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_borrowck/src/dataflow.rs

@@ -641,6 +641,7 @@ impl<'tcx> rustc_mir_dataflow::Analysis<'tcx> for Borrows<'_, 'tcx> {
             | mir::StatementKind::Coverage(..)
             | mir::StatementKind::Intrinsic(..)
             | mir::StatementKind::ConstEvalCounter
+            | mir::StatementKind::BackwardIncompatibleDropHint { .. }
             | mir::StatementKind::Nop => {}
         }
     }

compiler/rustc_borrowck/src/lib.rs

@@ -652,6 +652,8 @@ 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(..) => {}
             StatementKind::StorageDead(local) => {
                 self.access_place(

compiler/rustc_borrowck/src/polonius/loan_invalidations.rs

@@ -88,6 +88,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LoanInvalidationsGenerator<'a, 'tcx> {
             | StatementKind::Nop
             | StatementKind::Retag { .. }
             | StatementKind::Deinit(..)
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::SetDiscriminant { .. } => {
                 bug!("Statement not allowed in this MIR phase")
             }

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -1252,6 +1252,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             | StatementKind::Coverage(..)
             | StatementKind::ConstEvalCounter
             | StatementKind::PlaceMention(..)
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => {}
             StatementKind::Deinit(..) | StatementKind::SetDiscriminant { .. } => {
                 bug!("Statement not allowed in this MIR phase")

compiler/rustc_codegen_cranelift/src/base.rs

@@ -924,6 +924,7 @@ fn codegen_stmt<'tcx>(
         | StatementKind::FakeRead(..)
         | StatementKind::Retag { .. }
         | StatementKind::PlaceMention(..)
+        | StatementKind::BackwardIncompatibleDropHint { .. }
         | StatementKind::AscribeUserType(..) => {}
 
         StatementKind::Coverage { .. } => unreachable!(),

compiler/rustc_codegen_cranelift/src/constant.rs

@@ -583,6 +583,7 @@ pub(crate) fn mir_operand_get_const_val<'tcx>(
                         | StatementKind::PlaceMention(..)
                         | StatementKind::Coverage(_)
                         | StatementKind::ConstEvalCounter
+                        | StatementKind::BackwardIncompatibleDropHint { .. }
                         | 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::AscribeUserType(..)
             | mir::StatementKind::ConstEvalCounter
             | mir::StatementKind::PlaceMention(..)
+            | mir::StatementKind::BackwardIncompatibleDropHint { .. }
             | mir::StatementKind::Nop => {}
         }
     }

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -609,6 +609,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
             | StatementKind::ConstEvalCounter
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => {}
         }
     }

compiler/rustc_const_eval/src/interpret/step.rs

@@ -143,6 +143,9 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             // Defined to do nothing. These are added by optimization passes, to avoid changing the
             // size of MIR constantly.
             Nop => {}
+
+            // Only used for temporary lifetime lints
+            BackwardIncompatibleDropHint { .. } => {}
         }
 
         interp_ok(())

compiler/rustc_hir_analysis/src/check/region.rs

@@ -17,6 +17,7 @@ use rustc_index::Idx;
 use rustc_middle::bug;
 use rustc_middle::middle::region::*;
 use rustc_middle::ty::TyCtxt;
+use rustc_session::lint;
 use rustc_span::source_map;
 use tracing::debug;
 
@@ -167,8 +168,23 @@ fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx h
             }
         }
         if let Some(tail_expr) = blk.expr {
-            if blk.span.edition().at_least_rust_2024() {
-                visitor.terminating_scopes.insert(tail_expr.hir_id.local_id);
+            let local_id = tail_expr.hir_id.local_id;
+            let edition = blk.span.edition();
+            if edition.at_least_rust_2024() {
+                visitor.terminating_scopes.insert(local_id);
+            } else if !visitor
+                .tcx
+                .lints_that_dont_need_to_run(())
+                .contains(&lint::LintId::of(lint::builtin::TAIL_EXPR_DROP_ORDER))
+            {
+                // If this temporary scope will be changing once the codebase adopts Rust 2024,
+                // and we are linting about possible semantic changes that would result,
+                // then record this node-id in the field `backwards_incompatible_scope`
+                // for future reference.
+                visitor
+                    .scope_tree
+                    .backwards_incompatible_scope
+                    .insert(local_id, Scope { id: local_id, data: ScopeData::Node });
             }
             visitor.visit_expr(tail_expr);
         }

