Auto merge of #101168 - jachris:dataflow-const-prop, r=oli-obk

Add new MIR constant propagation based on dataflow analysis

The current constant propagation in `rustc_mir_transform/src/const_prop.rs` fails to handle many cases that would be expected from a constant propagation optimization. For example:
```rust
let x = if true { 0 } else { 0 };
```
This pull request adds a new constant propagation MIR optimization pass based on the existing dataflow analysis framework. Since most of the analysis is not unique to constant propagation, a generic framework has been extracted. It works on top of the existing framework and could be reused for other optimzations.

Closes #80038. Closes #81605.

## Todo
### Essential
- [x] [Writes to inactive enum variants](https://github.com/rust-lang/rust/pull/101168#pullrequestreview-1089493974). Resolved by rejecting the registration of places with downcast projections for now. Could be improved by flooding other variants if mutable access to a variant is observed.
- [X] Handle [`StatementKind::CopyNonOverlapping`](https://github.com/rust-lang/rust/pull/101168#discussion_r957774914). Resolved by flooding the destination.
- [x] Handle `UnsafeCell` / `!Freeze` correctly.
- [X] Overflow propagation of `CheckedBinaryOp`: Decided to not propagate if overflow flag is `true` (`false` will still be propagated)
- [x] More documentation in general.
- [x] Arguments for correctness, documentation of necessary assumptions.
- [x] Better performance, or alternatively, require `-Zmir-opt-level=3` for now.

### Extra
- [x]  Add explicit unreachability, i.e. upgrading the lattice from $\mathbb{P} \to \mathbb{V}$ to $\set{\bot} \cup (\mathbb{P} \to \mathbb{V})$.
- [x] Use storage statements to improve precision.
- [ ] Consider opening issue for duplicate diagnostics: https://github.com/rust-lang/rust/pull/101168#issuecomment-1276609950
- [ ] Flood moved-from places with $\bot$ (requires some changes for places with tracked projections).
- [ ] Add downcast projections back in.
- [ ] [Algebraic simplifications](https://github.com/rust-lang/rust/pull/101168#discussion_r957967878) (possibly with a shared API; done by old const prop).
- [ ] Propagation through slices / arrays.
- [ ] Find other optimizations that are done by old `const_prop.rs`, but not by this one.

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -0,0 +1,530 @@
+//! A constant propagation optimization pass based on dataflow analysis.
+//!
+//! Currently, this pass only propagates scalar values.
+
+use rustc_const_eval::interpret::{ConstValue, ImmTy, Immediate, InterpCx, Scalar};
+use rustc_data_structures::fx::FxHashMap;
+use rustc_middle::mir::visit::{MutVisitor, Visitor};
+use rustc_middle::mir::*;
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_mir_dataflow::value_analysis::{Map, State, TrackElem, ValueAnalysis, ValueOrPlace};
+use rustc_mir_dataflow::{lattice::FlatSet, Analysis, ResultsVisitor, SwitchIntEdgeEffects};
+use rustc_span::DUMMY_SP;
+
+use crate::MirPass;
+
+// These constants are somewhat random guesses and have not been optimized.
+// If `tcx.sess.mir_opt_level() >= 4`, we ignore the limits (this can become very expensive).
+const BLOCK_LIMIT: usize = 100;
+const PLACE_LIMIT: usize = 100;
+
+pub struct DataflowConstProp;
+
+impl<'tcx> MirPass<'tcx> for DataflowConstProp {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        sess.mir_opt_level() >= 3
+    }
+
+    #[instrument(skip_all level = "debug")]
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        if tcx.sess.mir_opt_level() < 4 && body.basic_blocks.len() > BLOCK_LIMIT {
+            debug!("aborted dataflow const prop due too many basic blocks");
+            return;
+        }
+
+        // Decide which places to track during the analysis.
+        let map = Map::from_filter(tcx, body, Ty::is_scalar);
+
+        // We want to have a somewhat linear runtime w.r.t. the number of statements/terminators.
