Rollup merge of #133155 - nnethercote:yet-more-rustc_mir_dataflow-cleanups, r=cjgillot

Yet more `rustc_mir_dataflow` cleanups

A few more cleanups.

r? `@cjgillot`

9 files changed, 242 insertions, 213 deletions

diff --git a/compiler/rustc_mir_dataflow/src/framework/cursor.rs b/compiler/rustc_mir_dataflow/src/framework/cursor.rs
index 5ebb343f4e1..11cf8c3e898 100644
--- a/compiler/rustc_mir_dataflow/src/framework/cursor.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/cursor.rs
@@ -1,6 +1,7 @@
 //! Random access inspection of the results of a dataflow analysis.
 
 use std::cmp::Ordering;
+use std::ops::{Deref, DerefMut};
 
 #[cfg(debug_assertions)]
 use rustc_index::bit_set::BitSet;
@@ -8,18 +9,63 @@ use rustc_middle::mir::{self, BasicBlock, Location};
 
 use super::{Analysis, Direction, Effect, EffectIndex, Results};
 
-/// Allows random access inspection of the results of a dataflow analysis.
+/// Some `ResultsCursor`s want to own a `Results`, and some want to borrow a `Results`, either
+/// mutable or immutably. This type allows all of the above. It's similar to `Cow`.
+pub enum ResultsHandle<'a, 'tcx, A>
+where
+    A: Analysis<'tcx>,
+{
+    Borrowed(&'a Results<'tcx, A>),
+    BorrowedMut(&'a mut Results<'tcx, A>),
+    Owned(Results<'tcx, A>),
+}
+
+impl<'tcx, A> Deref for ResultsHandle<'_, 'tcx, A>
+where
+    A: Analysis<'tcx>,
+{
+    type Target = Results<'tcx, A>;
+
+    fn deref(&self) -> &Results<'tcx, A> {
+        match self {
+            ResultsHandle::Borrowed(borrowed) => borrowed,
+            ResultsHandle::BorrowedMut(borrowed) => borrowed,
+            ResultsHandle::Owned(owned) => owned,
+        }
+    }
+}
+
+impl<'tcx, A> DerefMut for ResultsHandle<'_, 'tcx, A>
+where
+    A: Analysis<'tcx>,
+{
+    fn deref_mut(&mut self) -> &mut Results<'tcx, A> {
+        match self {
+            ResultsHandle::Borrowed(_borrowed) => {
+                panic!("tried to deref_mut a `ResultsHandle::Borrowed")
+            }
+            ResultsHandle::BorrowedMut(borrowed) => borrowed,
+            ResultsHandle::Owned(owned) => owned,
+        }
+    }
+}
+
+/// Allows random access inspection of the results of a dataflow analysis. Use this when you want
+/// to inspect domain values only in certain locations; use `ResultsVisitor` if you want to inspect
+/// domain values in many or all locations.
 ///
-/// This cursor only has linear performance within a basic block when its statements are visited in
-/// the same order as the `DIRECTION` of the analysis. In the worst case—when statements are
-/// visited in *reverse* order—performance will be quadratic in the number of statements in the
-/// block. The order in which basic blocks are inspected has no impact on performance.
+/// Because `Results` only has domain values for the entry of each basic block, these inspections
+/// involve some amount of domain value recomputations. This cursor only has linear performance
+/// within a basic block when its statements are visited in the same order as the `DIRECTION` of
+/// the analysis. In the worst case—when statements are visited in *reverse* order—performance will
+/// be quadratic in the number of statements in the block. The order in which basic blocks are
+/// inspected has no impact on performance.
 pub struct ResultsCursor<'mir, 'tcx, A>
 where
     A: Analysis<'tcx>,
 {
     body: &'mir mir::Body<'tcx>,
-    results: Results<'tcx, A>,
+    results: ResultsHandle<'mir, 'tcx, A>,
     state: A::Domain,
 
