Auto merge of #76044 - ecstatic-morse:dataflow-lattice, r=oli-obk

Support dataflow problems on arbitrary lattices

This PR implements last of the proposed extensions I mentioned in the design meeting for the original dataflow refactor. It extends the current dataflow framework to work with arbitrary lattices, not just `BitSet`s. This is a prerequisite for dataflow-enabled MIR const-propagation. Personally, I am skeptical of the usefulness of doing const-propagation pre-monomorphization, since many useful constants only become known after monomorphization (e.g. `size_of::<T>()`) and users have a natural tendency to hand-optimize the rest. It's probably worth exprimenting with, however, and others have shown interest cc `@rust-lang/wg-mir-opt.`

The `Idx` associated type is moved from `AnalysisDomain` to `GenKillAnalysis` and replaced with an associated `Domain` type that must implement `JoinSemiLattice`. Like before, each `Analysis` defines the "bottom value" for its domain, but can no longer override the dataflow join operator. Analyses that want to use set intersection must now use the `lattice::Dual` newtype. `GenKillAnalysis` impls have an additional requirement that `Self::Domain: BorrowMut<BitSet<Self::Idx>>`, which effectively means that they must use `BitSet<Self::Idx>` or `lattice::Dual<BitSet<Self::Idx>>` as their domain.

Most of these changes were mechanical. However, because a `Domain` is no longer always a powerset of some index type, we can no longer use an `IndexVec<BasicBlock, GenKillSet<A::Idx>>>` to store cached block transfer functions. Instead, we use a boxed `dyn Fn` trait object. I discuss a few alternatives to the current approach in a commit message.

The majority of new lines of code are to preserve existing Graphviz diagrams for those unlucky enough to have to debug dataflow analyses. I find these diagrams incredibly useful when things are going wrong and considered regressing them unacceptable, especially the pretty-printing of `MovePathIndex`s, which are used in many dataflow analyses. This required a parallel `fmt` trait used only for printing dataflow domains, as well as a refactoring of the `graphviz` module now that we cannot expect the domain to be a `BitSet`. Some features did have to be removed, such as the gen/kill display mode (which I didn't use but existed to mirror the output of the old dataflow framework) and line wrapping. Since I had to rewrite much of it anyway, I took the opportunity to switch to a `Visitor` for printing dataflow state diffs instead of using cursors, which are error prone for code that must be generic over both forward and backward analyses. As a side-effect of this change, we no longer have quadratic behavior when writing graphviz diagrams for backward dataflow analyses.

r? `@pnkfelix`

compiler/rustc_mir/src/transform/rustc_peek.rs

@@ -1,4 +1,6 @@
-use rustc_ast as ast;
+use std::borrow::Borrow;
+
+use rustc_ast::ast;
 use rustc_span::symbol::sym;
 use rustc_span::Span;
 use rustc_target::spec::abi::Abi;
@@ -16,7 +18,7 @@ use crate::dataflow::impls::{
 use crate::dataflow::move_paths::{HasMoveData, MoveData};
 use crate::dataflow::move_paths::{LookupResult, MovePathIndex};
 use crate::dataflow::MoveDataParamEnv;
-use crate::dataflow::{Analysis, Results, ResultsCursor};
+use crate::dataflow::{Analysis, JoinSemiLattice, Results, ResultsCursor};
 
 pub struct SanityCheck;
 
@@ -248,25 +250,26 @@ pub trait RustcPeekAt<'tcx>: Analysis<'tcx> {
         &self,
         tcx: TyCtxt<'tcx>,
         place: mir::Place<'tcx>,
-        flow_state: &BitSet<Self::Idx>,
+        flow_state: &Self::Domain,
         call: PeekCall,
     );
 }
 
-impl<'tcx, A> RustcPeekAt<'tcx> for A
+impl<'tcx, A, D> RustcPeekAt<'tcx> for A
 where
-    A: Analysis<'tcx, Idx = MovePathIndex> + HasMoveData<'tcx>,
+    A: Analysis<'tcx, Domain = D> + HasMoveData<'tcx>,
+    D: JoinSemiLattice + Clone + Borrow<BitSet<MovePathIndex>>,
 {
     fn peek_at(
         &self,
         tcx: TyCtxt<'tcx>,
         place: mir::Place<'tcx>,
-        flow_state: &BitSet<Self::Idx>,
+        flow_state: &Self::Domain,
         call: PeekCall,
     ) {
         match self.move_data().rev_lookup.find(place.as_ref()) {
             LookupResult::Exact(peek_mpi) => {
-                let bit_state = flow_state.contains(peek_mpi);
+                let bit_state = flow_state.borrow().contains(peek_mpi);
                 debug!("rustc_peek({:?} = &{:?}) bit_state: {}", call.arg, place, bit_state);
                 if !bit_state {
                     tcx.sess.span_err(call.span, "rustc_peek: bit not set");