Address unwind drop order review feedback

Author: sladyn98

compiler/rustc_borrowck/src/polonius/legacy/loan_invalidations.rs

@@ -70,7 +70,9 @@ impl<'a, 'tcx> Visitor<'tcx> for LoanInvalidationsGenerator<'a, 'tcx> {
             // Doesn't have any language semantics
             | StatementKind::Coverage(..)
             // Does not actually affect borrowck
-            | StatementKind::StorageLive(..) => {}
+            | StatementKind::StorageLive(..)
+            // Future-compatibility drop hints are lint markers, not loan invalidations.
+            | StatementKind::BackwardIncompatibleDropHint { .. } => {}
             StatementKind::StorageDead(local) => {
                 self.access_place(
                     location,
@@ -81,7 +83,6 @@ impl<'a, 'tcx> Visitor<'tcx> for LoanInvalidationsGenerator<'a, 'tcx> {
             }
             StatementKind::ConstEvalCounter
             | StatementKind::Nop
-            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::SetDiscriminant { .. } => {
                 bug!("Statement not allowed in this MIR phase")
             }

compiler/rustc_middle/src/mir/pretty.rs

@@ -879,10 +879,8 @@ impl Debug for StatementKind<'_> {
             Intrinsic(ref intrinsic) => write!(fmt, "{intrinsic}"),
             ConstEvalCounter => write!(fmt, "ConstEvalCounter"),
             Nop => write!(fmt, "nop"),
-            BackwardIncompatibleDropHint { ref place, reason: _ } => {
-                // The reason is intentionally omitted to keep MIR diffs stable
-                // across future-compatibility lint implementation details.
-                write!(fmt, "BackwardIncompatibleDropHint({place:?})")
+            BackwardIncompatibleDropHint { ref place, reason } => {
+                write!(fmt, "BackwardIncompatibleDropHint({place:?}, {reason:?})")
             }
         }
     }

compiler/rustc_middle/src/mir/syntax.rs

@@ -445,8 +445,7 @@ pub enum StatementKind<'tcx> {
     /// Marker statement indicating where `place` would be dropped.
     /// This is semantically equivalent to `Nop`, so codegen and MIRI should interpret this
     /// statement as such.
-    /// The only use case of this statement is for linting in MIR to detect temporary lifetime
-    /// changes.
+    /// The only use case of this statement is for MIR future-compatibility linting.
     BackwardIncompatibleDropHint {
         /// Place to drop
         place: Box<Place<'tcx>>,

compiler/rustc_mir_build/src/builder/scope.rs

@@ -207,7 +207,11 @@ pub(crate) fn run_lint<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId, body: &Body<
     let mut ty_dropped_components = UnordMap::default();
     for (block, data) in body.basic_blocks.iter_enumerated() {
         for (statement_index, stmt) in data.statements.iter().enumerate() {
-            if let StatementKind::BackwardIncompatibleDropHint { place, reason: _ } = &stmt.kind {
+            if let StatementKind::BackwardIncompatibleDropHint {
+                place,
+                reason: mir::BackwardIncompatibleDropReason::Edition2024,
+            } = &stmt.kind
+            {
                 let ty = place.ty(body, tcx).ty;
                 if ty_dropped_components
                     .entry(ty)

compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs

@@ -14,8 +14,8 @@ LL |     let local = 0;
 help: ALLOC was deallocated here:
   --> tests/fail/tail_calls/dangling-local-var.rs:LL:CC
    |
-LL |     let _val = unsafe { *x };
-   |         ^^^^
+LL |     become g(ptr)
+   |                 ^
    = note: stack backtrace:
            0: g
                at tests/fail/tail_calls/dangling-local-var.rs:LL:CC

src/tools/miri/tests/fail/tail_calls/dangling-local-var.stderr

@@ -73,19 +73,19 @@ fn main() -> () {
     }
 
     bb6 (cleanup): {
-        StorageDead(_6);
+        BackwardIncompatibleDropHint(_6, UnwindStorageDead);
         drop(_5) -> [return: bb7, unwind terminate(cleanup)];
     }
 
     bb7 (cleanup): {
-        StorageDead(_5);
+        BackwardIncompatibleDropHint(_5, UnwindStorageDead);
         drop(_4) -> [return: bb8, unwind terminate(cleanup)];
     }
 
     bb8 (cleanup): {
-        StorageDead(_4);
-        StorageDead(_2);
-        StorageDead(_1);
+        BackwardIncompatibleDropHint(_4, UnwindStorageDead);
+        BackwardIncompatibleDropHint(_2, UnwindStorageDead);
+        BackwardIncompatibleDropHint(_1, UnwindStorageDead);
         resume;
     }
 }

tests/mir-opt/basic_assignment.main.SimplifyCfg-initial.after.mir

@@ -48,6 +48,10 @@ fn _f(_1: !, _2: !) -> () {
     }
 
     bb5 (cleanup): {
+        BackwardIncompatibleDropHint(_7, UnwindStorageDead);
+        BackwardIncompatibleDropHint(_5, UnwindStorageDead);
+        BackwardIncompatibleDropHint(_8, UnwindStorageDead);
+        BackwardIncompatibleDropHint(_4, UnwindStorageDead);
         resume;
     }
 }

tests/mir-opt/building/eq_never_type._f.built.after.mir

@@ -26,6 +26,8 @@ fn index_array(_1: &[i32; 7], _2: usize) -> &i32 {
     }
 
     bb2 (cleanup): {
+        BackwardIncompatibleDropHint(_4, UnwindStorageDead);
+        BackwardIncompatibleDropHint(_3, UnwindStorageDead);
         resume;
     }
 }

tests/mir-opt/building/index_array_and_slice.index_array.built.after.mir

@@ -26,6 +26,8 @@ fn index_const_generic_array(_1: &[i32; N], _2: usize) -> &i32 {
     }
 
     bb2 (cleanup): {
+        BackwardIncompatibleDropHint(_4, UnwindStorageDead);
+        BackwardIncompatibleDropHint(_3, UnwindStorageDead);
         resume;
     }
 }