+        // Let's call this number `n`. Dataflow analysis has `O(h*n)` transfer function
+        // applications, where `h` is the height of the lattice. Because the height of our lattice
+        // is linear w.r.t. the number of tracked places, this is `O(tracked_places * n)`. However,
+        // because every transfer function application could traverse the whole map, this becomes
+        // `O(num_nodes * tracked_places * n)` in terms of time complexity. Since the number of
+        // map nodes is strongly correlated to the number of tracked places, this becomes more or
+        // less `O(n)` if we place a constant limit on the number of tracked places.
+        if tcx.sess.mir_opt_level() < 4 && map.tracked_places() > PLACE_LIMIT {
+            debug!("aborted dataflow const prop due to too many tracked places");
+            return;
+        }
+
+        // Perform the actual dataflow analysis.
+        let analysis = ConstAnalysis::new(tcx, body, map);
+        let results = debug_span!("analyze")
+            .in_scope(|| analysis.wrap().into_engine(tcx, body).iterate_to_fixpoint());
+
+        // Collect results and patch the body afterwards.
+        let mut visitor = CollectAndPatch::new(tcx, &results.analysis.0.map);
+        debug_span!("collect").in_scope(|| results.visit_reachable_with(body, &mut visitor));
+        debug_span!("patch").in_scope(|| visitor.visit_body(body));
+    }
+}
+
+struct ConstAnalysis<'tcx> {
+    map: Map,
+    tcx: TyCtxt<'tcx>,
+    ecx: InterpCx<'tcx, 'tcx, DummyMachine>,
+    param_env: ty::ParamEnv<'tcx>,
+}
+
+impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'tcx> {
+    type Value = FlatSet<ScalarTy<'tcx>>;
+
+    const NAME: &'static str = "ConstAnalysis";
+
+    fn map(&self) -> &Map {
+        &self.map
+    }
+
+    fn handle_assign(
+        &self,
+        target: Place<'tcx>,
+        rvalue: &Rvalue<'tcx>,
+        state: &mut State<Self::Value>,
+    ) {
+        match rvalue {
+            Rvalue::CheckedBinaryOp(op, box (left, right)) => {
+                let target = self.map().find(target.as_ref());
+                if let Some(target) = target {
+                    // We should not track any projections other than
+                    // what is overwritten below, but just in case...
+                    state.flood_idx(target, self.map());
+                }
+
+                let value_target = target
+                    .and_then(|target| self.map().apply(target, TrackElem::Field(0_u32.into())));
+                let overflow_target = target
+                    .and_then(|target| self.map().apply(target, TrackElem::Field(1_u32.into())));
+
+                if value_target.is_some() || overflow_target.is_some() {
+                    let (val, overflow) = self.binary_op(state, *op, left, right);
+
+                    if let Some(value_target) = value_target {
+                        state.assign_idx(value_target, ValueOrPlace::Value(val), self.map());
+                    }
+                    if let Some(overflow_target) = overflow_target {
+                        let overflow = match overflow {
+                            FlatSet::Top => FlatSet::Top,
+                            FlatSet::Elem(overflow) => {
+                                if overflow {
+                                    // Overflow cannot be reliably propagated. See: https://github.com/rust-lang/rust/pull/101168#issuecomment-1288091446
+                                    FlatSet::Top
+                                } else {
+                                    self.wrap_scalar(Scalar::from_bool(false), self.tcx.types.bool)
+                                }
+                            }
+                            FlatSet::Bottom => FlatSet::Bottom,
+                        };
+                        state.assign_idx(
+                            overflow_target,
+                            ValueOrPlace::Value(overflow),
+                            self.map(),
+                        );
+                    }
+                }
+            }
+            _ => self.super_assign(target, rvalue, state),
+        }
+    }
+
+    fn handle_rvalue(
+        &self,
+        rvalue: &Rvalue<'tcx>,
+        state: &mut State<Self::Value>,
+    ) -> ValueOrPlace<Self::Value> {
+        match rvalue {
+            Rvalue::Cast(
+                kind @ (CastKind::IntToInt
+                | CastKind::FloatToInt
+                | CastKind::FloatToFloat
+                | CastKind::IntToFloat),
+                operand,
+                ty,
+            ) => match self.eval_operand(operand, state) {
+                FlatSet::Elem(op) => match kind {
+                    CastKind::IntToInt | CastKind::IntToFloat => {
+                        self.ecx.int_to_int_or_float(&op, *ty)
+                    }
+                    CastKind::FloatToInt | CastKind::FloatToFloat => {
+                        self.ecx.float_to_float_or_int(&op, *ty)
+                    }
+                    _ => unreachable!(),
+                }
+                .map(|result| ValueOrPlace::Value(self.wrap_immediate(result, *ty)))
+                .unwrap_or(ValueOrPlace::top()),
+                _ => ValueOrPlace::top(),
+            },
+            Rvalue::BinaryOp(op, box (left, right)) => {
+                // Overflows must be ignored here.
