reduce false positives of tail-expr-drop-order from consumed values

take 2

open up coroutines

tweak the wordings

the lint works up until 2021

We were missing one case, for ADTs, which was
causing `Result` to yield incorrect results.

only include field spans with significant types

deduplicate and eliminate field spans

switch to emit spans to impl Drops

Co-authored-by: Niko Matsakis <nikomat@amazon.com>

collect drops instead of taking liveness diff

apply some suggestions and add explantory notes

small fix on the cache

let the query recurse through coroutine

new suggestion format with extracted variable name

fine-tune the drop span and messages

bugfix on runtime borrows

tweak message wording

filter out ecosystem types earlier

apply suggestions

clippy

check lint level at session level

further restrict applicability of the lint

translate bid into nop for stable mir

detect cycle in type structure

compiler/rustc_mir_transform/src/coroutine.rs

@@ -1772,6 +1772,7 @@ impl<'tcx> Visitor<'tcx> for EnsureCoroutineFieldAssignmentsNeverAlias<'_> {
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
             | StatementKind::ConstEvalCounter
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => {}
         }
     }

compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs

@@ -97,6 +97,7 @@ fn filtered_statement_span(statement: &Statement<'_>) -> Option<Span> {
         StatementKind::StorageLive(_)
         | StatementKind::StorageDead(_)
         | StatementKind::ConstEvalCounter
+        | StatementKind::BackwardIncompatibleDropHint { .. }
         | StatementKind::Nop => None,
 
         // FIXME(#78546): MIR InstrumentCoverage - Can the source_info.span for `FakeRead`

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -186,7 +186,8 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             | StatementKind::FakeRead(..)
             | StatementKind::PlaceMention(..)
             | StatementKind::Coverage(..)
-            | StatementKind::AscribeUserType(..) => (),
+            | StatementKind::BackwardIncompatibleDropHint { .. }
+            | StatementKind::AscribeUserType(..) => {}
         }
     }
 

compiler/rustc_mir_transform/src/dead_store_elimination.rs

@@ -99,7 +99,8 @@ fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
                 | StatementKind::Intrinsic(_)
                 | StatementKind::ConstEvalCounter
                 | StatementKind::PlaceMention(_)
-                | StatementKind::Nop => (),
+                | StatementKind::BackwardIncompatibleDropHint { .. }
+                | StatementKind::Nop => {}
 
                 StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
                     bug!("{:?} not found in this MIR phase!", statement.kind)

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -581,7 +581,7 @@ impl WriteInfo {
                     | Rvalue::RawPtr(_, _)
                     | Rvalue::Len(_)
                     | Rvalue::Discriminant(_)
-                    | Rvalue::CopyForDeref(_) => (),
+                    | Rvalue::CopyForDeref(_) => {}
                 }
             }
             // Retags are technically also reads, but reporting them as a write suffices
@@ -596,7 +596,8 @@ impl WriteInfo {
             | StatementKind::Coverage(_)
             | StatementKind::StorageLive(_)
             | StatementKind::StorageDead(_)
-            | StatementKind::PlaceMention(_) => (),
+            | StatementKind::BackwardIncompatibleDropHint { .. }
+            | StatementKind::PlaceMention(_) => {}
             StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
                 bug!("{:?} not found in this MIR phase", statement)
             }

compiler/rustc_mir_transform/src/jump_threading.rs

@@ -352,6 +352,7 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
             | StatementKind::FakeRead(..)
             | StatementKind::ConstEvalCounter
             | StatementKind::PlaceMention(..)
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => None,
         }
     }

compiler/rustc_mir_transform/src/lib.rs

@@ -50,6 +50,7 @@ mod deduce_param_attrs;
 mod errors;
 mod ffi_unwind_calls;
 mod lint;
+mod lint_tail_expr_drop_order;
 mod shim;
 mod ssa;
 