     pos: CursorPosition,
@@ -47,13 +93,8 @@ where
         self.body
     }
 
-    /// Unwraps this cursor, returning the underlying `Results`.
-    pub fn into_results(self) -> Results<'tcx, A> {
-        self.results
-    }
-
     /// Returns a new cursor that can inspect `results`.
-    pub fn new(body: &'mir mir::Body<'tcx>, results: Results<'tcx, A>) -> Self {
+    pub fn new(body: &'mir mir::Body<'tcx>, results: ResultsHandle<'mir, 'tcx, A>) -> Self {
         let bottom_value = results.analysis.bottom_value(body);
         ResultsCursor {
             body,
diff --git a/compiler/rustc_mir_dataflow/src/framework/direction.rs b/compiler/rustc_mir_dataflow/src/framework/direction.rs
index 9a5cf9d4e84..566a6b09b2b 100644
--- a/compiler/rustc_mir_dataflow/src/framework/direction.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/direction.rs
@@ -9,30 +9,35 @@ use super::{Analysis, Effect, EffectIndex, Results, SwitchIntTarget};
 
 pub trait Direction {
     const IS_FORWARD: bool;
-
     const IS_BACKWARD: bool = !Self::IS_FORWARD;
 
-    /// Applies all effects between the given `EffectIndex`s.
-    ///
-    /// `effects.start()` must precede or equal `effects.end()` in this direction.
-    fn apply_effects_in_range<'tcx, A>(
+    /// Called by `iterate_to_fixpoint` during initial analysis computation.
+    fn apply_effects_in_block<'mir, 'tcx, A>(
         analysis: &mut A,
+        body: &mir::Body<'tcx>,
         state: &mut A::Domain,
         block: BasicBlock,
-        block_data: &mir::BasicBlockData<'tcx>,
-        effects: RangeInclusive<EffectIndex>,
+        block_data: &'mir mir::BasicBlockData<'tcx>,
+        propagate: impl FnMut(BasicBlock, &A::Domain),
     ) where
         A: Analysis<'tcx>;
 
-    fn apply_effects_in_block<'mir, 'tcx, A>(
+    /// Called by `ResultsCursor` to recompute the domain value for a location
+    /// in a basic block. Applies all effects between the given `EffectIndex`s.
+    ///
+    /// `effects.start()` must precede or equal `effects.end()` in this direction.
+    fn apply_effects_in_range<'tcx, A>(
         analysis: &mut A,
         state: &mut A::Domain,
         block: BasicBlock,
-        block_data: &'mir mir::BasicBlockData<'tcx>,
-    ) -> TerminatorEdges<'mir, 'tcx>
-    where
+        block_data: &mir::BasicBlockData<'tcx>,
+        effects: RangeInclusive<EffectIndex>,
+    ) where
         A: Analysis<'tcx>;
 
+    /// Called by `ResultsVisitor` to recompute the analysis domain values for
+    /// all locations in a basic block (starting from the entry value stored
+    /// in `Results`) and to visit them with `vis`.
     fn visit_results_in_block<'mir, 'tcx, A>(
         state: &mut A::Domain,
         block: BasicBlock,
@@ -41,16 +46,6 @@ pub trait Direction {
         vis: &mut impl ResultsVisitor<'mir, 'tcx, A>,
     ) where
         A: Analysis<'tcx>;
-
-    fn join_state_into_successors_of<'tcx, A>(
-        analysis: &mut A,
-        body: &mir::Body<'tcx>,
-        exit_state: &mut A::Domain,
-        block: BasicBlock,
-        edges: TerminatorEdges<'_, 'tcx>,
-        propagate: impl FnMut(BasicBlock, &A::Domain),
-    ) where
-        A: Analysis<'tcx>;
 }
 
