                  scope 7 {
+                      debug x => _11;
+                  }
+                  scope 8 {
+                      debug x => _12;
+                  }
+              }
+          }
+          scope 3 {
+              debug x => _5;
+          }
+          scope 4 {
+              debug x => _6;
+          }
+      }
+  
+      bb0: {
+          StorageLive(_1);
+          StorageLive(_2);
+          _2 = const {alloc1: &E};
+          _1 = (*_2);
+          StorageDead(_2);
+          StorageLive(_3);
+-         _4 = discriminant(_1);
+-         switchInt(move _4) -> [0: bb3, 1: bb1, otherwise: bb2];
++         _4 = const 0_isize;
++         switchInt(const 0_isize) -> [0: bb3, 1: bb1, otherwise: bb2];
+      }
+  
+      bb1: {
+          StorageLive(_6);
+          _6 = ((_1 as V2).0: i32);
+          _3 = _6;
+          StorageDead(_6);
+          goto -> bb4;
+      }
+  
+      bb2: {
+          unreachable;
+      }
+  
+      bb3: {
+          StorageLive(_5);
+-         _5 = ((_1 as V1).0: i32);
+-         _3 = _5;
++         _5 = const 0_i32;
++         _3 = const 0_i32;
+          StorageDead(_5);
+          goto -> bb4;
+      }
+  
+      bb4: {
+          StorageLive(_7);
+          StorageLive(_8);
+          _8 = const {alloc2: &&E};
+          _7 = (*_8);
+          StorageDead(_8);
+          StorageLive(_9);
+          _10 = discriminant((*_7));
+          switchInt(move _10) -> [0: bb6, 1: bb5, otherwise: bb2];
+      }
+  
+      bb5: {
+          StorageLive(_12);
+          _12 = &(((*_7) as V2).0: i32);
+          _9 = &(*_12);
+          StorageDead(_12);
+          goto -> bb7;
+      }
+  
+      bb6: {
+          StorageLive(_11);
+          _11 = &(((*_7) as V1).0: i32);
+          _9 = _11;
+          StorageDead(_11);
+          goto -> bb7;
+      }
+  
+      bb7: {
+          _0 = const ();
+          StorageDead(_9);
+          StorageDead(_7);
+          StorageDead(_3);
+          StorageDead(_1);
+          return;
+      }
+  }
+  
+  alloc2 (static: RC, size: 8, align: 8) {
+      ╾───────alloc14───────╼                         │ ╾──────╼
+  }
+  
+  alloc14 (size: 8, align: 4) {
+      01 00 00 00 04 00 00 00                         │ ........
+  }
+  
+  alloc1 (static: statics::C, size: 8, align: 4) {
+      00 00 00 00 00 00 00 00                         │ ........
+  }
+  

tests/mir-opt/dataflow-const-prop/slice_len.main.DataflowConstProp.32bit.panic-abort.diff

@@ -55,11 +55,12 @@
           _10 = const _;
           StorageLive(_11);
           _11 = const 1_usize;
-          _12 = Len((*_10));
+-         _12 = Len((*_10));
 -         _13 = Lt(_11, _12);
 -         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, _11) -> [success: bb2, unwind unreachable];
-+         _13 = Lt(const 1_usize, _12);
++         _12 = const 3_usize;
-+         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, const 1_usize) -> [success: bb2, unwind unreachable];
++         _13 = const true;
++         assert(const true, "index out of bounds: the length is {} but the index is {}", const 3_usize, const 1_usize) -> [success: bb2, unwind unreachable];
       }
   
       bb2: {

tests/mir-opt/dataflow-const-prop/slice_len.main.DataflowConstProp.32bit.panic-unwind.diff

@@ -55,11 +55,12 @@
           _10 = const _;
           StorageLive(_11);
           _11 = const 1_usize;
-          _12 = Len((*_10));
+-         _12 = Len((*_10));
 -         _13 = Lt(_11, _12);
 -         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, _11) -> [success: bb2, unwind continue];
-+         _13 = Lt(const 1_usize, _12);
++         _12 = const 3_usize;
-+         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, const 1_usize) -> [success: bb2, unwind continue];
++         _13 = const true;
++         assert(const true, "index out of bounds: the length is {} but the index is {}", const 3_usize, const 1_usize) -> [success: bb2, unwind continue];
       }
   
       bb2: {

tests/mir-opt/dataflow-const-prop/slice_len.main.DataflowConstProp.64bit.panic-abort.diff

