Desugaring of drop and replace at MIR build

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -1467,6 +1467,32 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     /// Reports StorageDeadOrDrop of `place` conflicts with `borrow`.
     ///
+    /// Depending on the origin of the StorageDeadOrDrop, this may be
+    /// reported as either a drop or an illegal mutation of a borrowed value.
+    /// The latter is preferred when the this is a drop triggered by a
+    /// reassignment, as it's more user friendly to report a problem with the
+    /// explicit assignment than the implicit drop.
+    #[instrument(level = "debug", skip(self))]
+    pub(crate) fn report_storage_dead_or_drop_of_borrowed(
+        &mut self,
+        location: Location,
+        place_span: (Place<'tcx>, Span),
+        borrow: &BorrowData<'tcx>,
+    ) {
+        // It's sufficient to check the last desugaring as Replace is the last
+        // one to be applied.
+        if let Some(DesugaringKind::Replace) = place_span.1.desugaring_kind() {
+            self.report_illegal_mutation_of_borrowed(location, place_span, borrow)
+        } else {
+            self.report_borrowed_value_does_not_live_long_enough(
+                location,
+                borrow,
+                place_span,
+                Some(WriteKind::StorageDeadOrDrop),
+            )
+        }
+    }
+
     /// This means that some data referenced by `borrow` needs to live
     /// past the point where the StorageDeadOrDrop of `place` occurs.
     /// This is usually interpreted as meaning that `place` has too

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -925,7 +925,20 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             return OtherUse(use_span);
         }
 
-        for stmt in &self.body[location.block].statements[location.statement_index + 1..] {
+        // drop and replace might have moved the assignment to the next block
+        let maybe_additional_statement =
+            if let TerminatorKind::Drop { target: drop_target, .. } =
+                self.body[location.block].terminator().kind
+            {
+                self.body[drop_target].statements.first()
+            } else {
+                None
+            };
+
+        let statements =
+            self.body[location.block].statements[location.statement_index + 1..].iter();
+
+        for stmt in statements.chain(maybe_additional_statement) {
             if let StatementKind::Assign(box (_, Rvalue::Aggregate(kind, places))) = &stmt.kind {
                 let (&def_id, is_generator) = match kind {
                     box AggregateKind::Closure(def_id, _) => (def_id, false),

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -828,7 +828,13 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             let Some(hir::Node::Item(item)) = node else { return; };
             let hir::ItemKind::Fn(.., body_id) = item.kind else { return; };
             let body = self.infcx.tcx.hir().body(body_id);
-            let mut v = V { assign_span: span, err, ty, suggested: false };
+            let mut assign_span = span;
+            // Drop desugaring is done at MIR build so it's not in the HIR
+            if let Some(DesugaringKind::Replace) = span.desugaring_kind() {
+                assign_span.remove_mark();
+            }
+
+            let mut v = V { assign_span, err, ty, suggested: false };
             v.visit_body(body);
             if !v.suggested {
                 err.help(&format!(

compiler/rustc_borrowck/src/lib.rs

@@ -1185,12 +1185,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                             this.buffer_error(err);
                         }
                         WriteKind::StorageDeadOrDrop => this
-                            .report_borrowed_value_does_not_live_long_enough(
-                                location,
-                                borrow,
-                                place_span,
-                                Some(kind),
-                            ),
+                            .report_storage_dead_or_drop_of_borrowed(location, place_span, borrow),
                         WriteKind::Mutate => {
                             this.report_illegal_mutation_of_borrowed(location, place_span, borrow)
                         }

compiler/rustc_mir_build/src/build/expr/stmt.rs

@@ -40,7 +40,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 // Generate better code for things that don't need to be
                 // dropped.
                 if lhs.ty.needs_drop(this.tcx, this.param_env) {
-                    let rhs = unpack!(block = this.as_local_operand(block, rhs));
+                    let rhs = unpack!(block = this.as_local_rvalue(block, rhs));
                     let lhs = unpack!(block = this.as_place(block, lhs));
                     unpack!(block = this.build_drop_and_replace(block, lhs_span, lhs, rhs));
                 } else {

compiler/rustc_mir_build/src/build/scope.rs

@@ -91,7 +91,7 @@ use rustc_middle::middle::region;
 use rustc_middle::mir::*;
 use rustc_middle::thir::{Expr, LintLevel};
 
-use rustc_span::{Span, DUMMY_SP};
+use rustc_span::{DesugaringKind, Span, DUMMY_SP};
 