+                let (val, _overflow) = self.binary_op(state, *op, left, right);
+                ValueOrPlace::Value(val)
+            }
+            Rvalue::UnaryOp(op, operand) => match self.eval_operand(operand, state) {
+                FlatSet::Elem(value) => self
+                    .ecx
+                    .unary_op(*op, &value)
+                    .map(|val| ValueOrPlace::Value(self.wrap_immty(val)))
+                    .unwrap_or(ValueOrPlace::Value(FlatSet::Top)),
+                FlatSet::Bottom => ValueOrPlace::Value(FlatSet::Bottom),
+                FlatSet::Top => ValueOrPlace::Value(FlatSet::Top),
+            },
+            _ => self.super_rvalue(rvalue, state),
+        }
+    }
+
+    fn handle_constant(
+        &self,
+        constant: &Constant<'tcx>,
+        _state: &mut State<Self::Value>,
+    ) -> Self::Value {
+        constant
+            .literal
+            .eval(self.tcx, self.param_env)
+            .try_to_scalar()
+            .map(|value| FlatSet::Elem(ScalarTy(value, constant.ty())))
+            .unwrap_or(FlatSet::Top)
+    }
+
+    fn handle_switch_int(
+        &self,
+        discr: &Operand<'tcx>,
+        apply_edge_effects: &mut impl SwitchIntEdgeEffects<State<Self::Value>>,
+    ) {
+        // FIXME: The dataflow framework only provides the state if we call `apply()`, which makes
+        // this more inefficient than it has to be.
+        let mut discr_value = None;
+        let mut handled = false;
+        apply_edge_effects.apply(|state, target| {
+            let discr_value = match discr_value {
+                Some(value) => value,
+                None => {
+                    let value = match self.handle_operand(discr, state) {
+                        ValueOrPlace::Value(value) => value,
+                        ValueOrPlace::Place(place) => state.get_idx(place, self.map()),
+                    };
+                    let result = match value {
+                        FlatSet::Top => FlatSet::Top,
+                        FlatSet::Elem(ScalarTy(scalar, _)) => {
+                            let int = scalar.assert_int();
+                            FlatSet::Elem(int.assert_bits(int.size()))
+                        }
+                        FlatSet::Bottom => FlatSet::Bottom,
+                    };
+                    discr_value = Some(result);
+                    result
+                }
+            };
+
+            let FlatSet::Elem(choice) = discr_value else {
+                // Do nothing if we don't know which branch will be taken.
+                return
+            };
+
+            if target.value.map(|n| n == choice).unwrap_or(!handled) {
+                // Branch is taken. Has no effect on state.
+                handled = true;
+            } else {
+                // Branch is not taken.