@@ -55,11 +55,12 @@
           _10 = const _;
           StorageLive(_11);
           _11 = const 1_usize;
-          _12 = Len((*_10));
+-         _12 = Len((*_10));
 -         _13 = Lt(_11, _12);
 -         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, _11) -> [success: bb2, unwind unreachable];
-+         _13 = Lt(const 1_usize, _12);
++         _12 = const 3_usize;
-+         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, const 1_usize) -> [success: bb2, unwind unreachable];
++         _13 = const true;
++         assert(const true, "index out of bounds: the length is {} but the index is {}", const 3_usize, const 1_usize) -> [success: bb2, unwind unreachable];
       }
   
       bb2: {

tests/mir-opt/dataflow-const-prop/slice_len.main.DataflowConstProp.64bit.panic-unwind.diff

@@ -55,11 +55,12 @@
           _10 = const _;
           StorageLive(_11);
           _11 = const 1_usize;
-          _12 = Len((*_10));
+-         _12 = Len((*_10));
 -         _13 = Lt(_11, _12);
 -         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, _11) -> [success: bb2, unwind continue];
-+         _13 = Lt(const 1_usize, _12);
++         _12 = const 3_usize;
-+         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, const 1_usize) -> [success: bb2, unwind continue];
++         _13 = const true;
++         assert(const true, "index out of bounds: the length is {} but the index is {}", const 3_usize, const 1_usize) -> [success: bb2, unwind continue];
       }
   
       bb2: {

tests/mir-opt/dataflow-const-prop/struct.main.DataflowConstProp.32bit.diff

@@ -0,0 +1,129 @@
+- // MIR for `main` before DataflowConstProp
++ // MIR for `main` after DataflowConstProp
+  
+  fn main() -> () {
+      let mut _0: ();
+      let mut _1: S;
+      let mut _3: i32;
+      let mut _5: i32;
+      let mut _6: i32;
+      let mut _11: BigStruct;
+      let mut _16: &&BigStruct;
+      let mut _17: &BigStruct;
+      let mut _18: &BigStruct;
+      let mut _19: &BigStruct;
+      let mut _20: &BigStruct;
+      let mut _21: &BigStruct;
+      scope 1 {
+          debug s => _1;
+          let _2: i32;
+          scope 2 {
+              debug a => _2;
+              let _4: i32;
+              scope 3 {
+                  debug b => _4;
+                  let _7: S;
+                  let _8: u8;
+                  let _9: f32;
+                  let _10: S;
+                  scope 4 {
+                      debug a => _7;
+                      debug b => _8;
+                      debug c => _9;
+                      debug d => _10;
+                      let _12: S;
+                      let _13: u8;
+                      let _14: f32;
+                      let _15: S;
+                      scope 5 {
+                          debug a => _12;
+                          debug b => _13;
+                          debug c => _14;
+                          debug d => _15;
+                      }
+                  }
+              }
+          }
+      }
+  
+      bb0: {
+          StorageLive(_1);
+          _1 = S(const 1_i32);
+          StorageLive(_2);
+          StorageLive(_3);
+-         _3 = (_1.0: i32);
+-         _2 = Add(move _3, const 2_i32);
++         _3 = const 1_i32;
++         _2 = const 3_i32;
+          StorageDead(_3);
+          (_1.0: i32) = const 3_i32;
+          StorageLive(_4);
+          StorageLive(_5);
+-         _5 = _2;
++         _5 = const 3_i32;
+          StorageLive(_6);
+-         _6 = (_1.0: i32);
+-         _4 = Add(move _5, move _6);
++         _6 = const 3_i32;
++         _4 = const 6_i32;
+          StorageDead(_6);
+          StorageDead(_5);
+          StorageLive(_11);
+          _11 = const _;
+          StorageLive(_7);
+-         _7 = (_11.0: S);
++         _7 = const S(1_i32);
+          StorageLive(_8);
+-         _8 = (_11.1: u8);
++         _8 = const 5_u8;
+          StorageLive(_9);
+-         _9 = (_11.2: f32);
++         _9 = const 7f32;
+          StorageLive(_10);
+-         _10 = (_11.3: S);
++         _10 = const S(13_i32);
+          StorageDead(_11);
+          StorageLive(_16);
+          _16 = const {alloc1: &&BigStruct};
+          _17 = deref_copy (*_16);
+          StorageLive(_12);
+          _18 = deref_copy (*_16);
+-         _12 = ((*_18).0: S);
++         _12 = const S(1_i32);
+          StorageLive(_13);
+          _19 = deref_copy (*_16);
+-         _13 = ((*_19).1: u8);
++         _13 = const 5_u8;
+          StorageLive(_14);
+          _20 = deref_copy (*_16);
+-         _14 = ((*_20).2: f32);
++         _14 = const 7f32;
+          StorageLive(_15);
+          _21 = deref_copy (*_16);
+-         _15 = ((*_21).3: S);
++         _15 = const S(13_i32);
+          StorageDead(_16);
+          _0 = const ();
+          StorageDead(_15);
+          StorageDead(_14);
+          StorageDead(_13);
+          StorageDead(_12);
+          StorageDead(_10);
+          StorageDead(_9);
+          StorageDead(_8);
+          StorageDead(_7);
+          StorageDead(_4);
+          StorageDead(_2);
+          StorageDead(_1);
+          return;
+      }
+  }
+  
+  alloc1 (static: STAT, size: 4, align: 4) {
+      ╾─alloc15─╼                                     │ ╾──╼
+  }
+  
+  alloc15 (size: 16, align: 4) {
+      01 00 00 00 00 00 e0 40 0d 00 00 00 05 __ __ __ │ .......@.....░░░
+  }
+  