 /// Dataflow that runs from the exit of a block (terminator), to its entry (the first statement).
@@ -61,23 +56,84 @@ impl Direction for Backward {
 
     fn apply_effects_in_block<'mir, 'tcx, A>(
         analysis: &mut A,
+        body: &mir::Body<'tcx>,
         state: &mut A::Domain,
         block: BasicBlock,
         block_data: &'mir mir::BasicBlockData<'tcx>,
-    ) -> TerminatorEdges<'mir, 'tcx>
-    where
+        mut propagate: impl FnMut(BasicBlock, &A::Domain),
+    ) where
         A: Analysis<'tcx>,
     {
         let terminator = block_data.terminator();
         let location = Location { block, statement_index: block_data.statements.len() };
         analysis.apply_before_terminator_effect(state, terminator, location);
-        let edges = analysis.apply_terminator_effect(state, terminator, location);
+        analysis.apply_terminator_effect(state, terminator, location);
         for (statement_index, statement) in block_data.statements.iter().enumerate().rev() {
             let location = Location { block, statement_index };
             analysis.apply_before_statement_effect(state, statement, location);
             analysis.apply_statement_effect(state, statement, location);
         }
-        edges
+
+        let exit_state = state;
+        for pred in body.basic_blocks.predecessors()[block].iter().copied() {
+            match body[pred].terminator().kind {
+                // Apply terminator-specific edge effects.
+                //
+                // FIXME(ecstaticmorse): Avoid cloning the exit state unconditionally.
+                mir::TerminatorKind::Call { destination, target: Some(dest), .. }
+                    if dest == block =>
+                {
+                    let mut tmp = exit_state.clone();
+                    analysis.apply_call_return_effect(
+                        &mut tmp,
+                        pred,
+                        CallReturnPlaces::Call(destination),
+                    );
+                    propagate(pred, &tmp);
+                }
+
+                mir::TerminatorKind::InlineAsm { ref targets, ref operands, .. }
+                    if targets.contains(&block) =>
+                {
+                    let mut tmp = exit_state.clone();
+                    analysis.apply_call_return_effect(
+                        &mut tmp,
+                        pred,
+                        CallReturnPlaces::InlineAsm(operands),
+                    );
+                    propagate(pred, &tmp);
+                }
+
+                mir::TerminatorKind::Yield { resume, resume_arg, .. } if resume == block => {
+                    let mut tmp = exit_state.clone();
+                    analysis.apply_call_return_effect(
+                        &mut tmp,
+                        resume,
+                        CallReturnPlaces::Yield(resume_arg),
+                    );
+                    propagate(pred, &tmp);
+                }
+
+                mir::TerminatorKind::SwitchInt { targets: _, ref discr } => {
+                    let mut applier = BackwardSwitchIntEdgeEffectsApplier {
+                        body,
+                        pred,
+                        exit_state,
+                        block,
+                        propagate: &mut propagate,
+                        effects_applied: false,
+                    };
+
+                    analysis.apply_switch_int_edge_effects(pred, discr, &mut applier);
+
+                    if !applier.effects_applied {
+                        propagate(pred, exit_state)
+                    }
+                }
+
+                _ => propagate(pred, exit_state),
+            }
+        }
     }
 
     fn apply_effects_in_range<'tcx, A>(
@@ -170,7 +226,6 @@ impl Direction for Backward {
 
         vis.visit_block_end(state);
 
-        // Terminator
         let loc = Location { block, statement_index: block_data.statements.len() };
         let term = block_data.terminator();
         results.analysis.apply_before_terminator_effect(state, term, loc);
@@ -188,82 +243,13 @@ impl Direction for Backward {
 