+                state.mark_unreachable();
+            }
+        })
+    }
+}
+
+#[derive(Clone, PartialEq, Eq)]
+struct ScalarTy<'tcx>(Scalar, Ty<'tcx>);
+
+impl<'tcx> std::fmt::Debug for ScalarTy<'tcx> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        // This is used for dataflow visualization, so we return something more concise.
+        std::fmt::Display::fmt(&ConstantKind::Val(ConstValue::Scalar(self.0), self.1), f)
+    }
+}
+
+impl<'tcx> ConstAnalysis<'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, map: Map) -> Self {
+        let param_env = tcx.param_env(body.source.def_id());
+        Self {
+            map,
+            tcx,
+            ecx: InterpCx::new(tcx, DUMMY_SP, param_env, DummyMachine),
+            param_env: param_env,
+        }
+    }
+
+    fn binary_op(
+        &self,
+        state: &mut State<FlatSet<ScalarTy<'tcx>>>,
+        op: BinOp,
+        left: &Operand<'tcx>,
+        right: &Operand<'tcx>,
+    ) -> (FlatSet<ScalarTy<'tcx>>, FlatSet<bool>) {
+        let left = self.eval_operand(left, state);
+        let right = self.eval_operand(right, state);
+        match (left, right) {
+            (FlatSet::Elem(left), FlatSet::Elem(right)) => {
+                match self.ecx.overflowing_binary_op(op, &left, &right) {
+                    Ok((val, overflow, ty)) => (self.wrap_scalar(val, ty), FlatSet::Elem(overflow)),
+                    _ => (FlatSet::Top, FlatSet::Top),
+                }
+            }
+            (FlatSet::Bottom, _) | (_, FlatSet::Bottom) => (FlatSet::Bottom, FlatSet::Bottom),
+            (_, _) => {
+                // Could attempt some algebraic simplifcations here.
+                (FlatSet::Top, FlatSet::Top)
+            }
+        }
+    }
+
+    fn eval_operand(
+        &self,
+        op: &Operand<'tcx>,
+        state: &mut State<FlatSet<ScalarTy<'tcx>>>,
+    ) -> FlatSet<ImmTy<'tcx>> {
+        let value = match self.handle_operand(op, state) {
+            ValueOrPlace::Value(value) => value,
+            ValueOrPlace::Place(place) => state.get_idx(place, &self.map),
+        };
+        match value {
+            FlatSet::Top => FlatSet::Top,
+            FlatSet::Elem(ScalarTy(scalar, ty)) => self
+                .tcx
+                .layout_of(self.param_env.and(ty))
+                .map(|layout| FlatSet::Elem(ImmTy::from_scalar(scalar, layout)))
+                .unwrap_or(FlatSet::Top),
+            FlatSet::Bottom => FlatSet::Bottom,
+        }
+    }
+
+    fn wrap_scalar(&self, scalar: Scalar, ty: Ty<'tcx>) -> FlatSet<ScalarTy<'tcx>> {
+        FlatSet::Elem(ScalarTy(scalar, ty))
+    }
+
+    fn wrap_immediate(&self, imm: Immediate, ty: Ty<'tcx>) -> FlatSet<ScalarTy<'tcx>> {
+        match imm {
+            Immediate::Scalar(scalar) => self.wrap_scalar(scalar, ty),
+            _ => FlatSet::Top,
+        }
+    }
+
+    fn wrap_immty(&self, val: ImmTy<'tcx>) -> FlatSet<ScalarTy<'tcx>> {
+        self.wrap_immediate(*val, val.layout.ty)
+    }
+}
+
+struct CollectAndPatch<'tcx, 'map> {
+    tcx: TyCtxt<'tcx>,
+    map: &'map Map,
+
+    /// For a given MIR location, this stores the values of the operands used by that location. In
+    /// particular, this is before the effect, such that the operands of `_1 = _1 + _2` are
+    /// properly captured. (This may become UB soon, but it is currently emitted even by safe code.)
+    before_effect: FxHashMap<(Location, Place<'tcx>), ScalarTy<'tcx>>,
+
+    /// Stores the assigned values for assignments where the Rvalue is constant.