@@ -490,6 +491,7 @@ fn mir_drops_elaborated_and_const_checked(tcx: TyCtxt<'_>, def: LocalDefId) -> &
     }
 
     let (body, _) = tcx.mir_promoted(def);
+    lint_tail_expr_drop_order::run_lint(tcx, def, &body.borrow());
     let mut body = body.steal();
 
     if let Some(error_reported) = tainted_by_errors {

compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs

@@ -0,0 +1,701 @@
+use std::cell::RefCell;
+use std::collections::hash_map;
+use std::rc::Rc;
+
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::unord::{UnordMap, UnordSet};
+use rustc_errors::Subdiagnostic;
+use rustc_hir::CRATE_HIR_ID;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_index::bit_set::ChunkedBitSet;
+use rustc_index::{IndexSlice, IndexVec};
+use rustc_macros::{LintDiagnostic, Subdiagnostic};
+use rustc_middle::bug;
+use rustc_middle::mir::{
+    self, BasicBlock, Body, ClearCrossCrate, Local, Location, Place, StatementKind, TerminatorKind,
+    dump_mir,
+};
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_mir_dataflow::impls::MaybeInitializedPlaces;
+use rustc_mir_dataflow::move_paths::{LookupResult, MoveData, MovePathIndex};
+use rustc_mir_dataflow::{Analysis, MaybeReachable, ResultsCursor};
+use rustc_session::lint::builtin::TAIL_EXPR_DROP_ORDER;
+use rustc_session::lint::{self};
+use rustc_span::{DUMMY_SP, Span, Symbol};
+use rustc_type_ir::data_structures::IndexMap;
+use smallvec::{SmallVec, smallvec};
+use tracing::{debug, instrument};
+
+fn place_has_common_prefix<'tcx>(left: &Place<'tcx>, right: &Place<'tcx>) -> bool {
+    left.local == right.local
+        && left.projection.iter().zip(right.projection).all(|(left, right)| left == right)
+}
+
+/// Cache entry of `drop` at a `BasicBlock`
+#[derive(Debug, Clone, Copy)]
+enum MovePathIndexAtBlock {
+    /// We know nothing yet
+    Unknown,
+    /// We know that the `drop` here has no effect
+    None,
+    /// We know that the `drop` here will invoke a destructor
+    Some(MovePathIndex),
+}
+
+struct DropsReachable<'a, 'mir, 'tcx> {
+    body: &'a Body<'tcx>,
+    place: &'a Place<'tcx>,
+    drop_span: &'a mut Option<Span>,
+    move_data: &'a MoveData<'tcx>,
+    maybe_init: &'a mut ResultsCursor<'mir, 'tcx, MaybeInitializedPlaces<'mir, 'tcx>>,
+    block_drop_value_info: &'a mut IndexSlice<BasicBlock, MovePathIndexAtBlock>,
+    collected_drops: &'a mut ChunkedBitSet<MovePathIndex>,
+    visited: FxHashMap<BasicBlock, Rc<RefCell<ChunkedBitSet<MovePathIndex>>>>,
+}
+
+impl<'a, 'mir, 'tcx> DropsReachable<'a, 'mir, 'tcx> {
+    fn visit(&mut self, block: BasicBlock) {
+        let move_set_size = self.move_data.move_paths.len();
+        let make_new_path_set = || Rc::new(RefCell::new(ChunkedBitSet::new_empty(move_set_size)));
+
+        let data = &self.body.basic_blocks[block];
+        let Some(terminator) = &data.terminator else { return };
+        // Given that we observe these dropped locals here at `block` so far,
+        // we will try to update the successor blocks.
+        // An occupied entry at `block` in `self.visited` signals that we have visited `block` before.
+        let dropped_local_here =
+            Rc::clone(self.visited.entry(block).or_insert_with(make_new_path_set));
+        // We could have invoked reverse lookup for a `MovePathIndex` every time, but unfortunately it is expensive.
+        // Let's cache them in `self.block_drop_value_info`.
+        match self.block_drop_value_info[block] {
+            MovePathIndexAtBlock::Some(dropped) => {
+                dropped_local_here.borrow_mut().insert(dropped);
+            }
+            MovePathIndexAtBlock::Unknown => {
+                if let TerminatorKind::Drop { place, .. } = &terminator.kind
+                    && let LookupResult::Exact(idx) | LookupResult::Parent(Some(idx)) =
+                        self.move_data.rev_lookup.find(place.as_ref())
+                {
+                    // Since we are working with MIRs at a very early stage,
+                    // observing a `drop` terminator is not indicative enough that
+                    // the drop will definitely happen.
+                    // That is decided in the drop elaboration pass instead.
+                    // Therefore, we need to consult with the maybe-initialization information.
+                    self.maybe_init.seek_before_primary_effect(Location {
+                        block,
+                        statement_index: data.statements.len(),
+                    });
+
+                    // Check if the drop of `place` under inspection is really in effect.
+                    // This is true only when `place` may have been initialized along a control flow path from a BID to the drop program point today.
+                    // In other words, this is where the drop of `place` will happen in the future instead.
+                    if let MaybeReachable::Reachable(maybe_init) = self.maybe_init.get()
+                        && maybe_init.contains(idx)
+                    {