         vis.visit_block_start(state);
     }
-
-    fn join_state_into_successors_of<'tcx, A>(
-        analysis: &mut A,
-        body: &mir::Body<'tcx>,
-        exit_state: &mut A::Domain,
-        bb: BasicBlock,
-        _edges: TerminatorEdges<'_, 'tcx>,
-        mut propagate: impl FnMut(BasicBlock, &A::Domain),
-    ) where
-        A: Analysis<'tcx>,
-    {
-        for pred in body.basic_blocks.predecessors()[bb].iter().copied() {
-            match body[pred].terminator().kind {
-                // Apply terminator-specific edge effects.
-                //
-                // FIXME(ecstaticmorse): Avoid cloning the exit state unconditionally.
-                mir::TerminatorKind::Call { destination, target: Some(dest), .. } if dest == bb => {
-                    let mut tmp = exit_state.clone();
-                    analysis.apply_call_return_effect(
-                        &mut tmp,
-                        pred,
-                        CallReturnPlaces::Call(destination),
-                    );
-                    propagate(pred, &tmp);
-                }
-
-                mir::TerminatorKind::InlineAsm { ref targets, ref operands, .. }
-                    if targets.contains(&bb) =>
-                {
-                    let mut tmp = exit_state.clone();
-                    analysis.apply_call_return_effect(
-                        &mut tmp,
-                        pred,
-                        CallReturnPlaces::InlineAsm(operands),
-                    );
-                    propagate(pred, &tmp);
-                }
-
-                mir::TerminatorKind::Yield { resume, resume_arg, .. } if resume == bb => {
-                    let mut tmp = exit_state.clone();
-                    analysis.apply_call_return_effect(
-                        &mut tmp,
-                        resume,
-                        CallReturnPlaces::Yield(resume_arg),
-                    );
-                    propagate(pred, &tmp);
-                }
-
-                mir::TerminatorKind::SwitchInt { targets: _, ref discr } => {
-                    let mut applier = BackwardSwitchIntEdgeEffectsApplier {
-                        body,
-                        pred,
-                        exit_state,
-                        bb,
-                        propagate: &mut propagate,
-                        effects_applied: false,
-                    };
-
-                    analysis.apply_switch_int_edge_effects(pred, discr, &mut applier);
-
-                    if !applier.effects_applied {
-                        propagate(pred, exit_state)
-                    }
-                }
-
-                _ => propagate(pred, exit_state),
-            }
-        }
-    }
 }
 
 struct BackwardSwitchIntEdgeEffectsApplier<'mir, 'tcx, D, F> {
     body: &'mir mir::Body<'tcx>,
     pred: BasicBlock,
     exit_state: &'mir mut D,
-    bb: BasicBlock,
+    block: BasicBlock,
     propagate: &'mir mut F,
     effects_applied: bool,
 }
@@ -276,8 +262,8 @@ where
     fn apply(&mut self, mut apply_edge_effect: impl FnMut(&mut D, SwitchIntTarget)) {
         assert!(!self.effects_applied);
 
-        let values = &self.body.basic_blocks.switch_sources()[&(self.bb, self.pred)];
-        let targets = values.iter().map(|&value| SwitchIntTarget { value, target: self.bb });
+        let values = &self.body.basic_blocks.switch_sources()[&(self.block, self.pred)];
+        let targets = values.iter().map(|&value| SwitchIntTarget { value, target: self.block });
 
         let mut tmp = None;
         for target in targets {
@@ -298,11 +284,12 @@ impl Direction for Forward {
 
     fn apply_effects_in_block<'mir, 'tcx, A>(
         analysis: &mut A,
+        _body: &mir::Body<'tcx>,
         state: &mut A::Domain,
         block: BasicBlock,
         block_data: &'mir mir::BasicBlockData<'tcx>,
-    ) -> TerminatorEdges<'mir, 'tcx>
-    where
+        mut propagate: impl FnMut(BasicBlock, &A::Domain),
+    ) where
         A: Analysis<'tcx>,
     {
         for (statement_index, statement) in block_data.statements.iter().enumerate() {
@@ -313,7 +300,53 @@ impl Direction for Forward {
         let terminator = block_data.terminator();
         let location = Location { block, statement_index: block_data.statements.len() };
         analysis.apply_before_terminator_effect(state, terminator, location);
-        analysis.apply_terminator_effect(state, terminator, location)
+        let edges = analysis.apply_terminator_effect(state, terminator, location);
+
+        let exit_state = state;
+        match edges {
+            TerminatorEdges::None => {}
+            TerminatorEdges::Single(target) => propagate(target, exit_state),
+            TerminatorEdges::Double(target, unwind) => {
+                propagate(target, exit_state);
+                propagate(unwind, exit_state);
+            }
+            TerminatorEdges::AssignOnReturn { return_, cleanup, place } => {
+                // This must be done *first*, otherwise the unwind path will see the assignments.
+                if let Some(cleanup) = cleanup {
+                    propagate(cleanup, exit_state);
+                }
+
+                if !return_.is_empty() {
+                    analysis.apply_call_return_effect(exit_state, block, place);
+                    for &target in return_ {
+                        propagate(target, exit_state);
+                    }
+                }
+            }
+            TerminatorEdges::SwitchInt { targets, discr } => {
+                let mut applier = ForwardSwitchIntEdgeEffectsApplier {
+                    exit_state,
+                    targets,
+                    propagate,
+                    effects_applied: false,
+                };
+
+                analysis.apply_switch_int_edge_effects(block, discr, &mut applier);
+
+                let ForwardSwitchIntEdgeEffectsApplier {
+                    exit_state,
+                    mut propagate,
+                    effects_applied,
+                    ..
+                } = applier;
+
+                if !effects_applied {
+                    for target in targets.all_targets() {
+                        propagate(*target, exit_state);
+                    }
+                }
+            }
+        }
     }
 