compiler/rustc_index/src/bit_set.rs

@@ -460,6 +460,10 @@ impl<T: Idx> ChunkedBitSet<T> {
         self.chunks.iter().map(|chunk| chunk.count()).sum()
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.chunks.iter().all(|chunk| matches!(chunk, Chunk::Zeros(..) | Chunk::Ones(0)))
+    }
+
     /// Returns `true` if `self` contains `elem`.
     #[inline]
     pub fn contains(&self, elem: T) -> bool {
@@ -668,12 +672,140 @@ impl<T: Idx> BitRelations<ChunkedBitSet<T>> for ChunkedBitSet<T> {
         changed
     }
 
-    fn subtract(&mut self, _other: &ChunkedBitSet<T>) -> bool {
-        unimplemented!("implement if/when necessary");
+    fn subtract(&mut self, other: &ChunkedBitSet<T>) -> bool {
+        assert_eq!(self.domain_size, other.domain_size);
+        debug_assert_eq!(self.chunks.len(), other.chunks.len());
+
+        let mut changed = false;
+        for (mut self_chunk, other_chunk) in self.chunks.iter_mut().zip(other.chunks.iter()) {
+            match (&mut self_chunk, &other_chunk) {
+                (Zeros(..), _) | (_, Zeros(..)) => {}
+                (
+                    Ones(self_chunk_domain_size) | Mixed(self_chunk_domain_size, _, _),
+                    Ones(other_chunk_domain_size),
+                ) => {
+                    debug_assert_eq!(self_chunk_domain_size, other_chunk_domain_size);
+                    changed = true;
+                    *self_chunk = Zeros(*self_chunk_domain_size);
+                }
+                (
+                    Ones(self_chunk_domain_size),
+                    Mixed(other_chunk_domain_size, other_chunk_count, other_chunk_words),
+                ) => {
+                    debug_assert_eq!(self_chunk_domain_size, other_chunk_domain_size);
+                    changed = true;
+                    let num_words = num_words(*self_chunk_domain_size as usize);
+                    debug_assert!(num_words > 0 && num_words <= CHUNK_WORDS);
+                    let mut tail_mask =
+                        1 << (*other_chunk_domain_size - ((num_words - 1) * WORD_BITS) as u16) - 1;
+                    let mut self_chunk_words = **other_chunk_words;
+                    for word in self_chunk_words[0..num_words].iter_mut().rev() {
+                        *word = !*word & tail_mask;
+                        tail_mask = u64::MAX;
+                    }
+                    let self_chunk_count = *self_chunk_domain_size - *other_chunk_count;
+                    debug_assert_eq!(
+                        self_chunk_count,
+                        self_chunk_words[0..num_words]
+                            .iter()
+                            .map(|w| w.count_ones() as ChunkSize)
+                            .sum()
+                    );
+                    *self_chunk =
+                        Mixed(*self_chunk_domain_size, self_chunk_count, Rc::new(self_chunk_words));
+                }
+                (
+                    Mixed(
+                        self_chunk_domain_size,
+                        ref mut self_chunk_count,
+                        ref mut self_chunk_words,
+                    ),
+                    Mixed(_other_chunk_domain_size, _other_chunk_count, other_chunk_words),
+                ) => {
+                    // See [`<Self as BitRelations<ChunkedBitSet<T>>>::union`] for the explanation
+                    let op = |a: u64, b: u64| a & !b;
+                    let num_words = num_words(*self_chunk_domain_size as usize);
+                    if bitwise_changes(
+                        &self_chunk_words[0..num_words],
+                        &other_chunk_words[0..num_words],
+                        op,
+                    ) {
+                        let self_chunk_words = Rc::make_mut(self_chunk_words);
+                        let has_changed = bitwise(
+                            &mut self_chunk_words[0..num_words],
+                            &other_chunk_words[0..num_words],
+                            op,
+                        );
+                        debug_assert!(has_changed);
+                        *self_chunk_count = self_chunk_words[0..num_words]
+                            .iter()
+                            .map(|w| w.count_ones() as ChunkSize)
+                            .sum();
+                        if *self_chunk_count == 0 {
+                            *self_chunk = Zeros(*self_chunk_domain_size);
+                        }
+                        changed = true;
+                    }
+                }
+            }
+        }
+        changed
     }
 