 #[derive(Debug)]
 pub struct Scopes<'tcx> {
@@ -1118,24 +1118,35 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     }
 
     /// Utility function for *non*-scope code to build their own drops
+    /// Force a drop at this point in the MIR by creating a new block.
     pub(crate) fn build_drop_and_replace(
         &mut self,
         block: BasicBlock,
         span: Span,
         place: Place<'tcx>,
-        value: Operand<'tcx>,
+        value: Rvalue<'tcx>,
     ) -> BlockAnd<()> {
+        let span = self.tcx.with_stable_hashing_context(|hcx| {
+            span.mark_with_reason(None, DesugaringKind::Replace, self.tcx.sess.edition(), hcx)
+        });
         let source_info = self.source_info(span);
-        let next_target = self.cfg.start_new_block();
+
+        // create the new block for the assignment
+        let assign = self.cfg.start_new_block();
+        self.cfg.push_assign(assign, source_info, place, value.clone());
+
+        // create the new block for the assignment in the case of unwinding
+        let assign_unwind = self.cfg.start_new_cleanup_block();
+        self.cfg.push_assign(assign_unwind, source_info, place, value.clone());
 
         self.cfg.terminate(
             block,
             source_info,
-            TerminatorKind::DropAndReplace { place, value, target: next_target, unwind: None },
+            TerminatorKind::Drop { place, target: assign, unwind: Some(assign_unwind) },
         );
         self.diverge_from(block);
 
-        next_target.unit()
+        assign.unit()
     }
 
     /// Creates an `Assert` terminator and return the success block.
@@ -1413,8 +1424,15 @@ impl<'tcx> DropTreeBuilder<'tcx> for Unwind {
     fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
         let term = &mut cfg.block_data_mut(from).terminator_mut();
         match &mut term.kind {
-            TerminatorKind::Drop { unwind, .. }
-            | TerminatorKind::DropAndReplace { unwind, .. }
+            TerminatorKind::Drop { unwind, .. } => {
+                if let Some(unwind) = *unwind {
+                    let source_info = term.source_info;
+                    cfg.terminate(unwind, source_info, TerminatorKind::Goto { target: to });
+                } else {
+                    *unwind = Some(to);
+                }
+            }
+            TerminatorKind::DropAndReplace { unwind, .. }
             | TerminatorKind::FalseUnwind { unwind, .. }
             | TerminatorKind::Call { cleanup: unwind, .. }
             | TerminatorKind::Assert { cleanup: unwind, .. }

compiler/rustc_mir_transform/src/elaborate_drops.rs

@@ -14,7 +14,7 @@ use rustc_mir_dataflow::un_derefer::UnDerefer;
 use rustc_mir_dataflow::MoveDataParamEnv;
 use rustc_mir_dataflow::{on_all_children_bits, on_all_drop_children_bits};
 use rustc_mir_dataflow::{Analysis, ResultsCursor};
-use rustc_span::Span;
+use rustc_span::{DesugaringKind, Span};
 use rustc_target::abi::VariantIdx;
 use std::fmt;
 
@@ -425,10 +425,19 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
                             bb,
                         ),
                         LookupResult::Parent(..) => {
-                            self.tcx.sess.delay_span_bug(
-                                terminator.source_info.span,
-                                &format!("drop of untracked value {:?}", bb),
-                            );
+                            if !matches!(
+                                terminator.source_info.span.desugaring_kind(),
+                                Some(DesugaringKind::Replace),
+                            ) {
+                                self.tcx.sess.delay_span_bug(
+                                    terminator.source_info.span,
+                                    &format!("drop of untracked value {:?}", bb),
+                                );
+                            }
+                            // A drop and replace behind a pointer/array/whatever.
+                            // The borrow checker requires that these locations are initialized before the assignment,
+                            // so we just leave an unconditional drop.
+                            assert!(!data.is_cleanup);
                         }
                     }
                 }

compiler/rustc_span/src/hygiene.rs

@@ -1151,6 +1151,7 @@ pub enum DesugaringKind {
     Await,
     ForLoop,
     WhileLoop,
+    Replace,
 }
 
 impl DesugaringKind {
@@ -1166,6 +1167,7 @@ impl DesugaringKind {
             DesugaringKind::OpaqueTy => "`impl Trait`",
             DesugaringKind::ForLoop => "`for` loop",
             DesugaringKind::WhileLoop => "`while` loop",
+            DesugaringKind::Replace => "drop and replace",
         }
     }
 }

tests/mir-opt/basic_assignment.main.ElaborateDrops.diff

@@ -0,0 +1,85 @@
+- // MIR for `main` before ElaborateDrops
++ // MIR for `main` after ElaborateDrops
+
+  fn main() -> () {
+      let mut _0: ();                      // return place in scope 0 at $DIR/basic_assignment.rs:+0:11: +0:11
+      let _1: bool;                        // in scope 0 at $DIR/basic_assignment.rs:+1:9: +1:17
+      let mut _3: bool;                    // in scope 0 at $DIR/basic_assignment.rs:+6:16: +6:24
+      let mut _6: std::option::Option<std::boxed::Box<u32>>; // in scope 0 at $DIR/basic_assignment.rs:+13:14: +13:20
+      scope 1 {
+          debug nodrop_x => _1;            // in scope 1 at $DIR/basic_assignment.rs:+1:9: +1:17
+          let _2: bool;                    // in scope 1 at $DIR/basic_assignment.rs:+2:9: +2:17
+          scope 2 {
+              debug nodrop_y => _2;        // in scope 2 at $DIR/basic_assignment.rs:+2:9: +2:17
+              let _4: std::option::Option<std::boxed::Box<u32>>; // in scope 2 at $DIR/basic_assignment.rs:+8:9: +8:15
+              scope 3 {
+                  debug drop_x => _4;      // in scope 3 at $DIR/basic_assignment.rs:+8:9: +8:15
+                  let _5: std::option::Option<std::boxed::Box<u32>>; // in scope 3 at $DIR/basic_assignment.rs:+9:9: +9:15
+                  scope 4 {
+                      debug drop_y => _5;  // in scope 4 at $DIR/basic_assignment.rs:+9:9: +9:15
+                  }
+              }
+          }
+      }
+
+      bb0: {
+          StorageLive(_1);                 // scope 0 at $DIR/basic_assignment.rs:+1:9: +1:17
+          _1 = const false;                // scope 0 at $DIR/basic_assignment.rs:+1:20: +1:25
+          StorageLive(_2);                 // scope 1 at $DIR/basic_assignment.rs:+2:9: +2:17
+          StorageLive(_3);                 // scope 2 at $DIR/basic_assignment.rs:+6:16: +6:24
+          _3 = _1;                         // scope 2 at $DIR/basic_assignment.rs:+6:16: +6:24
+          _2 = move _3;                    // scope 2 at $DIR/basic_assignment.rs:+6:5: +6:24
+          StorageDead(_3);                 // scope 2 at $DIR/basic_assignment.rs:+6:23: +6:24
+          StorageLive(_4);                 // scope 2 at $DIR/basic_assignment.rs:+8:9: +8:15
+          _4 = Option::<Box<u32>>::None;   // scope 2 at $DIR/basic_assignment.rs:+8:36: +8:40
+          StorageLive(_5);                 // scope 3 at $DIR/basic_assignment.rs:+9:9: +9:15
+          StorageLive(_6);                 // scope 4 at $DIR/basic_assignment.rs:+13:14: +13:20
+          _6 = move _4;                    // scope 4 at $DIR/basic_assignment.rs:+13:14: +13:20
+-         drop(_5) -> [return: bb1, unwind: bb2]; // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
++         goto -> bb1;                     // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
+      }
+
+      bb1: {
+          _5 = move _6;                    // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
+-         drop(_6) -> [return: bb3, unwind: bb6]; // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
++         goto -> bb3;                     // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
+      }
+
+      bb2 (cleanup): {
+          _5 = move _6;                    // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
+          drop(_6) -> bb6;                 // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
+      }
+
+      bb3: {
+          StorageDead(_6);                 // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
+          _0 = const ();                   // scope 0 at $DIR/basic_assignment.rs:+0:11: +14:2
+          drop(_5) -> [return: bb4, unwind: bb7]; // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
+      }
+
+      bb4: {
+          StorageDead(_5);                 // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
+-         drop(_4) -> bb5;                 // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
++         goto -> bb5;                     // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
+      }
+
+      bb5: {
+          StorageDead(_4);                 // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
+          StorageDead(_2);                 // scope 1 at $DIR/basic_assignment.rs:+14:1: +14:2
+          StorageDead(_1);                 // scope 0 at $DIR/basic_assignment.rs:+14:1: +14:2
+          return;                          // scope 0 at $DIR/basic_assignment.rs:+14:2: +14:2
+      }
+
+      bb6 (cleanup): {
+          drop(_5) -> bb7;                 // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
+      }
+
+      bb7 (cleanup): {
+-         drop(_4) -> bb8;                 // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
++         goto -> bb8;                     // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
+      }
+
+      bb8 (cleanup): {
+          resume;                          // scope 0 at $DIR/basic_assignment.rs:+0:1: +14:2
+      }
+  }
+