     fn apply_effects_in_range<'tcx, A>(
@@ -351,7 +384,8 @@ impl Direction for Forward {
                 let statement = &block_data.statements[from.statement_index];
                 analysis.apply_statement_effect(state, statement, location);
 
-                // If we only needed to apply the after effect of the statement at `idx`, we are done.
+                // If we only needed to apply the after effect of the statement at `idx`, we are
+                // done.
                 if from == to {
                     return;
                 }
@@ -419,62 +453,6 @@ impl Direction for Forward {
 
         vis.visit_block_end(state);
     }
-
-    fn join_state_into_successors_of<'tcx, A>(
-        analysis: &mut A,
-        _body: &mir::Body<'tcx>,
-        exit_state: &mut A::Domain,
-        bb: BasicBlock,
-        edges: TerminatorEdges<'_, 'tcx>,
-        mut propagate: impl FnMut(BasicBlock, &A::Domain),
-    ) where
-        A: Analysis<'tcx>,
-    {
-        match edges {
-            TerminatorEdges::None => {}
-            TerminatorEdges::Single(target) => propagate(target, exit_state),
-            TerminatorEdges::Double(target, unwind) => {
-                propagate(target, exit_state);
-                propagate(unwind, exit_state);
-            }
-            TerminatorEdges::AssignOnReturn { return_, cleanup, place } => {
-                // This must be done *first*, otherwise the unwind path will see the assignments.
-                if let Some(cleanup) = cleanup {
-                    propagate(cleanup, exit_state);
-                }
-
-                if !return_.is_empty() {
-                    analysis.apply_call_return_effect(exit_state, bb, place);
-                    for &target in return_ {
-                        propagate(target, exit_state);
-                    }
-                }
-            }
-            TerminatorEdges::SwitchInt { targets, discr } => {
-                let mut applier = ForwardSwitchIntEdgeEffectsApplier {
-                    exit_state,
-                    targets,
-                    propagate,
-                    effects_applied: false,
-                };
-
-                analysis.apply_switch_int_edge_effects(bb, discr, &mut applier);
-
-                let ForwardSwitchIntEdgeEffectsApplier {
-                    exit_state,
-                    mut propagate,
-                    effects_applied,
-                    ..
-                } = applier;
-
-                if !effects_applied {
-                    for target in targets.all_targets() {
-                        propagate(*target, exit_state);
-                    }
-                }
-            }
-        }
-    }
 }
 
 struct ForwardSwitchIntEdgeEffectsApplier<'mir, D, F> {
diff --git a/compiler/rustc_mir_dataflow/src/framework/graphviz.rs b/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
index 98a4f58cb5d..6e4994af8b4 100644
--- a/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
@@ -47,20 +47,16 @@ where
 {
     pub(crate) fn new(
         body: &'mir Body<'tcx>,
-        results: Results<'tcx, A>,
+        results: &'mir Results<'tcx, A>,
         style: OutputStyle,
     ) -> Self {
         let reachable = mir::traversal::reachable_as_bitset(body);
-        Formatter { cursor: results.into_results_cursor(body).into(), style, reachable }
+        Formatter { cursor: results.as_results_cursor(body).into(), style, reachable }
     }
 
     fn body(&self) -> &'mir Body<'tcx> {
         self.cursor.borrow().body()
     }
-
-    pub(crate) fn into_results(self) -> Results<'tcx, A> {
-        self.cursor.into_inner().into_results()
-    }
 }
 
 /// A pair of a basic block and an index into that basic blocks `successors`.
diff --git a/compiler/rustc_mir_dataflow/src/framework/mod.rs b/compiler/rustc_mir_dataflow/src/framework/mod.rs
index 244dfe26ad3..88bab250781 100644
--- a/compiler/rustc_mir_dataflow/src/framework/mod.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/mod.rs
@@ -8,8 +8,9 @@
 //! The `impls` module contains several examples of dataflow analyses.
 //!
 //! Then call `iterate_to_fixpoint` on your type that impls `Analysis` to get a `Results`. From
-//! there, you can use a `ResultsCursor` to inspect the fixpoint solution to your dataflow problem,
-//! or implement the `ResultsVisitor` interface and use `visit_results`. The following example uses
+//! there, you can use a `ResultsCursor` to inspect the fixpoint solution to your dataflow problem
+//! (good for inspecting a small number of locations), or implement the `ResultsVisitor` interface
+//! and use `visit_results` (good for inspecting many or all locations). The following example uses
 //! the `ResultsCursor` approach.
 //!
 //! ```ignore (cross-crate-imports)
@@ -278,22 +279,17 @@ pub trait Analysis<'tcx> {
         // every iteration.
         let mut state = self.bottom_value(body);
         while let Some(bb) = dirty_queue.pop() {
-            let bb_data = &body[bb];
-
             // Set the state to the entry state of the block.
             // This is equivalent to `state = entry_sets[bb].clone()`,
             // but it saves an allocation, thus improving compile times.
             state.clone_from(&entry_sets[bb]);
 
-            // Apply the block transfer function, using the cached one if it exists.
-            let edges = Self::Direction::apply_effects_in_block(&mut self, &mut state, bb, bb_data);
-
-            Self::Direction::join_state_into_successors_of(
+            Self::Direction::apply_effects_in_block(
                 &mut self,
                 body,
                 &mut state,
                 bb,
-                edges,
+                &body[bb],
                 |target: BasicBlock, state: &Self::Domain| {
                     let set_changed = entry_sets[target].join(state);
                     if set_changed {
@@ -306,14 +302,13 @@ pub trait Analysis<'tcx> {
         let results = Results { analysis: self, entry_sets };
 
         if tcx.sess.opts.unstable_opts.dump_mir_dataflow {
-            let (res, results) = write_graphviz_results(tcx, body, results, pass_name);
+            let res = write_graphviz_results(tcx, body, &results, pass_name);
             if let Err(e) = res {
                 error!("Failed to write graphviz dataflow results: {}", e);
             }
-            results
-        } else {
-            results
         }
+
+        results
     }
 }
 
diff --git a/compiler/rustc_mir_dataflow/src/framework/results.rs b/compiler/rustc_mir_dataflow/src/framework/results.rs
index ff6cafbfbae..8493a7aa44b 100644
--- a/compiler/rustc_mir_dataflow/src/framework/results.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/results.rs
@@ -17,10 +17,13 @@ use super::{Analysis, ResultsCursor, ResultsVisitor, graphviz, visit_results};
 use crate::errors::{
     DuplicateValuesFor, PathMustEndInFilename, RequiresAnArgument, UnknownFormatter,
 };
+use crate::framework::cursor::ResultsHandle;
 
 pub type EntrySets<'tcx, A> = IndexVec<BasicBlock, <A as Analysis<'tcx>>::Domain>;
 
