Introduce a no-op PlaceMention statement for let _ =.

compiler/rustc_borrowck/src/dataflow.rs

@@ -390,6 +390,7 @@ impl<'tcx> rustc_mir_dataflow::GenKillAnalysis<'tcx> for Borrows<'_, 'tcx> {
  | mir::StatementKind::Deinit(..)
  | mir::StatementKind::StorageLive(..)
  | mir::StatementKind::Retag { .. }
+ | mir::StatementKind::PlaceMention(..)
  | mir::StatementKind::AscribeUserType(..)
  | mir::StatementKind::Coverage(..)
  | mir::StatementKind::Intrinsic(..)

compiler/rustc_borrowck/src/def_use.rs

@@ -72,6 +72,8 @@ pub fn categorize(context: PlaceContext) -> Option<DefUse> {
  PlaceContext::MutatingUse(MutatingUseContext::Drop) =>
  Some(DefUse::Drop),
 
+ // This statement exists to help unsafeck. It does not require the place to be live.
+ PlaceContext::NonUse(NonUseContext::PlaceMention) => None,
  // Debug info is neither def nor use.
  PlaceContext::NonUse(NonUseContext::VarDebugInfo) => None,
 

compiler/rustc_borrowck/src/invalidation.rs

@@ -79,6 +79,8 @@ impl<'cx, 'tcx> Visitor<'tcx> for InvalidationGenerator<'cx, 'tcx> {
  }
  // Only relevant for mir typeck
  StatementKind::AscribeUserType(..)
+ // Only relevant for unsafeck
+ | StatementKind::PlaceMention(..)
  // Doesn't have any language semantics
  | StatementKind::Coverage(..)
  // Does not actually affect borrowck

compiler/rustc_borrowck/src/lib.rs

@@ -690,6 +690,8 @@ impl<'cx, 'tcx> rustc_mir_dataflow::ResultsVisitor<'cx, 'tcx> for MirBorrowckCtx
  }
  // Only relevant for mir typeck
  StatementKind::AscribeUserType(..)
+ // Only relevant for unsafeck
+ | StatementKind::PlaceMention(..)
  // Doesn't have any language semantics
  | StatementKind::Coverage(..)
  // These do not actually affect borrowck

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -772,7 +772,9 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
 
  match context {
  PlaceContext::MutatingUse(_) => ty::Invariant,
- PlaceContext::NonUse(StorageDead | StorageLive | VarDebugInfo) => ty::Invariant,
+ PlaceContext::NonUse(StorageDead | StorageLive | PlaceMention | VarDebugInfo) => {
+ ty::Invariant
+ }
  PlaceContext::NonMutatingUse(
  Inspect | Copy | Move | SharedBorrow | ShallowBorrow | UniqueBorrow | AddressOf
  | Projection,
@@ -1282,6 +1284,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
  | StatementKind::Retag { .. }
  | StatementKind::Coverage(..)
  | StatementKind::ConstEvalCounter
+ | StatementKind::PlaceMention(..)
  | StatementKind::Nop => {}
  StatementKind::Deinit(..) | StatementKind::SetDiscriminant { .. } => {
  bug!("Statement not allowed in this MIR phase")

compiler/rustc_codegen_cranelift/src/base.rs

@@ -819,6 +819,7 @@ fn codegen_stmt<'tcx>(
  | StatementKind::Nop
  | StatementKind::FakeRead(..)
  | StatementKind::Retag { .. }
+ | StatementKind::PlaceMention(..)
  | StatementKind::AscribeUserType(..) => {}
 
  StatementKind::Coverage { .. } => fx.tcx.sess.fatal("-Zcoverage is unimplemented"),

compiler/rustc_codegen_cranelift/src/constant.rs

@@ -529,6 +529,7 @@ pub(crate) fn mir_operand_get_const_val<'tcx>(
  | StatementKind::StorageDead(_)
  | StatementKind::Retag(_, _)
  | StatementKind::AscribeUserType(_, _)
+ | StatementKind::PlaceMention(..)
  | StatementKind::Coverage(_)
  | StatementKind::ConstEvalCounter
  | StatementKind::Nop => {}

compiler/rustc_codegen_ssa/src/mir/statement.rs

@@ -92,6 +92,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
  | mir::StatementKind::Retag { .. }
  | mir::StatementKind::AscribeUserType(..)
  | mir::StatementKind::ConstEvalCounter
+ | mir::StatementKind::PlaceMention(..)
  | mir::StatementKind::Nop => {}
  }
  }

compiler/rustc_const_eval/src/interpret/step.rs

@@ -114,7 +114,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
  Intrinsic(box intrinsic) => self.emulate_nondiverging_intrinsic(intrinsic)?,
 