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

@@ -1,8 +1,8 @@
 // MIR for `main` after SimplifyCfg-initial
 
 | User Type Annotations
-| 0: user_ty: Canonical { max_universe: U0, variables: [], value: Ty(std::option::Option<std::boxed::Box<u32>>) }, span: $DIR/basic_assignment.rs:18:17: 18:33, inferred_ty: std::option::Option<std::boxed::Box<u32>>
-| 1: user_ty: Canonical { max_universe: U0, variables: [], value: Ty(std::option::Option<std::boxed::Box<u32>>) }, span: $DIR/basic_assignment.rs:18:17: 18:33, inferred_ty: std::option::Option<std::boxed::Box<u32>>
+| 0: user_ty: Canonical { max_universe: U0, variables: [], value: Ty(std::option::Option<std::boxed::Box<u32>>) }, span: $DIR/basic_assignment.rs:20:17: 20:33, inferred_ty: std::option::Option<std::boxed::Box<u32>>
+| 1: user_ty: Canonical { max_universe: U0, variables: [], value: Ty(std::option::Option<std::boxed::Box<u32>>) }, span: $DIR/basic_assignment.rs:20:17: 20:33, inferred_ty: std::option::Option<std::boxed::Box<u32>>
 |
 fn main() -> () {
     let mut _0: ();                      // return place in scope 0 at $DIR/basic_assignment.rs:+0:11: +0:11
@@ -41,35 +41,37 @@ fn main() -> () {
         StorageLive(_5);                 // scope 3 at $DIR/basic_assignment.rs:+9:9: +9:15
         StorageLive(_6);                 // scope 4 at $DIR/basic_assignment.rs:+13:14: +13:20
         _6 = move _4;                    // scope 4 at $DIR/basic_assignment.rs:+13:14: +13:20
-        replace(_5 <- move _6) -> [return: bb1, unwind: bb5]; // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
+        drop(_5) -> [return: bb1, unwind: bb2]; // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
     }
 
     bb1: {
-        drop(_6) -> [return: bb2, unwind: bb6]; // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
+        _5 = move _6;                    // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
+        drop(_6) -> [return: bb3, unwind: bb6]; // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
     }
 
-    bb2: {
+    bb2 (cleanup): {
+        _5 = move _6;                    // scope 4 at $DIR/basic_assignment.rs:+13:5: +13:11
+        drop(_6) -> bb6;                 // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
+    }
+
+    bb3: {
         StorageDead(_6);                 // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
         _0 = const ();                   // scope 0 at $DIR/basic_assignment.rs:+0:11: +14:2
-        drop(_5) -> [return: bb3, unwind: bb7]; // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
+        drop(_5) -> [return: bb4, unwind: bb7]; // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
     }
 
-    bb3: {
+    bb4: {
         StorageDead(_5);                 // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
-        drop(_4) -> [return: bb4, unwind: bb8]; // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
+        drop(_4) -> [return: bb5, unwind: bb8]; // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
     }
 
-    bb4: {
+    bb5: {
         StorageDead(_4);                 // scope 2 at $DIR/basic_assignment.rs:+14:1: +14:2
         StorageDead(_2);                 // scope 1 at $DIR/basic_assignment.rs:+14:1: +14:2
         StorageDead(_1);                 // scope 0 at $DIR/basic_assignment.rs:+14:1: +14:2
         return;                          // scope 0 at $DIR/basic_assignment.rs:+14:2: +14:2
     }
 
-    bb5 (cleanup): {
-        drop(_6) -> bb6;                 // scope 4 at $DIR/basic_assignment.rs:+13:19: +13:20
-    }
-
     bb6 (cleanup): {
         drop(_5) -> bb7;                 // scope 3 at $DIR/basic_assignment.rs:+14:1: +14:2
     }

tests/mir-opt/basic_assignment.rs

@@ -1,5 +1,7 @@
+// needs-unwind
 // this tests move up progration, which is not yet implemented
 
+// EMIT_MIR basic_assignment.main.ElaborateDrops.diff
 // EMIT_MIR basic_assignment.main.SimplifyCfg-initial.after.mir
 
 // Check codegen for assignments (`a = b`) where the left-hand-side is