tests/mir-opt/dataflow-const-prop/struct.main.DataflowConstProp.64bit.diff

@@ -0,0 +1,129 @@
+- // MIR for `main` before DataflowConstProp
++ // MIR for `main` after DataflowConstProp
+  
+  fn main() -> () {
+      let mut _0: ();
+      let mut _1: S;
+      let mut _3: i32;
+      let mut _5: i32;
+      let mut _6: i32;
+      let mut _11: BigStruct;
+      let mut _16: &&BigStruct;
+      let mut _17: &BigStruct;
+      let mut _18: &BigStruct;
+      let mut _19: &BigStruct;
+      let mut _20: &BigStruct;
+      let mut _21: &BigStruct;
+      scope 1 {
+          debug s => _1;
+          let _2: i32;
+          scope 2 {
+              debug a => _2;
+              let _4: i32;
+              scope 3 {
+                  debug b => _4;
+                  let _7: S;
+                  let _8: u8;
+                  let _9: f32;
+                  let _10: S;
+                  scope 4 {
+                      debug a => _7;
+                      debug b => _8;
+                      debug c => _9;
+                      debug d => _10;
+                      let _12: S;
+                      let _13: u8;
+                      let _14: f32;
+                      let _15: S;
+                      scope 5 {
+                          debug a => _12;
+                          debug b => _13;
+                          debug c => _14;
+                          debug d => _15;
+                      }
+                  }
+              }
+          }
+      }
+  
+      bb0: {
+          StorageLive(_1);
+          _1 = S(const 1_i32);
+          StorageLive(_2);
+          StorageLive(_3);
+-         _3 = (_1.0: i32);
+-         _2 = Add(move _3, const 2_i32);
++         _3 = const 1_i32;
++         _2 = const 3_i32;
+          StorageDead(_3);
+          (_1.0: i32) = const 3_i32;
+          StorageLive(_4);
+          StorageLive(_5);
+-         _5 = _2;
++         _5 = const 3_i32;
+          StorageLive(_6);
+-         _6 = (_1.0: i32);
+-         _4 = Add(move _5, move _6);
++         _6 = const 3_i32;
++         _4 = const 6_i32;
+          StorageDead(_6);
+          StorageDead(_5);
+          StorageLive(_11);
+          _11 = const _;
+          StorageLive(_7);
+-         _7 = (_11.0: S);
++         _7 = const S(1_i32);
+          StorageLive(_8);
+-         _8 = (_11.1: u8);
++         _8 = const 5_u8;
+          StorageLive(_9);
+-         _9 = (_11.2: f32);
++         _9 = const 7f32;
+          StorageLive(_10);
+-         _10 = (_11.3: S);
++         _10 = const S(13_i32);
+          StorageDead(_11);
+          StorageLive(_16);
+          _16 = const {alloc1: &&BigStruct};
+          _17 = deref_copy (*_16);
+          StorageLive(_12);
+          _18 = deref_copy (*_16);
+-         _12 = ((*_18).0: S);
++         _12 = const S(1_i32);
+          StorageLive(_13);
+          _19 = deref_copy (*_16);
+-         _13 = ((*_19).1: u8);
++         _13 = const 5_u8;
+          StorageLive(_14);
+          _20 = deref_copy (*_16);
+-         _14 = ((*_20).2: f32);
++         _14 = const 7f32;
+          StorageLive(_15);
+          _21 = deref_copy (*_16);
+-         _15 = ((*_21).3: S);
++         _15 = const S(13_i32);
+          StorageDead(_16);
+          _0 = const ();
+          StorageDead(_15);
+          StorageDead(_14);
+          StorageDead(_13);
+          StorageDead(_12);
+          StorageDead(_10);
+          StorageDead(_9);
+          StorageDead(_8);
+          StorageDead(_7);
+          StorageDead(_4);
+          StorageDead(_2);
+          StorageDead(_1);
+          return;
+      }
+  }
+  
+  alloc1 (static: STAT, size: 8, align: 8) {
+      ╾───────alloc15───────╼                         │ ╾──────╼
+  }
+  
+  alloc15 (size: 16, align: 4) {
+      01 00 00 00 00 00 e0 40 0d 00 00 00 05 __ __ __ │ .......@.....░░░
+  }
+  