+                        // We also cache the drop information, so that we do not need to check on data-flow cursor again
+                        self.block_drop_value_info[block] = MovePathIndexAtBlock::Some(idx);
+                        dropped_local_here.borrow_mut().insert(idx);
+                    } else {
+                        self.block_drop_value_info[block] = MovePathIndexAtBlock::None;
+                    }
+                }
+            }
+            MovePathIndexAtBlock::None => {}
+        }
+
+        for succ in terminator.successors() {
+            let target = &self.body.basic_blocks[succ];
+            if target.is_cleanup {
+                continue;
+            }
+
+            // As long as we are passing through a new block, or new dropped places to propagate,
+            // we will proceed with `succ`
+            let dropped_local_there = match self.visited.entry(succ) {
+                hash_map::Entry::Occupied(occupied_entry) => {
+                    if succ == block
+                        || !occupied_entry.get().borrow_mut().union(&*dropped_local_here.borrow())
+                    {
+                        // `succ` has been visited but no new drops observed so far,
+                        // so we can bail on `succ` until new drop information arrives
+                        continue;
+                    }
+                    Rc::clone(occupied_entry.get())
+                }
+                hash_map::Entry::Vacant(vacant_entry) => Rc::clone(
+                    vacant_entry.insert(Rc::new(RefCell::new(dropped_local_here.borrow().clone()))),
+                ),
+            };
+            if let Some(terminator) = &target.terminator
+                && let TerminatorKind::Drop {
+                    place: dropped_place,
+                    target: _,
+                    unwind: _,
+                    replace: _,
+                } = &terminator.kind
+                && place_has_common_prefix(dropped_place, self.place)
+            {
+                // We have now reached the current drop of the `place`.
+                // Let's check the observed dropped places in.
+                self.collected_drops.union(&*dropped_local_there.borrow());
+                if self.drop_span.is_none() {
+                    // FIXME(@dingxiangfei2009): it turns out that `self.body.source_scopes` are still a bit wonky.
+                    // There is a high chance that this span still points to a block rather than a statement semicolon.
+                    *self.drop_span = Some(terminator.source_info.span);
+                }
+                // Now we have discovered a simple control flow path from a future drop point
+                // to the current drop point.
+                // We will not continue from there.
+            } else {
+                self.visit(succ)
+            }
+        }
+    }
+}
+
+/// An additional filter to exclude well-known types from the ecosystem
+/// because their drops are trivial.
+/// This returns additional types to check if the drops are delegated to those.
+/// A typical example is `hashbrown::HashMap<K, V>`, whose drop is delegated to `K` and `V`.
+fn true_significant_drop_ty<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    ty: Ty<'tcx>,
+) -> Option<SmallVec<[Ty<'tcx>; 2]>> {
+    if let ty::Adt(def, args) = ty.kind() {
+        let mut did = def.did();
+        let mut name_rev = vec![];
+        loop {
+            let key = tcx.def_key(did);
+
+            match key.disambiguated_data.data {
+                rustc_hir::definitions::DefPathData::CrateRoot => {
+                    name_rev.push(tcx.crate_name(did.krate))
+                }
+                rustc_hir::definitions::DefPathData::TypeNs(symbol) => name_rev.push(symbol),
+                _ => return None,
+            }
+            if let Some(parent) = key.parent {
+                did = DefId { krate: did.krate, index: parent };
+            } else {
+                break;
+            }
+        }
+        let name_str: Vec<_> = name_rev.iter().rev().map(|x| x.as_str()).collect();
+        debug!(?name_str);
+        match name_str[..] {
+            // These are the types from Rust core ecosystem
+            ["sym" | "proc_macro2", ..]
+            | ["core" | "std", "task", "LocalWaker" | "Waker"]
+            | ["core" | "std", "task", "wake", "LocalWaker" | "Waker"] => Some(smallvec![]),
+            // These are important types from Rust ecosystem
+            ["tracing", "instrument", "Instrumented"] | ["bytes", "Bytes"] => Some(smallvec![]),
+            ["hashbrown", "raw", "RawTable" | "RawIntoIter"] => {
+                if let [ty, ..] = &***args
+                    && let Some(ty) = ty.as_type()
+                {
+                    Some(smallvec![ty])
+                } else {
+                    None
+                }
+            }
+            ["hashbrown", "raw", "RawDrain"] => {
