Actually keep PlaceMention if requested.

compiler/rustc_const_eval/src/transform/validate.rs

@@ -757,14 +757,6 @@ 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(
@@ -874,6 +866,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
             StatementKind::StorageDead(_)
             | StatementKind::Coverage(_)
             | StatementKind::ConstEvalCounter
+            | StatementKind::PlaceMention(..)
             | StatementKind::Nop => {}
         }
 

compiler/rustc_interface/src/tests.rs

@@ -768,6 +768,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(merge_functions, Some(MergeFunctions::Disabled));
     tracked!(mir_emit_retag, true);
     tracked!(mir_enable_passes, vec![("DestProp".to_string(), false)]);
+    tracked!(mir_keep_place_mention, true);
     tracked!(mir_opt_level, Some(4));
     tracked!(move_size_limit, Some(4096));
     tracked!(mutable_noalias, false);

compiler/rustc_middle/src/mir/syntax.rs

@@ -333,8 +333,6 @@ pub enum StatementKind<'tcx> {
     ///
     /// When executed at runtime, this computes the given place, but then discards
     /// it without doing a load. It is UB if the place is not pointing to live memory.
-    ///
-    /// Disallowed after drop elaboration.
     PlaceMention(Box<Place<'tcx>>),
 
     /// Encodes a user's type ascription. These need to be preserved

compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs

@@ -24,7 +24,6 @@ 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/dead_store_elimination.rs

@@ -54,11 +54,10 @@ pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, borrowed: &BitS
                 | StatementKind::Coverage(_)
                 | StatementKind::Intrinsic(_)
                 | StatementKind::ConstEvalCounter
+                | StatementKind::PlaceMention(_)
                 | StatementKind::Nop => (),
 
-                StatementKind::FakeRead(_)
+                StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
-                | StatementKind::PlaceMention(_)
-                | StatementKind::AscribeUserType(_, _) => {
                     bug!("{:?} not found in this MIR phase!", &statement.kind)
                 }
             }

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -582,10 +582,9 @@ impl WriteInfo {
             | StatementKind::Nop
             | StatementKind::Coverage(_)
             | StatementKind::StorageLive(_)
-            | StatementKind::StorageDead(_) => (),
+            | StatementKind::StorageDead(_)
-            StatementKind::FakeRead(_)
+            | StatementKind::PlaceMention(_) => (),
-            | StatementKind::AscribeUserType(_, _)
+            StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
-            | StatementKind::PlaceMention(_) => {
                 bug!("{:?} not found in this MIR phase", statement)
             }
         }

compiler/rustc_mir_transform/src/lib.rs

@@ -48,6 +48,7 @@ mod add_retag;
 mod check_const_item_mutation;
 mod check_packed_ref;
 pub mod check_unsafety;
+mod remove_place_mention;
 // This pass is public to allow external drivers to perform MIR cleanup
 pub mod cleanup_post_borrowck;
 mod const_debuginfo;
@@ -460,8 +461,11 @@ fn run_runtime_lowering_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 
 /// Returns the sequence of passes that do the initial cleanup of runtime MIR.
 fn run_runtime_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
-    let passes: &[&dyn MirPass<'tcx>] =
+    let passes: &[&dyn MirPass<'tcx>] = &[
-        &[&lower_intrinsics::LowerIntrinsics, &simplify::SimplifyCfg::ElaborateDrops];
+        &lower_intrinsics::LowerIntrinsics,
+        &remove_place_mention::RemovePlaceMention,
+        &simplify::SimplifyCfg::ElaborateDrops,
+    ];
 
     pm::run_passes(tcx, body, passes, Some(MirPhase::Runtime(RuntimePhase::PostCleanup)));
 

compiler/rustc_mir_transform/src/remove_place_mention.rs

@@ -0,0 +1,23 @@
+//! This pass removes `PlaceMention` statement, which has no effect at codegen.
+
+use crate::MirPass;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+
+pub struct RemovePlaceMention;
+
+impl<'tcx> MirPass<'tcx> for RemovePlaceMention {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        !sess.opts.unstable_opts.mir_keep_place_mention
+    }
+
+    fn run_pass(&self, _: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        trace!("Running RemovePlaceMention on {:?}", body.source);
+        for data in body.basic_blocks.as_mut_preserves_cfg() {
+            data.statements.retain(|statement| match statement.kind {
+                StatementKind::PlaceMention(..) | StatementKind::Nop => false,
+                _ => true,
+            })
+        }
+    }
+}

compiler/rustc_session/src/options.rs

@@ -1558,6 +1558,9 @@ options! {
         "use like `-Zmir-enable-passes=+DestProp,-InstCombine`. Forces the specified passes to be \
         enabled, overriding all other checks. Passes that are not specified are enabled or \
         disabled by other flags as usual."),
+    mir_keep_place_mention: bool = (false, parse_bool, [TRACKED],
+        "keep place mention MIR statements, interpreted e.g., by miri; implies -Zmir-opt-level=0 \
+        (default: no)"),
     #[rustc_lint_opt_deny_field_access("use `Session::mir_opt_level` instead of this field")]
     mir_opt_level: Option<usize> = (None, parse_opt_number, [TRACKED],
         "MIR optimization level (0-4; default: 1 in non optimized builds and 2 in optimized builds)"),

src/tools/miri/src/lib.rs

@@ -130,6 +130,7 @@ pub const MIRI_DEFAULT_ARGS: &[&str] = &[
     "-Zalways-encode-mir",
     "-Zextra-const-ub-checks",
     "-Zmir-emit-retag",
+    "-Zmir-keep-place-mention",
     "-Zmir-opt-level=0",
     "-Zmir-enable-passes=-CheckAlignment",
 ];

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_deref_underscore.rs

@@ -6,6 +6,8 @@ fn main() {
         let b = Box::new(42);
         &*b as *const i32
     };
-    let _ = unsafe { *p }; //~ ERROR: dereferenced after this allocation got freed
+    unsafe {
+        let _ = *p; //~ ERROR: dereferenced after this allocation got freed
+    }
     panic!("this should never print");
 }

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_deref_underscore.stderr

@@ -1,7 +1,7 @@
 error: Undefined Behavior: pointer to ALLOC was dereferenced after this allocation got freed
   --> $DIR/dangling_pointer_deref_underscore.rs:LL:CC
    |
-LL |     let _ = unsafe { *p };
+LL |         let _ = *p;
    |                 ^^ pointer to ALLOC was dereferenced after this allocation got freed
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior