Make it possible for `ResultsCursor` to borrow a `Results`.

`ResultsCursor` currently owns its `Results`. But sometimes the
`Results` is needed again afterwards. So there is
`ResultsCursor::into_results` for extracting the `Results`, which leads
to some awkwardness.

This commit adds `ResultsHandle`, a `Cow`-like type that can either
borrow or own a a `Results`. `ResultsCursor` now uses it. This is good
because some `ResultsCursor`s really want to own their `Results`, while
others just want to borrow it.

We end with with a few more lines of code, but get some nice cleanups.
- `ResultsCursor::into_results` and `Formatter::into_results` are
  removed.
- `write_graphviz_results` now just borrows a `Results`, instead of the
  awkward "take ownership of a `Results` and then return it unchanged"
  pattern.

This reinstates the cursor flexibility that was lost in #118230 -- which
removed the old `ResultsRefCursor` and `ResultsCloneCursor` types -- but
in a much simpler way. Hooray!

5 files changed, 81 insertions, 31 deletions

diff --git a/compiler/rustc_mir_dataflow/src/framework/cursor.rs b/compiler/rustc_mir_dataflow/src/framework/cursor.rs
index fbd9376917a..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;
@@ -8,6 +9,47 @@ use rustc_middle::mir::{self, BasicBlock, Location};
 
 use super::{Analysis, Direction, Effect, EffectIndex, Results};
 
+/// 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.
@@ -23,7 +65,7 @@ where
     A: Analysis<'tcx>,
 {
     body: &'mir mir::Body<'tcx>,
-    results: Results<'tcx, A>,
+    results: ResultsHandle<'mir, 'tcx, A>,
     state: A::Domain,
 
     pos: CursorPosition,
@@ -51,13 +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/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 f9e7ba743fc..88bab250781 100644
--- a/compiler/rustc_mir_dataflow/src/framework/mod.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/mod.rs
@@ -302,14 +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 7c775ae7f4a..8493a7aa44b 100644
--- a/compiler/rustc_mir_dataflow/src/framework/results.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/results.rs
@@ -17,6 +17,7 @@ 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>;
 
@@ -36,12 +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.
@@ -76,9 +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>,
@@ -89,7 +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 {
@@ -106,12 +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 {
@@ -134,7 +153,7 @@ where
         file.write_all(&buf)?;
     };
 
-    (lhs, graphviz.into_results())
+    lhs
 }
 
 #[derive(Default)]
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 {