+                if let [_, ty, ..] = &***args
+                    && let Some(ty) = ty.as_type()
+                {
+                    Some(smallvec![ty])
+                } else {
+                    None
+                }
+            }
+            _ => None,
+        }
+    } else {
+        None
+    }
+}
+
+/// Returns the list of types with a "potentially sigificant" that may be dropped
+/// by dropping a value of type `ty`.
+#[instrument(level = "debug", skip(tcx, param_env))]
+fn extract_component_raw<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    ty: Ty<'tcx>,
+    ty_seen: &mut UnordSet<Ty<'tcx>>,
+) -> SmallVec<[Ty<'tcx>; 4]> {
+    // Droppiness does not depend on regions, so let us erase them.
+    let ty = tcx
+        .try_normalize_erasing_regions(
+            ty::TypingEnv { param_env, typing_mode: ty::TypingMode::PostAnalysis },
+            ty,
+        )
+        .unwrap_or(ty);
+
+    let tys = tcx.list_significant_drop_tys(param_env.and(ty));
+    debug!(?ty, "components");
+    let mut out_tys = smallvec![];
+    for ty in tys {
+        if let Some(tys) = true_significant_drop_ty(tcx, ty) {
+            // Some types can be further opened up because the drop is simply delegated
+            for ty in tys {
+                if ty_seen.insert(ty) {
+                    out_tys.extend(extract_component_raw(tcx, param_env, ty, ty_seen));
+                }
+            }
+        } else {
+            if ty_seen.insert(ty) {
+                out_tys.push(ty);
+            }
+        }
+    }
+    out_tys
+}
+
+#[instrument(level = "debug", skip(tcx, param_env))]
+fn extract_component_with_significant_dtor<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    ty: Ty<'tcx>,
+) -> SmallVec<[Ty<'tcx>; 4]> {
+    let mut tys = extract_component_raw(tcx, param_env, ty, &mut Default::default());
+    let mut deduplicate = FxHashSet::default();
+    tys.retain(|oty| deduplicate.insert(*oty));
+    tys.into_iter().collect()
+}
+
+/// Extract the span of the custom destructor of a type
+/// especially the span of the `impl Drop` header or its entire block
+/// when we are working with current local crate.
+#[instrument(level = "debug", skip(tcx))]
+fn ty_dtor_span<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Option<Span> {
+    match ty.kind() {
+        ty::Bool
+        | ty::Char
+        | ty::Int(_)
+        | ty::Uint(_)
+        | ty::Float(_)
+        | ty::Error(_)
+        | ty::Str
+        | ty::Never
+        | ty::RawPtr(_, _)
+        | ty::Ref(_, _, _)
+        | ty::FnPtr(_, _)
+        | ty::Tuple(_)
+        | ty::Dynamic(_, _, _)
+        | ty::Alias(_, _)
+        | ty::Bound(_, _)
+        | ty::Pat(_, _)
+        | ty::Placeholder(_)
+        | ty::Infer(_)
+        | ty::Slice(_)
+        | ty::Array(_, _) => None,
+        ty::Adt(adt_def, _) => {
+            let did = adt_def.did();
+            let try_local_did_span = |did: DefId| {
+                if let Some(local) = did.as_local() {
+                    tcx.source_span(local)
+                } else {
+                    tcx.def_span(did)
+                }
+            };
+            let dtor = if let Some(dtor) = tcx.adt_destructor(did) {
+                dtor.did
+            } else if let Some(dtor) = tcx.adt_async_destructor(did) {
+                dtor.future
+            } else {
+                return Some(try_local_did_span(did));
+            };
+            let def_key = tcx.def_key(dtor);
+            let Some(parent_index) = def_key.parent else { return Some(try_local_did_span(dtor)) };
+            let parent_did = DefId { index: parent_index, krate: dtor.krate };
+            Some(try_local_did_span(parent_did))
+        }
+        ty::Coroutine(did, _)
+        | ty::CoroutineWitness(did, _)
+        | ty::CoroutineClosure(did, _)
+        | ty::Closure(did, _)
+        | ty::FnDef(did, _)
+        | ty::Foreign(did) => Some(tcx.def_span(did)),
+        ty::Param(_) => None,
+    }
+}
+
+/// Check if a moved place at `idx` is a part of a BID.
+/// The use of this check is that we will consider drops on these
+/// as a drop of the overall BID and, thus, we can exclude it from the diagnosis.
+fn place_descendent_of_bids<'tcx>(
+    mut idx: MovePathIndex,
+    move_data: &MoveData<'tcx>,
+    bids: &UnordSet<&Place<'tcx>>,
+) -> bool {
+    loop {
+        let path = &move_data.move_paths[idx];
+        if bids.contains(&path.place) {
+            return true;
+        }
+        if let Some(parent) = path.parent {
+            idx = parent;
+        } else {
+            return false;
+        }
+    }
+}
+
+/// The core of the lint `tail-expr-drop-order`
+pub(crate) fn run_lint<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId, body: &Body<'tcx>) {
+    if matches!(tcx.def_kind(def_id), rustc_hir::def::DefKind::SyntheticCoroutineBody) {
+        // A synthetic coroutine has no HIR body and it is enough to just analyse the original body
+        return;
+    }
+    if body.span.edition().at_least_rust_2024()
+        || tcx.lints_that_dont_need_to_run(()).contains(&lint::LintId::of(TAIL_EXPR_DROP_ORDER))
+    {
+        return;
+    }
+    // ## About BIDs in blocks ##
+    // Track the set of blocks that contain a backwards-incompatible drop (BID)
+    // and, for each block, the vector of locations.
+    //
+    // We group them per-block because they tend to scheduled in the same drop ladder block.
+    let mut bid_per_block = IndexMap::default();
+    let mut bid_places = UnordSet::new();
+    let param_env = tcx.param_env(def_id).with_reveal_all_normalized(tcx);
+    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 {
+                let ty = place.ty(body, tcx).ty;
+                if ty_dropped_components
+                    .entry(ty)
+                    .or_insert_with(|| extract_component_with_significant_dtor(tcx, param_env, ty))
+                    .is_empty()
+                {
+                    continue;
+                }
+                bid_per_block
+                    .entry(block)
+                    .or_insert(vec![])
+                    .push((Location { block, statement_index }, &**place));
+                bid_places.insert(&**place);
+            }
+        }
+    }
+    if bid_per_block.is_empty() {
+        return;
+    }
+
+    dump_mir(tcx, false, "lint_tail_expr_drop_order", &0 as _, body, |_, _| Ok(()));
+    let locals_with_user_names = collect_user_names(body);
+    let is_closure_like = tcx.is_closure_like(def_id.to_def_id());
+
+    // Compute the "maybe initialized" information for this body.
+    // When we encounter a DROP of some place P we only care
+    // about the drop if `P` may be initialized.
+    let move_data = MoveData::gather_moves(body, tcx, |_| true);
+    let maybe_init = MaybeInitializedPlaces::new(tcx, body, &move_data);
+    let mut maybe_init = maybe_init.iterate_to_fixpoint(tcx, body, None).into_results_cursor(body);
+    let mut block_drop_value_info =
+        IndexVec::from_elem_n(MovePathIndexAtBlock::Unknown, body.basic_blocks.len());
+    for (&block, candidates) in &bid_per_block {
+        // We will collect drops on locals on paths between BID points to their actual drop locations
+        // into `all_locals_dropped`.
+        let mut all_locals_dropped = ChunkedBitSet::new_empty(move_data.move_paths.len());
+        let mut drop_span = None;
+        for &(_, place) in candidates.iter() {
+            let mut collected_drops = ChunkedBitSet::new_empty(move_data.move_paths.len());
+            // ## On detecting change in relative drop order ##
+            // Iterate through each BID-containing block `block`.
+            // If the place `P` targeted by the BID is "maybe initialized",
+            // then search forward to find the actual `DROP(P)` point.
+            // Everything dropped between the BID and the actual drop point
+            // is something whose relative drop order will change.
+            DropsReachable {
+                body,
+                place,
+                drop_span: &mut drop_span,
+                move_data: &move_data,
+                maybe_init: &mut maybe_init,
+                block_drop_value_info: &mut block_drop_value_info,
+                collected_drops: &mut collected_drops,
+                visited: Default::default(),
+            }
+            .visit(block);
+            // Compute the set `all_locals_dropped` of local variables that are dropped
+            // after the BID point but before the current drop point.
+            //
+            // These are the variables whose drop impls will be reordered with respect
+            // to `place`.
+            all_locals_dropped.union(&collected_drops);
+        }
+
+        // We shall now exclude some local bindings for the following cases.
+        {
+            let mut to_exclude = ChunkedBitSet::new_empty(all_locals_dropped.domain_size());
+            // We will now do subtraction from the candidate dropped locals, because of the following reasons.
+            for path_idx in all_locals_dropped.iter() {
+                let move_path = &move_data.move_paths[path_idx];
+                let dropped_local = move_path.place.local;
+                // a) A return value _0 will eventually be used
+                // Example:
+                // fn f() -> Droppy {
+                //     let _x = Droppy;
+                //     Droppy
+                // }
+                // _0 holds the literal `Droppy` and rightfully `_x` has to be dropped first
+                if dropped_local == Local::ZERO {
+                    debug!(?dropped_local, "skip return value");
+                    to_exclude.insert(path_idx);
+                    continue;
+                }
+                // b) If we are analysing a closure, the captures are still dropped last.
+                // This is part of the closure capture lifetime contract.
+                // They are similar to the return value _0 with respect to lifetime rules.
