Run borrowck tests on BIDs and emit tail-expr-drop-order lints for violations

compiler/rustc_borrowck/messages.ftl

@@ -213,6 +213,10 @@ borrowck_suggest_create_fresh_reborrow =
 borrowck_suggest_iterate_over_slice =
     consider iterating over a slice of the `{$ty}`'s content to avoid moving into the `for` loop
 
+borrowck_tail_expr_drop_order = relative drop order changing in Rust 2024
+    .label = this temporary value will be dropped at the end of the block
+    .note = consider using a `let` binding to ensure the value will live long enough
+
 borrowck_ty_no_impl_copy =
     {$is_partial_move ->
         [true] partial move

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -5,7 +5,7 @@
 
 use std::assert_matches::assert_matches;
 
-use rustc_errors::{Applicability, Diag};
+use rustc_errors::{Applicability, Diag, EmissionGuarantee};
 use rustc_hir as hir;
 use rustc_hir::intravisit::Visitor;
 use rustc_infer::infer::NllRegionVariableOrigin;
@@ -61,10 +61,10 @@ impl<'tcx> BorrowExplanation<'tcx> {
     pub(crate) fn is_explained(&self) -> bool {
         !matches!(self, BorrowExplanation::Unexplained)
     }
-    pub(crate) fn add_explanation_to_diagnostic(
+    pub(crate) fn add_explanation_to_diagnostic<G: EmissionGuarantee>(
         &self,
         cx: &MirBorrowckCtxt<'_, '_, 'tcx>,
-        err: &mut Diag<'_>,
+        err: &mut Diag<'_, G>,
         borrow_desc: &str,
         borrow_span: Option<Span>,
         multiple_borrow_span: Option<(Span, Span)>,
@@ -346,10 +346,10 @@ impl<'tcx> BorrowExplanation<'tcx> {
         }
     }
 
-    fn add_object_lifetime_default_note(
+    fn add_object_lifetime_default_note<G: EmissionGuarantee>(
         &self,
         tcx: TyCtxt<'tcx>,
-        err: &mut Diag<'_>,
+        err: &mut Diag<'_, G>,
         unsize_ty: Ty<'tcx>,
     ) {
         if let ty::Adt(def, args) = unsize_ty.kind() {
@@ -403,9 +403,9 @@ impl<'tcx> BorrowExplanation<'tcx> {
         }
     }
 
-    fn add_lifetime_bound_suggestion_to_diagnostic(
+    fn add_lifetime_bound_suggestion_to_diagnostic<G: EmissionGuarantee>(
         &self,
-        err: &mut Diag<'_>,
+        err: &mut Diag<'_, G>,
         category: &ConstraintCategory<'tcx>,
         span: Span,
         region_name: &RegionName,
@@ -432,14 +432,14 @@ impl<'tcx> BorrowExplanation<'tcx> {
     }
 }
 
-fn suggest_rewrite_if_let(
+fn suggest_rewrite_if_let<G: EmissionGuarantee>(
     tcx: TyCtxt<'_>,
     expr: &hir::Expr<'_>,
     pat: &str,
     init: &hir::Expr<'_>,
     conseq: &hir::Expr<'_>,
     alt: Option<&hir::Expr<'_>>,
-    err: &mut Diag<'_>,
+    err: &mut Diag<'_, G>,
 ) {
     let source_map = tcx.sess.source_map();
     err.span_note(

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -4,7 +4,7 @@ use std::collections::BTreeMap;
 
 use rustc_abi::{FieldIdx, VariantIdx};
 use rustc_data_structures::fx::FxIndexMap;
-use rustc_errors::{Applicability, Diag, MultiSpan};
+use rustc_errors::{Applicability, Diag, EmissionGuarantee, MultiSpan};
 use rustc_hir::def::{CtorKind, Namespace};
 use rustc_hir::{self as hir, CoroutineKind, LangItem};
 use rustc_index::IndexSlice;
@@ -626,9 +626,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
 
     /// Add a note to region errors and borrow explanations when higher-ranked regions in predicates
     /// implicitly introduce an "outlives `'static`" constraint.
-    fn add_placeholder_from_predicate_note(
+    fn add_placeholder_from_predicate_note<G: EmissionGuarantee>(
         &self,
-        err: &mut Diag<'_>,
+        err: &mut Diag<'_, G>,
         path: &[OutlivesConstraint<'tcx>],
     ) {
         let predicate_span = path.iter().find_map(|constraint| {
@@ -651,9 +651,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
 
