Auto merge of #71006 - ecstatic-morse:dataflow-bidi, r=ecstatic-morse

Use existing framework for backward dataflow analyses

This PR adds support for backward analyses to the dataflow framework and adds a new live variable analysis (based on the existing one in `librustc_mir/util/liveness.rs`). By adding these to the framework instead of having a separate API, all newly implemented backward dataflow analyses get cursors/visitors, `rustc_peek` tests, and graphviz visualizations for free. In the near-term, this makes it much easier to implement global dead-store elimination, and I believe that this will enable even more MIR optimizations in the future.

This PR makes many changes to the dataflow API, since some concepts and terminology only make sense in forward dataflow. Below is a list of the important changes.
- ~~`entry_set` -> `fixpoint` (the fixpoint for backward dataflow problems is after the block's terminator)~~
- `seek_{before,after}` -> `seek_{before,after}_primary_effect` (the unprefixed dataflow effect is now referred to as the "primary" effect instead of the "after" effect. The "before" effect remains the same, although I considered changing it to the "antecedent" effect. In both backward and forward dataflow, the "before" effect is applied prior to the "primary" effect. I feel very strongly that this is the correct choice, as it means consumers don't have to switch between `seek_before` and `seek_after` based on the direction of their analysis.
- `seek_after_assume_call_returns` is now gone. Users can use `ResultsCursor::apply_custom_effect` to emulate it.
- `visit_{statement,terminator}_exit` -> `visit_{statement,terminator}_after_primary_effect`
- `visit_{statement,terminator}` -> `visit_{statement,terminator}_before_primary_effect`

Implementing this also required refactoring the dataflow cursor implementation so it could work in both directions. This is a large percentage of the diff, since the cursor code is rather complex. The fact that the cursor is exhaustively tested in both directions should reassure whomever is unlucky enough to review this 🤣.

In order to avoid computing the reverse CFG for forward dataflow analyses, I've added some hacks to the existing `mir::BodyAndCache` interface. I've requested changes to this interface that would let me implement this more efficiently.

r? @eddyb (feel free to reassign)
cc @rust-lang/wg-mir-opt

src/librustc_codegen_ssa/mir/analyze.rs

@@ -269,7 +269,8 @@ impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
 
     fn visit_local(&mut self, &local: &mir::Local, context: PlaceContext, location: Location) {
         match context {
-            PlaceContext::MutatingUse(MutatingUseContext::Call) => {
+            PlaceContext::MutatingUse(MutatingUseContext::Call)
+            | PlaceContext::MutatingUse(MutatingUseContext::Yield) => {
                 self.assign(local, location);
             }
 

src/librustc_middle/mir/visit.rs

@@ -510,7 +510,7 @@ macro_rules! make_mir_visitor {
                         self.visit_operand(value, source_location);
                         self.visit_place(
                             resume_arg,
-                            PlaceContext::MutatingUse(MutatingUseContext::Store),
+                            PlaceContext::MutatingUse(MutatingUseContext::Yield),
                             source_location,
                         );
                     }
@@ -1052,6 +1052,8 @@ pub enum MutatingUseContext {
     AsmOutput,
     /// Destination of a call.
     Call,
+    /// Destination of a yield.
+    Yield,
     /// Being dropped.
     Drop,
     /// Mutable borrow.

src/librustc_mir/borrow_check/mod.rs

@@ -518,7 +518,7 @@ crate struct MirBorrowckCtxt<'cx, 'tcx> {
 impl<'cx, 'tcx> dataflow::ResultsVisitor<'cx, 'tcx> for MirBorrowckCtxt<'cx, 'tcx> {
     type FlowState = Flows<'cx, 'tcx>;
 
-    fn visit_statement(
+    fn visit_statement_before_primary_effect(
         &mut self,
         flow_state: &Flows<'cx, 'tcx>,
         stmt: &'cx Statement<'tcx>,
@@ -607,7 +607,7 @@ impl<'cx, 'tcx> dataflow::ResultsVisitor<'cx, 'tcx> for MirBorrowckCtxt<'cx, 'tc
         }
     }
 
-    fn visit_terminator(
+    fn visit_terminator_before_primary_effect(
         &mut self,
         flow_state: &Flows<'cx, 'tcx>,
         term: &'cx Terminator<'tcx>,
@@ -701,7 +701,7 @@ impl<'cx, 'tcx> dataflow::ResultsVisitor<'cx, 'tcx> for MirBorrowckCtxt<'cx, 'tc
         }
     }
 
-    fn visit_terminator_exit(
+    fn visit_terminator_after_primary_effect(
         &mut self,
         flow_state: &Flows<'cx, 'tcx>,
         term: &'cx Terminator<'tcx>,

src/librustc_mir/borrow_check/type_check/liveness/trace.rs

@@ -408,7 +408,7 @@ impl LivenessContext<'_, '_, '_, 'tcx> {
     /// DROP of some local variable will have an effect -- note that
     /// drops, as they may unwind, are always terminators.
     fn initialized_at_terminator(&mut self, block: BasicBlock, mpi: MovePathIndex) -> bool {
-        self.flow_inits.seek_before(self.body.terminator_loc(block));
+        self.flow_inits.seek_before_primary_effect(self.body.terminator_loc(block));
         self.initialized_at_curr_loc(mpi)
     }
 
@@ -418,7 +418,7 @@ impl LivenessContext<'_, '_, '_, 'tcx> {
     /// **Warning:** Does not account for the result of `Call`
     /// instructions.
     fn initialized_at_exit(&mut self, block: BasicBlock, mpi: MovePathIndex) -> bool {
-        self.flow_inits.seek_after(self.body.terminator_loc(block));
+        self.flow_inits.seek_after_primary_effect(self.body.terminator_loc(block));
         self.initialized_at_curr_loc(mpi)
     }