  // Statements we do not track.
- AscribeUserType(..) => {}
+ PlaceMention(..) | AscribeUserType(..) => {}
 
  // Currently, Miri discards Coverage statements. Coverage statements are only injected
  // via an optional compile time MIR pass and have no side effects. Since Coverage

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -690,6 +690,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
  | StatementKind::StorageLive(_)
  | StatementKind::StorageDead(_)
  | StatementKind::Retag { .. }
+ | StatementKind::PlaceMention(..)
  | StatementKind::AscribeUserType(..)
  | StatementKind::Coverage(..)
  | StatementKind::Intrinsic(..)

compiler/rustc_const_eval/src/transform/validate.rs

@@ -679,6 +679,14 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
  }
  }
  }
+ StatementKind::PlaceMention(..) => {
+ if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
+ self.fail(
+ location,
+ "`PlaceMention` should have been removed after drop lowering phase",
+ );
+ }
+ }
  StatementKind::AscribeUserType(..) => {
  if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
  self.fail(

compiler/rustc_middle/src/mir/mod.rs

@@ -1453,6 +1453,9 @@ impl Debug for Statement<'_> {
  write!(fmt, "discriminant({:?}) = {:?}", place, variant_index)
  }
  Deinit(ref place) => write!(fmt, "Deinit({:?})", place),
+ PlaceMention(ref place) => {
+ write!(fmt, "PlaceMention({:?})", place)
+ }
  AscribeUserType(box (ref place, ref c_ty), ref variance) => {
  write!(fmt, "AscribeUserType({:?}, {:?}, {:?})", place, variance, c_ty)
  }

compiler/rustc_middle/src/mir/spanview.rs

@@ -247,6 +247,7 @@ pub fn statement_kind_name(statement: &Statement<'_>) -> &'static str {
  StorageLive(..) => "StorageLive",
  StorageDead(..) => "StorageDead",
  Retag(..) => "Retag",
+ PlaceMention(..) => "PlaceMention",
  AscribeUserType(..) => "AscribeUserType",
  Coverage(..) => "Coverage",
  Intrinsic(..) => "Intrinsic",

compiler/rustc_middle/src/mir/syntax.rs

@@ -325,6 +325,15 @@ pub enum StatementKind<'tcx> {
  /// Only `RetagKind::Default` and `RetagKind::FnEntry` are permitted.
  Retag(RetagKind, Box<Place<'tcx>>),
 
+ /// This statement exists to preserve a trace of a scrutinee matched against a wildcard binding.
+ /// This is especially useful for `let _ = PLACE;` bindings that desugar to a single
+ /// `PlaceMention(PLACE)`.
+ ///
+ /// When executed at runtime this is a nop.
+ ///
+ /// Disallowed after drop elaboration.
+ PlaceMention(Box<Place<'tcx>>),
+
  /// Encodes a user's type ascription. These need to be preserved
  /// intact so that NLL can respect them. For example:
  /// ```ignore (illustrative)

compiler/rustc_middle/src/mir/visit.rs

@@ -405,6 +405,13 @@ macro_rules! make_mir_visitor {
  StatementKind::Retag(kind, place) => {
  self.visit_retag($(& $mutability)? *kind, place, location);
  }
+ StatementKind::PlaceMention(place) => {
+ self.visit_place(
+ place,
+ PlaceContext::NonUse(NonUseContext::PlaceMention),
+ location
+ );
+ }
  StatementKind::AscribeUserType(
  box (place, user_ty),
  variance
@@ -1288,6 +1295,8 @@ pub enum NonUseContext {
  AscribeUserTy,
  /// The data of a user variable, for debug info.
  VarDebugInfo,
+ /// PlaceMention statement.
+ PlaceMention,
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]

compiler/rustc_mir_build/src/build/cfg.rs

@@ -90,6 +90,17 @@ impl<'tcx> CFG<'tcx> {
  self.push(block, stmt);
  }
 
+ pub(crate) fn push_place_mention(
+ &mut self,
+ block: BasicBlock,
+ source_info: SourceInfo,
+ place: Place<'tcx>,
+ ) {
+ let kind = StatementKind::PlaceMention(Box::new(place));
+ let stmt = Statement { source_info, kind };
+ self.push(block, stmt);
+ }
+
  pub(crate) fn terminate(
  &mut self,
  block: BasicBlock,