     /// Add a label to region errors and borrow explanations when outlives constraints arise from
     /// proving a type implements `Sized` or `Copy`.
-    fn add_sized_or_copy_bound_info(
+    fn add_sized_or_copy_bound_info<G: EmissionGuarantee>(
         &self,
-        err: &mut Diag<'_>,
+        err: &mut Diag<'_, G>,
         blamed_category: ConstraintCategory<'tcx>,
         path: &[OutlivesConstraint<'tcx>],
     ) {
@@ -1042,6 +1042,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 kind,
             };
         }
+
         normal_ret
     }
 

compiler/rustc_borrowck/src/diagnostics/region_name.rs

@@ -5,7 +5,7 @@ use std::fmt::{self, Display};
 use std::iter;
 
 use rustc_data_structures::fx::IndexEntry;
-use rustc_errors::Diag;
+use rustc_errors::{Diag, EmissionGuarantee};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_middle::ty::print::RegionHighlightMode;
@@ -108,7 +108,7 @@ impl RegionName {
         }
     }
 
-    pub(crate) fn highlight_region_name(&self, diag: &mut Diag<'_>) {
+    pub(crate) fn highlight_region_name<G: EmissionGuarantee>(&self, diag: &mut Diag<'_, G>) {
         match &self.source {
             RegionNameSource::NamedLateParamRegion(span)
             | RegionNameSource::NamedEarlyParamRegion(span) => {

compiler/rustc_borrowck/src/lib.rs

@@ -16,14 +16,17 @@
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
+use std::borrow::Cow;
 use std::cell::RefCell;
 use std::marker::PhantomData;
 use std::ops::{ControlFlow, Deref};
 
 use rustc_abi::FieldIdx;
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_data_structures::graph::dominators::Dominators;
+use rustc_errors::LintDiagnostic;
 use rustc_hir as hir;
+use rustc_hir::CRATE_HIR_ID;
 use rustc_hir::def_id::LocalDefId;
 use rustc_index::bit_set::{BitSet, MixedBitSet};
 use rustc_index::{IndexSlice, IndexVec};
@@ -43,7 +46,7 @@ use rustc_mir_dataflow::move_paths::{
     InitIndex, InitLocation, LookupResult, MoveData, MovePathIndex,
 };
 use rustc_mir_dataflow::{Analysis, EntryStates, Results, ResultsVisitor, visit_results};
-use rustc_session::lint::builtin::UNUSED_MUT;
+use rustc_session::lint::builtin::{TAIL_EXPR_DROP_ORDER, UNUSED_MUT};
 use rustc_span::{Span, Symbol};
 use smallvec::SmallVec;
 use tracing::{debug, instrument};
@@ -636,9 +639,11 @@ impl<'a, 'tcx> ResultsVisitor<'a, 'tcx, Borrowck<'a, 'tcx>> for MirBorrowckCtxt<
             | StatementKind::Coverage(..)
             // These do not actually affect borrowck
             | StatementKind::ConstEvalCounter
-            // This do not affect borrowck
-            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::StorageLive(..) => {}
+            // This does not affect borrowck
+            StatementKind::BackwardIncompatibleDropHint { place, reason: BackwardIncompatibleDropReason::Edition2024 } => {
+                self.check_backward_incompatible_drop(location, (**place, span), state);
+            }
             StatementKind::StorageDead(local) => {
                 self.access_place(
                     location,
@@ -1007,6 +1012,24 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
         }
     }
 
+    fn borrows_in_scope<'s>(
+        &self,
+        location: Location,
+        state: &'s BorrowckDomain,
+    ) -> Cow<'s, BitSet<BorrowIndex>> {
+        if let Some(polonius) = &self.polonius_output {
+            // Use polonius output if it has been enabled.
+            let location = self.location_table.start_index(location);
+            let mut polonius_output = BitSet::new_empty(self.borrow_set.len());
+            for &idx in polonius.errors_at(location) {
+                polonius_output.insert(idx);
+            }
+            Cow::Owned(polonius_output)
+        } else {
+            Cow::Borrowed(&state.borrows)
+        }
+    }
+
     #[instrument(level = "debug", skip(self, state))]
     fn check_access_for_conflict(
         &mut self,
@@ -1018,18 +1041,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
     ) -> bool {
         let mut error_reported = false;
 
-        // Use polonius output if it has been enabled.
-        let mut polonius_output;
-        let borrows_in_scope = if let Some(polonius) = &self.polonius_output {
-            let location = self.location_table.start_index(location);
-            polonius_output = BitSet::new_empty(self.borrow_set.len());
-            for &idx in polonius.errors_at(location) {
-                polonius_output.insert(idx);
-            }
-            &polonius_output
-        } else {
-            &state.borrows
-        };
+        let borrows_in_scope = self.borrows_in_scope(location, state);
 
         each_borrow_involving_path(
             self,
@@ -1149,6 +1161,61 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
         error_reported
     }
 