-    fn intersect(&mut self, _other: &ChunkedBitSet<T>) -> bool {
-        unimplemented!("implement if/when necessary");
+    fn intersect(&mut self, other: &ChunkedBitSet<T>) -> bool {
+        assert_eq!(self.domain_size, other.domain_size);
+        debug_assert_eq!(self.chunks.len(), other.chunks.len());
+
+        let mut changed = false;
+        for (mut self_chunk, other_chunk) in self.chunks.iter_mut().zip(other.chunks.iter()) {
+            match (&mut self_chunk, &other_chunk) {
+                (Zeros(..), _) | (_, Ones(..)) => {}
+                (
+                    Ones(self_chunk_domain_size),
+                    Zeros(other_chunk_domain_size) | Mixed(other_chunk_domain_size, ..),
+                )
+                | (Mixed(self_chunk_domain_size, ..), Zeros(other_chunk_domain_size)) => {
+                    debug_assert_eq!(self_chunk_domain_size, other_chunk_domain_size);
+                    changed = true;
+                    *self_chunk = other_chunk.clone();
+                }
+                (
+                    Mixed(
+                        self_chunk_domain_size,
+                        ref mut self_chunk_count,
+                        ref mut self_chunk_words,
+                    ),
+                    Mixed(_other_chunk_domain_size, _other_chunk_count, other_chunk_words),
+                ) => {
+                    // See [`<Self as BitRelations<ChunkedBitSet<T>>>::union`] for the explanation
+                    let op = |a, b| a & b;
+                    let num_words = num_words(*self_chunk_domain_size as usize);
+                    if bitwise_changes(
+                        &self_chunk_words[0..num_words],
+                        &other_chunk_words[0..num_words],
+                        op,
+                    ) {
+                        let self_chunk_words = Rc::make_mut(self_chunk_words);
+                        let has_changed = bitwise(
+                            &mut self_chunk_words[0..num_words],
+                            &other_chunk_words[0..num_words],
+                            op,
+                        );
+                        debug_assert!(has_changed);
+                        *self_chunk_count = self_chunk_words[0..num_words]
+                            .iter()
+                            .map(|w| w.count_ones() as ChunkSize)
+                            .sum();
+                        if *self_chunk_count == 0 {
+                            *self_chunk = Zeros(*self_chunk_domain_size);
+                        }
+                        changed = true;
+                    }
+                }
+            }
+        }
+
+        changed
     }
 }
 

compiler/rustc_lint/messages.ftl

@@ -772,9 +772,6 @@ lint_suspicious_double_ref_clone =
 lint_suspicious_double_ref_deref =
     using `.deref()` on a double reference, which returns `{$ty}` instead of dereferencing the inner type
 
-lint_tail_expr_drop_order = these values and local bindings have significant drop implementation that will have a different drop order from that of Edition 2021
-    .label = these values have significant drop implementation and will observe changes in drop order under Edition 2024
-
 lint_trailing_semi_macro = trailing semicolon in macro used in expression position
     .note1 = macro invocations at the end of a block are treated as expressions
     .note2 = to ignore the value produced by the macro, add a semicolon after the invocation of `{$name}`

compiler/rustc_lint/src/lib.rs

@@ -75,7 +75,6 @@ mod redundant_semicolon;
 mod reference_casting;
 mod shadowed_into_iter;
 mod static_mut_refs;
-mod tail_expr_drop_order;
 mod traits;
 mod types;
 mod unit_bindings;
@@ -116,7 +115,6 @@ use rustc_middle::ty::TyCtxt;
 use shadowed_into_iter::ShadowedIntoIter;
 pub use shadowed_into_iter::{ARRAY_INTO_ITER, BOXED_SLICE_INTO_ITER};
 use static_mut_refs::*;
-use tail_expr_drop_order::TailExprDropOrder;
 use traits::*;
 use types::*;
 use unit_bindings::*;
@@ -240,7 +238,6 @@ late_lint_methods!(
             AsyncFnInTrait: AsyncFnInTrait,
             NonLocalDefinitions: NonLocalDefinitions::default(),
             ImplTraitOvercaptures: ImplTraitOvercaptures,
-            TailExprDropOrder: TailExprDropOrder,
             IfLetRescope: IfLetRescope::default(),
             StaticMutRefs: StaticMutRefs,
             UnqualifiedLocalImports: UnqualifiedLocalImports,