+                if is_closure_like && matches!(dropped_local, ty::CAPTURE_STRUCT_LOCAL) {
+                    debug!(?dropped_local, "skip closure captures");
+                    to_exclude.insert(path_idx);
+                    continue;
+                }
+                // c) Sometimes we collect places that are projections into the BID locals,
+                // so they are considered dropped now.
+                // Example:
+                // struct NotVeryDroppy(Droppy);
+                // impl Drop for Droppy {..}
+                // fn f() -> NotVeryDroppy {
+                //    let x = NotVeryDroppy(droppy());
+                //    {
+                //        let y: Droppy = x.0;
+                //        NotVeryDroppy(y)
+                //    }
+                // }
+                // `y` takes `x.0`, which invalidates `x` as a complete `NotVeryDroppy`
+                // so there is no point in linting against `x` any more.
+                if place_descendent_of_bids(path_idx, &move_data, &bid_places) {
+                    debug!(?dropped_local, "skip descendent of bids");
+                    to_exclude.insert(path_idx);
+                    continue;
+                }
+                let observer_ty = move_path.place.ty(body, tcx).ty;
+                // d) The collected local has no custom destructor that passes our ecosystem filter.
+                if ty_dropped_components
+                    .entry(observer_ty)
+                    .or_insert_with(|| {
+                        extract_component_with_significant_dtor(tcx, param_env, observer_ty)
+                    })
+                    .is_empty()
+                {
+                    debug!(?dropped_local, "skip non-droppy types");
+                    to_exclude.insert(path_idx);
+                    continue;
+                }
+            }
+            // Suppose that all BIDs point into the same local,
+            // we can remove the this local from the observed drops,
+            // so that we can focus our diagnosis more on the others.
+            if candidates.iter().all(|&(_, place)| candidates[0].1.local == place.local) {
+                for path_idx in all_locals_dropped.iter() {
+                    if move_data.move_paths[path_idx].place.local == candidates[0].1.local {
+                        to_exclude.insert(path_idx);
+                    }
+                }
+            }
+            all_locals_dropped.subtract(&to_exclude);
+        }
+        if all_locals_dropped.is_empty() {
+            // No drop effect is observable, so let us move on.
+            continue;
+        }
+
+        // ## The final work to assemble the diagnosis ##
+        // First collect or generate fresh names for local variable bindings and temporary values.
+        let local_names = assign_observables_names(
+            all_locals_dropped
+                .iter()
+                .map(|path_idx| move_data.move_paths[path_idx].place.local)
+                .chain(candidates.iter().map(|(_, place)| place.local)),
+            &locals_with_user_names,
+        );
+
+        let mut lint_root = None;
+        let mut local_labels = vec![];
+        // We now collect the types with custom destructors.
+        for &(_, place) in candidates {
+            let linted_local_decl = &body.local_decls[place.local];
+            let Some(&(ref name, is_generated_name)) = local_names.get(&place.local) else {
+                bug!("a name should have been assigned")
+            };
+            let name = name.as_str();
+
+            if lint_root.is_none()
+                && let ClearCrossCrate::Set(data) =
+                    &body.source_scopes[linted_local_decl.source_info.scope].local_data
+            {
+                lint_root = Some(data.lint_root);
+            }
+
+            // Collect spans of the custom destructors.
+            let mut seen_dyn = false;
+            let destructors = ty_dropped_components
+                .get(&linted_local_decl.ty)
+                .unwrap()
+                .iter()
+                .filter_map(|&ty| {
+                    if let Some(span) = ty_dtor_span(tcx, ty) {
+                        Some(DestructorLabel { span, name, dtor_kind: "concrete" })
+                    } else if matches!(ty.kind(), ty::Dynamic(..)) {
+                        if seen_dyn {
+                            None
+                        } else {
+                            seen_dyn = true;
+                            Some(DestructorLabel { span: DUMMY_SP, name, dtor_kind: "dyn" })
+                        }
+                    } else {
+                        None
+                    }
+                })
+                .collect();
+            local_labels.push(LocalLabel {
+                span: linted_local_decl.source_info.span,
+                destructors,
+                name,
+                is_generated_name,