+    assignments: FxHashMap<Location, ScalarTy<'tcx>>,
+}
+
+impl<'tcx, 'map> CollectAndPatch<'tcx, 'map> {
+    fn new(tcx: TyCtxt<'tcx>, map: &'map Map) -> Self {
+        Self { tcx, map, before_effect: FxHashMap::default(), assignments: FxHashMap::default() }
+    }
+
+    fn make_operand(&self, scalar: ScalarTy<'tcx>) -> Operand<'tcx> {
+        Operand::Constant(Box::new(Constant {
+            span: DUMMY_SP,
+            user_ty: None,
+            literal: ConstantKind::Val(ConstValue::Scalar(scalar.0), scalar.1),
+        }))
+    }
+}
+
+impl<'mir, 'tcx, 'map> ResultsVisitor<'mir, 'tcx> for CollectAndPatch<'tcx, 'map> {
+    type FlowState = State<FlatSet<ScalarTy<'tcx>>>;
+
+    fn visit_statement_before_primary_effect(
+        &mut self,
+        state: &Self::FlowState,
+        statement: &'mir Statement<'tcx>,
+        location: Location,
+    ) {
+        match &statement.kind {
+            StatementKind::Assign(box (_, rvalue)) => {
+                OperandCollector { state, visitor: self }.visit_rvalue(rvalue, location);
+            }
+            _ => (),
+        }
+    }
+
+    fn visit_statement_after_primary_effect(
+        &mut self,
+        state: &Self::FlowState,
+        statement: &'mir Statement<'tcx>,
+        location: Location,
+    ) {
+        match statement.kind {
+            StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(_)))) => {
+                // 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(), self.map) {
+                FlatSet::Top => (),
+                FlatSet::Elem(value) => {
+                    self.assignments.insert(location, value);
+                }
+                FlatSet::Bottom => {
+                    // This assignment is either unreachable, or an uninitialized value is assigned.
+                }
+            },
+            _ => (),
+        }
+    }
+
+    fn visit_terminator_before_primary_effect(
+        &mut self,
+        state: &Self::FlowState,
+        terminator: &'mir Terminator<'tcx>,
+        location: Location,
+    ) {
+        OperandCollector { state, visitor: self }.visit_terminator(terminator, location);
+    }
+}
+
+impl<'tcx, 'map> MutVisitor<'tcx> for CollectAndPatch<'tcx, 'map> {
+    fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
+        if let Some(value) = self.assignments.get(&location) {
+            match &mut statement.kind {
+                StatementKind::Assign(box (_, rvalue)) => {
+                    *rvalue = Rvalue::Use(self.make_operand(value.clone()));
+                }
+                _ => bug!("found assignment info for non-assign statement"),
+            }
+        } else {
+            self.super_statement(statement, location);
+        }
+    }
+
+    fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
+        match operand {
+            Operand::Copy(place) | Operand::Move(place) => {
+                if let Some(value) = self.before_effect.get(&(location, *place)) {
+                    *operand = self.make_operand(value.clone());
+                }
+            }
+            _ => (),
+        }
+    }
+}
+
+struct OperandCollector<'tcx, 'map, 'a> {
+    state: &'a State<FlatSet<ScalarTy<'tcx>>>,
+    visitor: &'a mut CollectAndPatch<'tcx, 'map>,
+}
+
+impl<'tcx, 'map, 'a> Visitor<'tcx> for OperandCollector<'tcx, 'map, 'a> {
+    fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
+        match operand {
+            Operand::Copy(place) | Operand::Move(place) => {
+                match self.state.get(place.as_ref(), self.visitor.map) {
+                    FlatSet::Top => (),
+                    FlatSet::Elem(value) => {
+                        self.visitor.before_effect.insert((location, *place), value);
+                    }
+                    FlatSet::Bottom => (),