tests/mir-opt/dataflow-const-prop/struct.main.DataflowConstProp.diff

@@ -1,51 +0,0 @@
-- // MIR for `main` before DataflowConstProp
-+ // MIR for `main` after DataflowConstProp
-  
-  fn main() -> () {
-      let mut _0: ();
-      let mut _1: S;
-      let mut _3: i32;
-      let mut _5: i32;
-      let mut _6: i32;
-      scope 1 {
-          debug s => _1;
-          let _2: i32;
-          scope 2 {
-              debug a => _2;
-              let _4: i32;
-              scope 3 {
-                  debug b => _4;
-              }
-          }
-      }
-  
-      bb0: {
-          StorageLive(_1);
-          _1 = S(const 1_i32);
-          StorageLive(_2);
-          StorageLive(_3);
--         _3 = (_1.0: i32);
--         _2 = Add(move _3, const 2_i32);
-+         _3 = const 1_i32;
-+         _2 = const 3_i32;
-          StorageDead(_3);
-          (_1.0: i32) = const 3_i32;
-          StorageLive(_4);
-          StorageLive(_5);
--         _5 = _2;
-+         _5 = const 3_i32;
-          StorageLive(_6);
--         _6 = (_1.0: i32);
--         _4 = Add(move _5, move _6);
-+         _6 = const 3_i32;
-+         _4 = const 6_i32;
-          StorageDead(_6);
-          StorageDead(_5);
-          _0 = const ();
-          StorageDead(_4);
-          StorageDead(_2);
-          StorageDead(_1);
-          return;
-      }
-  }
-  

tests/mir-opt/dataflow-const-prop/struct.rs

@@ -1,11 +1,22 @@
 // unit-test: DataflowConstProp
+// EMIT_MIR_FOR_EACH_BIT_WIDTH
 
+#[derive(Copy, Clone)]
 struct S(i32);
 
+#[derive(Copy, Clone)]
+struct BigStruct(S, u8, f32, S);
+
 // EMIT_MIR struct.main.DataflowConstProp.diff
 fn main() {
     let mut s = S(1);
     let a = s.0 + 2;
     s.0 = 3;
     let b = a + s.0;
+
+    const VAL: BigStruct = BigStruct(S(1), 5, 7., S(13));
+    let BigStruct(a, b, c, d) = VAL;
+
+    static STAT: &BigStruct = &BigStruct(S(1), 5, 7., S(13));
+    let BigStruct(a, b, c, d) = *STAT;
 }