+                is_dropped_first_edition_2024: true,
+            });
+        }
+
+        // Similarly, custom destructors of the observed drops.
+        for path_idx in all_locals_dropped.iter() {
+            let place = &move_data.move_paths[path_idx].place;
+            // We are not using the type of the local because the drop may be partial.
+            let observer_ty = place.ty(body, tcx).ty;
+
+            let observer_local_decl = &body.local_decls[place.local];
+            let Some(&(ref name, is_generated_name)) = local_names.get(&place.local) else {
+                bug!("a name should have been assigned")
+            };
+            let name = name.as_str();
+
+            let mut seen_dyn = false;
+            let destructors = extract_component_with_significant_dtor(tcx, param_env, observer_ty)
+                .into_iter()
+                .filter_map(|ty| {
+                    if let Some(span) = ty_dtor_span(tcx, ty) {
+                        Some(DestructorLabel { span, name, dtor_kind: "concrete" })
+                    } else if matches!(ty.kind(), ty::Dynamic(..)) {
+                        if seen_dyn {
+                            None
+                        } else {
+                            seen_dyn = true;
+                            Some(DestructorLabel { span: DUMMY_SP, name, dtor_kind: "dyn" })
+                        }
+                    } else {
+                        None
+                    }
+                })
+                .collect();
+            local_labels.push(LocalLabel {
+                span: observer_local_decl.source_info.span,
+                destructors,
+                name,
+                is_generated_name,
+                is_dropped_first_edition_2024: false,
+            });
+        }
+
+        let span = local_labels[0].span;
+        tcx.emit_node_span_lint(
+            lint::builtin::TAIL_EXPR_DROP_ORDER,
+            lint_root.unwrap_or(CRATE_HIR_ID),
+            span,
+            TailExprDropOrderLint { local_labels, drop_span, _epilogue: () },
+        );
+    }
+}
+
+/// Extract binding names if available for diagnosis
+fn collect_user_names(body: &Body<'_>) -> IndexMap<Local, Symbol> {
+    let mut names = IndexMap::default();
+    for var_debug_info in &body.var_debug_info {
+        if let mir::VarDebugInfoContents::Place(place) = &var_debug_info.value
+            && let Some(local) = place.local_or_deref_local()
+        {
+            names.entry(local).or_insert(var_debug_info.name);
+        }
+    }
+    names
+}
+
+/// Assign names for anonymous or temporary values for diagnosis
+fn assign_observables_names(
+    locals: impl IntoIterator<Item = Local>,
+    user_names: &IndexMap<Local, Symbol>,
+) -> IndexMap<Local, (String, bool)> {
+    let mut names = IndexMap::default();
+    let mut assigned_names = FxHashSet::default();
+    let mut idx = 0u64;
+    let mut fresh_name = || {
+        idx += 1;
+        (format!("#{idx}"), true)
+    };
+    for local in locals {
+        let name = if let Some(name) = user_names.get(&local) {
+            let name = name.as_str();
+            if assigned_names.contains(name) { fresh_name() } else { (name.to_owned(), false) }
+        } else {
+            fresh_name()
+        };
+        assigned_names.insert(name.0.clone());
+        names.insert(local, name);
+    }
+    names
+}
+
+#[derive(LintDiagnostic)]
+#[diag(mir_transform_tail_expr_drop_order)]
+struct TailExprDropOrderLint<'a> {
+    #[subdiagnostic]
+    local_labels: Vec<LocalLabel<'a>>,
+    #[label(mir_transform_drop_location)]
+    drop_span: Option<Span>,
+    #[note(mir_transform_note_epilogue)]
+    _epilogue: (),
+}
+
+struct LocalLabel<'a> {
+    span: Span,
+    name: &'a str,
+    is_generated_name: bool,
+    is_dropped_first_edition_2024: bool,
+    destructors: Vec<DestructorLabel<'a>>,
+}
+
+/// A custom `Subdiagnostic` implementation so that the notes are delivered in a specific order
+impl Subdiagnostic for LocalLabel<'_> {
+    fn add_to_diag_with<
+        G: rustc_errors::EmissionGuarantee,
+        F: rustc_errors::SubdiagMessageOp<G>,
+    >(
+        self,
+        diag: &mut rustc_errors::Diag<'_, G>,
+        f: &F,
+    ) {
+        diag.arg("name", self.name);
+        diag.arg("is_generated_name", self.is_generated_name);
+        diag.arg("is_dropped_first_edition_2024", self.is_dropped_first_edition_2024);
+        let msg = f(diag, crate::fluent_generated::mir_transform_tail_expr_local.into());
+        diag.span_label(self.span, msg);
+        for dtor in self.destructors {
+            dtor.add_to_diag_with(diag, f);
+        }
+        let msg = f(diag, crate::fluent_generated::mir_transform_label_local_epilogue.into());
+        diag.span_label(self.span, msg);
+    }
+}
+
+#[derive(Subdiagnostic)]
+#[note(mir_transform_tail_expr_dtor)]
+struct DestructorLabel<'a> {
+    #[primary_span]
+    span: Span,
+    dtor_kind: &'static str,
+    name: &'a str,
+}

compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs

@@ -92,6 +92,7 @@ impl RemoveNoopLandingPads {
                 | StatementKind::AscribeUserType(..)
                 | StatementKind::Coverage(..)
                 | StatementKind::ConstEvalCounter
+                | StatementKind::BackwardIncompatibleDropHint { .. }
                 | StatementKind::Nop => {
                     // These are all noops in a landing pad
                 }

compiler/rustc_mir_transform/src/remove_zsts.rs

@@ -125,6 +125,7 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'_, 'tcx> {
             StatementKind::Coverage(_)
             | StatementKind::Intrinsic(_)
             | StatementKind::Nop
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::ConstEvalCounter => None,
         };
         if let Some(place_for_ty) = place_for_ty

compiler/rustc_mir_transform/src/simplify.rs

@@ -523,7 +523,8 @@ impl<'tcx> Visitor<'tcx> for UsedLocals {
             }
 
             StatementKind::SetDiscriminant { ref place, variant_index: _ }
-            | StatementKind::Deinit(ref place) => {
+            | StatementKind::Deinit(ref place)
+            | StatementKind::BackwardIncompatibleDropHint { ref place, reason: _ } => {
                 self.visit_lhs(place, location);
             }
         }
@@ -560,6 +561,7 @@ fn remove_unused_definitions_helper(used_locals: &mut UsedLocals, body: &mut Bod
                     StatementKind::Assign(box (place, _)) => used_locals.is_used(place.local),
 
                     StatementKind::SetDiscriminant { ref place, .. }
+                    | StatementKind::BackwardIncompatibleDropHint { ref place, reason: _ }
                     | StatementKind::Deinit(ref place) => used_locals.is_used(place.local),
                     StatementKind::Nop => false,
                     _ => true,
@@ -587,6 +589,20 @@ impl<'tcx> MutVisitor<'tcx> for LocalUpdater<'tcx> {
         self.tcx
     }
 
+    fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
+        if let StatementKind::BackwardIncompatibleDropHint { place, reason: _ } =
+            &mut statement.kind
+        {
+            self.visit_local(
+                &mut place.local,
+                PlaceContext::MutatingUse(MutatingUseContext::Store),
+                location,
+            );
+        } else {
+            self.super_statement(statement, location);
+        }
+    }
+
     fn visit_local(&mut self, l: &mut Local, _: PlaceContext, _: Location) {
         *l = self.map[*l].unwrap();
     }

compiler/rustc_mir_transform/src/validate.rs

@@ -343,6 +343,7 @@ impl<'a, 'tcx> Visitor<'tcx> for CfgChecker<'a, 'tcx> {
             | StatementKind::Intrinsic(_)
             | StatementKind::ConstEvalCounter
             | StatementKind::PlaceMention(..)
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => {}
         }
 
@@ -1493,6 +1494,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
             | StatementKind::Coverage(_)
             | StatementKind::ConstEvalCounter
             | StatementKind::PlaceMention(..)
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => {}
         }