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -556,6 +556,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
  _ => {
  let place_builder = unpack!(block = self.as_place_builder(block, initializer));
+
+ if let Some(place) = place_builder.try_to_place(self) {
+ let source_info = self.source_info(initializer.span);
+ self.cfg.push_place_mention(block, source_info, place);
+ }
+
  self.place_into_pattern(block, &irrefutable_pat, place_builder, true)
  }
  }
@@ -576,6 +582,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
  false,
  &mut [&mut candidate],
  );
+
  // For matches and function arguments, the place that is being matched
  // can be set when creating the variables. But the place for
  // let PATTERN = ... might not even exist until we do the assignment.

compiler/rustc_mir_dataflow/src/impls/liveness.rs

@@ -263,6 +263,7 @@ impl<'a, 'tcx> Analysis<'tcx> for MaybeTransitiveLiveLocals<'a> {
  | StatementKind::StorageDead(_)
  | StatementKind::Retag(..)
  | StatementKind::AscribeUserType(..)
+ | StatementKind::PlaceMention(..)
  | StatementKind::Coverage(..)
  | StatementKind::Intrinsic(..)
  | StatementKind::ConstEvalCounter

compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs

@@ -139,6 +139,7 @@ impl<'mir, 'tcx> crate::GenKillAnalysis<'tcx> for MaybeRequiresStorage<'mir, 'tc
  // Nothing to do for these. Match exhaustively so this fails to compile when new
  // variants are added.
  StatementKind::AscribeUserType(..)
+ | StatementKind::PlaceMention(..)
  | StatementKind::Coverage(..)
  | StatementKind::FakeRead(..)
  | StatementKind::ConstEvalCounter

compiler/rustc_mir_dataflow/src/move_paths/builder.rs

@@ -329,6 +329,7 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
  }
  StatementKind::Retag { .. }
  | StatementKind::AscribeUserType(..)
+ | StatementKind::PlaceMention(..)
  | StatementKind::Coverage(..)
  | StatementKind::Intrinsic(..)
  | StatementKind::ConstEvalCounter

compiler/rustc_mir_dataflow/src/value_analysis.rs

@@ -86,6 +86,7 @@ pub trait ValueAnalysis<'tcx> {
  StatementKind::ConstEvalCounter
  | StatementKind::Nop
  | StatementKind::FakeRead(..)
+ | StatementKind::PlaceMention(..)
  | StatementKind::Coverage(..)
  | StatementKind::AscribeUserType(..) => (),
  }

compiler/rustc_mir_transform/src/check_unsafety.rs

@@ -100,13 +100,16 @@ impl<'tcx> Visitor<'tcx> for UnsafetyChecker<'_, 'tcx> {
  | StatementKind::StorageLive(..)
  | StatementKind::StorageDead(..)
  | StatementKind::Retag { .. }
- | StatementKind::AscribeUserType(..)
+ | StatementKind::PlaceMention(..)
  | StatementKind::Coverage(..)
  | StatementKind::Intrinsic(..)
  | StatementKind::ConstEvalCounter
  | StatementKind::Nop => {
  // safe (at least as emitted during MIR construction)
  }
+ // `AscribeUserType` just exists to help MIR borrowck.
+ // It has no semantics, and everything is already reported by `PlaceMention`.
+ StatementKind::AscribeUserType(..) => return,
  }
  self.super_statement(statement, location);
  }

compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs

@@ -24,6 +24,7 @@ impl<'tcx> MirPass<'tcx> for CleanupPostBorrowck {
  for statement in basic_block.statements.iter_mut() {
  match statement.kind {
  StatementKind::AscribeUserType(..)
+ | StatementKind::PlaceMention(..)
  | StatementKind::Assign(box (_, Rvalue::Ref(_, BorrowKind::Shallow, _)))
  | StatementKind::FakeRead(..) => statement.make_nop(),
  _ => (),