-/// A dataflow analysis that has converged to fixpoint.
+/// A dataflow analysis that has converged to fixpoint. It only holds the domain values at the
+/// entry of each basic block. Domain values in other parts of the block are recomputed on the fly
+/// by visitors (i.e. `ResultsCursor`, or `ResultsVisitor` impls).
 #[derive(Clone)]
 pub struct Results<'tcx, A>
 where
@@ -34,12 +37,30 @@ impl<'tcx, A> Results<'tcx, A>
 where
     A: Analysis<'tcx>,
 {
-    /// Creates a `ResultsCursor` that can inspect these `Results`.
+    /// Creates a `ResultsCursor` that can inspect these `Results`. Immutably borrows the `Results`,
+    /// which is appropriate when the `Results` is used outside the cursor.
+    pub fn as_results_cursor<'mir>(
+        &'mir self,
+        body: &'mir mir::Body<'tcx>,
+    ) -> ResultsCursor<'mir, 'tcx, A> {
+        ResultsCursor::new(body, ResultsHandle::Borrowed(self))
+    }
+
+    /// Creates a `ResultsCursor` that can mutate these `Results`. Mutably borrows the `Results`,
+    /// which is appropriate when the `Results` is used outside the cursor.
+    pub fn as_results_cursor_mut<'mir>(
+        &'mir mut self,
+        body: &'mir mir::Body<'tcx>,
+    ) -> ResultsCursor<'mir, 'tcx, A> {
+        ResultsCursor::new(body, ResultsHandle::BorrowedMut(self))
+    }
+
+    /// Creates a `ResultsCursor` that takes ownership of the `Results`.
     pub fn into_results_cursor<'mir>(
         self,
         body: &'mir mir::Body<'tcx>,
     ) -> ResultsCursor<'mir, 'tcx, A> {
-        ResultsCursor::new(body, self)
+        ResultsCursor::new(body, ResultsHandle::Owned(self))
     }
 
     /// Gets the dataflow state for the given block.
@@ -74,9 +95,9 @@ where
 pub(super) fn write_graphviz_results<'tcx, A>(
     tcx: TyCtxt<'tcx>,
     body: &mir::Body<'tcx>,
-    results: Results<'tcx, A>,
+    results: &Results<'tcx, A>,
     pass_name: Option<&'static str>,
-) -> (std::io::Result<()>, Results<'tcx, A>)
+) -> std::io::Result<()>
 where
     A: Analysis<'tcx>,
     A::Domain: DebugWithContext<A>,
@@ -87,7 +108,7 @@ where
     let def_id = body.source.def_id();
     let Ok(attrs) = RustcMirAttrs::parse(tcx, def_id) else {
         // Invalid `rustc_mir` attrs are reported in `RustcMirAttrs::parse`
-        return (Ok(()), results);
+        return Ok(());
     };
 
     let file = try {
@@ -104,12 +125,12 @@ where
                 create_dump_file(tcx, "dot", false, A::NAME, &pass_name.unwrap_or("-----"), body)?
             }
 
-            _ => return (Ok(()), results),
+            _ => return Ok(()),
         }
     };
     let mut file = match file {
         Ok(f) => f,
-        Err(e) => return (Err(e), results),
+        Err(e) => return Err(e),
     };
 
     let style = match attrs.formatter {
@@ -132,7 +153,7 @@ where
         file.write_all(&buf)?;
     };
 
-    (lhs, graphviz.into_results())
+    lhs
 }
 
 #[derive(Default)]
diff --git a/compiler/rustc_mir_dataflow/src/framework/visitor.rs b/compiler/rustc_mir_dataflow/src/framework/visitor.rs
index 5c7539eed4d..bde41974d47 100644
--- a/compiler/rustc_mir_dataflow/src/framework/visitor.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/visitor.rs
@@ -26,7 +26,9 @@ pub fn visit_results<'mir, 'tcx, A>(
     }
 }
 