+    /// Through #123739, backward incompatible drops (BIDs) are introduced.
+    /// We would like to emit lints whether borrow checking fails at these future drop locations.
+    #[instrument(level = "debug", skip(self, state))]
+    fn check_backward_incompatible_drop(
+        &mut self,
+        location: Location,
+        (place, place_span): (Place<'tcx>, Span),
+        state: &BorrowckDomain,
+    ) {
+        let tcx = self.infcx.tcx;
+        // If this type does not need `Drop`, then treat it like a `StorageDead`.
+        // This is needed because we track the borrows of refs to thread locals,
+        // and we'll ICE because we don't track borrows behind shared references.
+        let sd = if place.ty(self.body, tcx).ty.needs_drop(tcx, self.body.typing_env(tcx)) {
+            AccessDepth::Drop
+        } else {
+            AccessDepth::Shallow(None)
+        };
+
+        let borrows_in_scope = self.borrows_in_scope(location, state);
+
+        // This is a very simplified version of `Self::check_access_for_conflict`.
+        // We are here checking on BIDs and specifically still-live borrows of data involving the BIDs.
+        each_borrow_involving_path(
+            self,
+            self.infcx.tcx,
+            self.body,
+            (sd, place),
+            self.borrow_set,
+            |borrow_index| borrows_in_scope.contains(borrow_index),
+            |this, _borrow_index, borrow| {
+                if matches!(borrow.kind, BorrowKind::Fake(_)) {
+                    return ControlFlow::Continue(());
+                }
+                let borrowed = this.retrieve_borrow_spans(borrow).var_or_use_path_span();
+                let explain = this.explain_why_borrow_contains_point(
+                    location,
+                    borrow,
+                    Some((WriteKind::StorageDeadOrDrop, place)),
+                );
+                this.infcx.tcx.node_span_lint(
+                    TAIL_EXPR_DROP_ORDER,
+                    CRATE_HIR_ID,
+                    borrowed,
+                    |diag| {
+                        session_diagnostics::TailExprDropOrder { borrowed }.decorate_lint(diag);
+                        explain.add_explanation_to_diagnostic(&this, diag, "", None, None);
+                    },
+                );
+                // We may stop at the first case
+                ControlFlow::Break(())
+            },
+        );
+    }
+
     fn mutate_place(
         &mut self,
         location: Location,

compiler/rustc_borrowck/src/session_diagnostics.rs

@@ -480,3 +480,10 @@ pub(crate) struct SimdIntrinsicArgConst {
     pub arg: usize,
     pub intrinsic: String,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(borrowck_tail_expr_drop_order)]
+pub(crate) struct TailExprDropOrder {
+    #[label]
+    pub borrowed: Span,
+}

compiler/rustc_mir_build/src/builder/scope.rs

@@ -1131,15 +1131,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
     /// Schedule emission of a backwards incompatible drop lint hint.
     /// Applicable only to temporary values for now.
+    #[instrument(level = "debug", skip(self))]
     pub(crate) fn schedule_backwards_incompatible_drop(
         &mut self,
         span: Span,
         region_scope: region::Scope,
         local: Local,
     ) {
-        if !self.local_decls[local].ty.has_significant_drop(self.tcx, self.typing_env()) {
-            return;
-        }
+        // Note that we are *not* gating BIDs here on whether they have significant destructor.
+        // We need to know all of them so that we can capture potential borrow-checking errors.
         for scope in self.scopes.scopes.iter_mut().rev() {
             // Since we are inserting linting MIR statement, we have to invalidate the caches
             scope.invalidate_cache();

compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs

@@ -351,14 +351,19 @@ pub(crate) fn run_lint<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId, body: &Body<
     {
         return;
     }
+
+    // FIXME(typing_env): This should be able to reveal the opaques local to the
+    // body using the typeck results.
+    let typing_env = ty::TypingEnv::non_body_analysis(tcx, def_id);
+
     // ## 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 typing_env = ty::TypingEnv::post_analysis(tcx, def_id);
+
     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() {

tests/mir-opt/tail_expr_drop_order_unwind.method_1.ElaborateDrops.after.panic-abort.mir

@@ -74,6 +74,7 @@ fn method_1(_1: Guard) -> () {
 
     bb7: {
         backward incompatible drop(_2);
+        backward incompatible drop(_4);
         backward incompatible drop(_5);
         goto -> bb21;
     }

tests/mir-opt/tail_expr_drop_order_unwind.method_1.ElaborateDrops.after.panic-unwind.mir

@@ -74,6 +74,7 @@ fn method_1(_1: Guard) -> () {
 
     bb7: {
         backward incompatible drop(_2);
+        backward incompatible drop(_4);
         backward incompatible drop(_5);
         goto -> bb21;
     }