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -832,6 +832,7 @@ pub(super) fn filtered_statement_span(statement: &Statement<'_>) -> Option<Span>
  | StatementKind::SetDiscriminant { .. }
  | StatementKind::Deinit(..)
  | StatementKind::Retag(_, _)
+ | StatementKind::PlaceMention(..)
  | StatementKind::AscribeUserType(_, _) => {
  Some(statement.source_info.span)
  }

compiler/rustc_mir_transform/src/dead_store_elimination.rs

@@ -56,7 +56,9 @@ pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, borrowed: &BitS
  | StatementKind::ConstEvalCounter
  | StatementKind::Nop => (),
 
- StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
+ StatementKind::FakeRead(_)
+ | StatementKind::PlaceMention(_)
+ | StatementKind::AscribeUserType(_, _) => {
  bug!("{:?} not found in this MIR phase!", &statement.kind)
  }
  }

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -583,7 +583,9 @@ impl WriteInfo {
  | StatementKind::Coverage(_)
  | StatementKind::StorageLive(_)
  | StatementKind::StorageDead(_) => (),
- StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
+ StatementKind::FakeRead(_)
+ | StatementKind::AscribeUserType(_, _)
+ | StatementKind::PlaceMention(_) => {
  bug!("{:?} not found in this MIR phase", statement)
  }
  }

compiler/rustc_mir_transform/src/generator.rs

@@ -1647,6 +1647,7 @@ impl<'tcx> Visitor<'tcx> for EnsureGeneratorFieldAssignmentsNeverAlias<'_> {
  | StatementKind::StorageDead(_)
  | StatementKind::Retag(..)
  | StatementKind::AscribeUserType(..)
+ | StatementKind::PlaceMention(..)
  | StatementKind::Coverage(..)
  | StatementKind::Intrinsic(..)
  | StatementKind::ConstEvalCounter

compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs

@@ -33,6 +33,7 @@ impl RemoveNoopLandingPads {
  StatementKind::FakeRead(..)
  | StatementKind::StorageLive(_)
  | StatementKind::StorageDead(_)
+ | StatementKind::PlaceMention(..)
  | StatementKind::AscribeUserType(..)
  | StatementKind::Coverage(..)
  | StatementKind::ConstEvalCounter

compiler/rustc_mir_transform/src/separate_const_switch.rs

@@ -245,6 +245,7 @@ fn is_likely_const<'tcx>(mut tracked_place: Place<'tcx>, block: &BasicBlockData<
  | StatementKind::StorageLive(_)
  | StatementKind::Retag(_, _)
  | StatementKind::AscribeUserType(_, _)
+ | StatementKind::PlaceMention(..)
  | StatementKind::Coverage(_)
  | StatementKind::StorageDead(_)
  | StatementKind::Intrinsic(_)
@@ -315,6 +316,7 @@ fn find_determining_place<'tcx>(
  | StatementKind::StorageDead(_)
  | StatementKind::Retag(_, _)
  | StatementKind::AscribeUserType(_, _)
+ | StatementKind::PlaceMention(..)
  | StatementKind::Coverage(_)
  | StatementKind::Intrinsic(_)
  | StatementKind::ConstEvalCounter

compiler/rustc_mir_transform/src/simplify.rs

@@ -525,6 +525,7 @@ impl<'tcx> Visitor<'tcx> for UsedLocals {
  | StatementKind::Retag(..)
  | StatementKind::Coverage(..)
  | StatementKind::FakeRead(..)
+ | StatementKind::PlaceMention(..)
  | StatementKind::AscribeUserType(..) => {
  self.super_statement(statement, location);
  }

src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs

@@ -241,6 +241,7 @@ fn check_statement<'tcx>(
  | StatementKind::StorageDead(_)
  | StatementKind::Retag { .. }
  | StatementKind::AscribeUserType(..)
+ | StatementKind::PlaceMention(..)
  | StatementKind::Coverage(..)
  | StatementKind::ConstEvalCounter
  | StatementKind::Nop => Ok(()),

tests/mir-opt/building/match_false_edges.full_tested_match.built.after.mir

@@ -100,6 +100,7 @@ fn full_tested_match() -> () {
  }
 
  bb11: {
+ PlaceMention(_1); // scope 0 at $DIR/match_false_edges.rs:+1:13: +5:6
  StorageDead(_2); // scope 0 at $DIR/match_false_edges.rs:+5:6: +5:7
  StorageDead(_1); // scope 0 at $DIR/match_false_edges.rs:+5:6: +5:7
  _0 = const (); // scope 0 at $DIR/match_false_edges.rs:+0:28: +6:2