-/// A visitor over the results of an `Analysis`.
+/// A visitor over the results of an `Analysis`. Use this when you want to inspect domain values in
+/// many or all locations; use `ResultsCursor` if you want to inspect domain values only in certain
+/// locations.
 pub trait ResultsVisitor<'mir, 'tcx, A>
 where
     A: Analysis<'tcx>,
diff --git a/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs b/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
index 859019fd1f6..cec654cac72 100644
--- a/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
@@ -15,7 +15,7 @@ use crate::{Analysis, GenKill};
 pub struct MaybeBorrowedLocals;
 
 impl MaybeBorrowedLocals {
-    pub(super) fn transfer_function<'a, T>(&'a self, trans: &'a mut T) -> TransferFunction<'a, T> {
+    pub(super) fn transfer_function<'a, T>(trans: &'a mut T) -> TransferFunction<'a, T> {
         TransferFunction { trans }
     }
 }
@@ -39,7 +39,7 @@ impl<'tcx> Analysis<'tcx> for MaybeBorrowedLocals {
         statement: &Statement<'tcx>,
         location: Location,
     ) {
-        self.transfer_function(trans).visit_statement(statement, location);
+        Self::transfer_function(trans).visit_statement(statement, location);
     }
 
     fn apply_terminator_effect<'mir>(
@@ -48,7 +48,7 @@ impl<'tcx> Analysis<'tcx> for MaybeBorrowedLocals {
         terminator: &'mir Terminator<'tcx>,
         location: Location,
     ) -> TerminatorEdges<'mir, 'tcx> {
-        self.transfer_function(trans).visit_terminator(terminator, location);
+        Self::transfer_function(trans).visit_terminator(terminator, location);
         terminator.edges()
     }
 }
diff --git a/compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs b/compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs
index 576289e228a..a6a4f849720 100644
--- a/compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs
@@ -135,7 +135,7 @@ impl<'tcx> Analysis<'tcx> for MaybeRequiresStorage<'_, 'tcx> {
         loc: Location,
     ) {
         // If a place is borrowed in a statement, it needs storage for that statement.
-        self.borrowed_locals.mut_analysis().apply_statement_effect(trans, stmt, loc);
+        MaybeBorrowedLocals::transfer_function(trans).visit_statement(stmt, loc);
 
         match &stmt.kind {
             StatementKind::StorageDead(l) => trans.kill(*l),
@@ -180,10 +180,7 @@ impl<'tcx> Analysis<'tcx> for MaybeRequiresStorage<'_, 'tcx> {
         loc: Location,
     ) {
         // If a place is borrowed in a terminator, it needs storage for that terminator.
-        self.borrowed_locals
-            .mut_analysis()
-            .transfer_function(trans)
-            .visit_terminator(terminator, loc);
+        MaybeBorrowedLocals::transfer_function(trans).visit_terminator(terminator, loc);
 
         match &terminator.kind {
             TerminatorKind::Call { destination, .. } => {
diff --git a/compiler/rustc_mir_transform/src/coroutine.rs b/compiler/rustc_mir_transform/src/coroutine.rs
index 8295a806d71..ae5506b05e7 100644
--- a/compiler/rustc_mir_transform/src/coroutine.rs
+++ b/compiler/rustc_mir_transform/src/coroutine.rs
@@ -676,12 +676,11 @@ fn locals_live_across_suspend_points<'tcx>(
 
     let mut borrowed_locals_cursor = borrowed_locals_results.clone().into_results_cursor(body);
 
-    // Calculate the MIR locals that we actually need to keep storage around
-    // for.
-    let mut requires_storage_cursor =
+    // Calculate the MIR locals that we need to keep storage around for.
+    let mut requires_storage_results =
         MaybeRequiresStorage::new(borrowed_locals_results.into_results_cursor(body))
-            .iterate_to_fixpoint(tcx, body, None)
-            .into_results_cursor(body);
+            .iterate_to_fixpoint(tcx, body, None);
+    let mut requires_storage_cursor = requires_storage_results.as_results_cursor_mut(body);
 
     // Calculate the liveness of MIR locals ignoring borrows.
     let mut liveness =
@@ -754,7 +753,7 @@ fn locals_live_across_suspend_points<'tcx>(
         body,
         &saved_locals,
         always_live_locals.clone(),
-        requires_storage_cursor.into_results(),
+        requires_storage_results,
     );
 
     LivenessInfo {