+                }
+            }
+            _ => (),
+        }
+    }
+}
+
+struct DummyMachine;
+
+impl<'mir, 'tcx> rustc_const_eval::interpret::Machine<'mir, 'tcx> for DummyMachine {
+    rustc_const_eval::interpret::compile_time_machine!(<'mir, 'tcx>);
+    type MemoryKind = !;
+    const PANIC_ON_ALLOC_FAIL: bool = true;
+
+    fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        unimplemented!()
+    }
+
+    fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        unimplemented!()
+    }
+
+    fn find_mir_or_eval_fn(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _instance: ty::Instance<'tcx>,
+        _abi: rustc_target::spec::abi::Abi,
+        _args: &[rustc_const_eval::interpret::OpTy<'tcx, Self::Provenance>],
+        _destination: &rustc_const_eval::interpret::PlaceTy<'tcx, Self::Provenance>,
+        _target: Option<BasicBlock>,
+        _unwind: rustc_const_eval::interpret::StackPopUnwind,
+    ) -> interpret::InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
+        unimplemented!()
+    }
+
+    fn call_intrinsic(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _instance: ty::Instance<'tcx>,
+        _args: &[rustc_const_eval::interpret::OpTy<'tcx, Self::Provenance>],
+        _destination: &rustc_const_eval::interpret::PlaceTy<'tcx, Self::Provenance>,
+        _target: Option<BasicBlock>,
+        _unwind: rustc_const_eval::interpret::StackPopUnwind,
+    ) -> interpret::InterpResult<'tcx> {
+        unimplemented!()
+    }
+
+    fn assert_panic(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _msg: &rustc_middle::mir::AssertMessage<'tcx>,
+        _unwind: Option<BasicBlock>,
+    ) -> interpret::InterpResult<'tcx> {
+        unimplemented!()
+    }
+
+    fn binary_ptr_op(
+        _ecx: &InterpCx<'mir, 'tcx, Self>,
+        _bin_op: BinOp,
+        _left: &rustc_const_eval::interpret::ImmTy<'tcx, Self::Provenance>,
+        _right: &rustc_const_eval::interpret::ImmTy<'tcx, Self::Provenance>,
+    ) -> interpret::InterpResult<'tcx, (interpret::Scalar<Self::Provenance>, bool, Ty<'tcx>)> {
+        throw_unsup!(Unsupported("".into()))
+    }
+
+    fn expose_ptr(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _ptr: interpret::Pointer<Self::Provenance>,
+    ) -> interpret::InterpResult<'tcx> {
+        unimplemented!()
+    }
+
+    fn init_frame_extra(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _frame: rustc_const_eval::interpret::Frame<'mir, 'tcx, Self::Provenance>,
+    ) -> interpret::InterpResult<
+        'tcx,
+        rustc_const_eval::interpret::Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>,
+    > {
+        unimplemented!()
+    }
+
+    fn stack<'a>(
+        _ecx: &'a InterpCx<'mir, 'tcx, Self>,
+    ) -> &'a [rustc_const_eval::interpret::Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>]
+    {
+        unimplemented!()
+    }
+
+    fn stack_mut<'a>(
+        _ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
+    ) -> &'a mut Vec<
+        rustc_const_eval::interpret::Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>,
+    > {
+        unimplemented!()
+    }
+}

compiler/rustc_mir_transform/src/lib.rs

@@ -54,6 +54,7 @@ mod const_goto;
 mod const_prop;
 mod const_prop_lint;
 mod coverage;
+mod dataflow_const_prop;
 mod dead_store_elimination;
 mod deaggregator;
 mod deduce_param_attrs;
@@ -569,6 +570,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             //
             // FIXME(#70073): This pass is responsible for both optimization as well as some lints.
             &const_prop::ConstProp,
+            &dataflow_const_prop::DataflowConstProp,
             //
             // Const-prop runs unconditionally, but doesn't mutate the MIR at mir-opt-level=0.
             &const_debuginfo::ConstDebugInfo,