tests/mir-opt/building/match_false_edges.full_tested_match2.built.after.mir

@@ -100,6 +100,7 @@ fn full_tested_match2() -> () {
  }
 
  bb11: {
+ PlaceMention(_1); // scope 0 at $DIR/match_false_edges.rs:+1:13: +5:6
  StorageDead(_2); // scope 0 at $DIR/match_false_edges.rs:+5:6: +5:7
  StorageDead(_1); // scope 0 at $DIR/match_false_edges.rs:+5:6: +5:7
  _0 = const (); // scope 0 at $DIR/match_false_edges.rs:+0:29: +6:2

tests/mir-opt/building/match_false_edges.main.built.after.mir

@@ -162,6 +162,7 @@ fn main() -> () {
  }
 
  bb19: {
+ PlaceMention(_1); // scope 0 at $DIR/match_false_edges.rs:+1:13: +6:6
  StorageDead(_2); // scope 0 at $DIR/match_false_edges.rs:+6:6: +6:7
  StorageDead(_1); // scope 0 at $DIR/match_false_edges.rs:+6:6: +6:7
  _0 = const (); // scope 0 at $DIR/match_false_edges.rs:+0:11: +7:2

tests/mir-opt/building/uniform_array_move_out.move_out_by_subslice.built.after.mir

@@ -77,6 +77,7 @@ fn move_out_by_subslice() -> () {
  bb6: {
  StorageDead(_2); // scope 0 at $DIR/uniform_array_move_out.rs:+1:26: +1:27
  FakeRead(ForLet(None), _1); // scope 0 at $DIR/uniform_array_move_out.rs:+1:9: +1:10
+ PlaceMention(_1); // scope 1 at $DIR/uniform_array_move_out.rs:+2:21: +2:22
  StorageLive(_12); // scope 1 at $DIR/uniform_array_move_out.rs:+2:10: +2:12
  _12 = move _1[0..2]; // scope 1 at $DIR/uniform_array_move_out.rs:+2:10: +2:12
  _0 = const (); // scope 0 at $DIR/uniform_array_move_out.rs:+0:27: +3:2

tests/mir-opt/building/uniform_array_move_out.move_out_from_end.built.after.mir

@@ -77,6 +77,7 @@ fn move_out_from_end() -> () {
  bb6: {
  StorageDead(_2); // scope 0 at $DIR/uniform_array_move_out.rs:+1:26: +1:27
  FakeRead(ForLet(None), _1); // scope 0 at $DIR/uniform_array_move_out.rs:+1:9: +1:10
+ PlaceMention(_1); // scope 1 at $DIR/uniform_array_move_out.rs:+2:20: +2:21
  StorageLive(_12); // scope 1 at $DIR/uniform_array_move_out.rs:+2:14: +2:16
  _12 = move _1[1 of 2]; // scope 1 at $DIR/uniform_array_move_out.rs:+2:14: +2:16
  _0 = const (); // scope 0 at $DIR/uniform_array_move_out.rs:+0:24: +3:2

tests/mir-opt/issue_72181.main.built.after.mir

@@ -29,6 +29,7 @@ fn main() -> () {
  }
 
  bb1: {
+ PlaceMention(_1); // scope 0 at $DIR/issue_72181.rs:+1:13: +1:34
  StorageDead(_1); // scope 0 at $DIR/issue_72181.rs:+1:34: +1:35
  StorageLive(_2); // scope 1 at $DIR/issue_72181.rs:+3:9: +3:10
  StorageLive(_3); // scope 1 at $DIR/issue_72181.rs:+3:14: +3:27
@@ -49,6 +50,7 @@ fn main() -> () {
 
  bb2: {
  _5 = (_2[_6].0: u64); // scope 4 at $DIR/issue_72181.rs:+4:22: +4:28
+ PlaceMention(_5); // scope 2 at $DIR/issue_72181.rs:+4:13: +4:30
  StorageDead(_6); // scope 2 at $DIR/issue_72181.rs:+4:30: +4:31
  StorageDead(_5); // scope 2 at $DIR/issue_72181.rs:+4:30: +4:31
  _0 = const (); // scope 0 at $DIR/issue_72181.rs:+0:11: +5:2