Auto merge of #120904 - Nadrieril:match-lowering-intermediate-repr, r…

…=matthewjasper

match lowering: eagerly simplify match pairs

This removes one important complication from match lowering. Before this, match pair simplification (which includes collecting bindings and type ascriptions) was intertwined with the whole match lowering algorithm.

I'm avoiding this by storing in each `MatchPair` the sub-`MatchPair`s that correspond to its subfields. This makes it possible to simplify everything (except or-patterns) in `Candidate::new()`.

This should open up further simplifications. It will also give us proper control over the order of bindings.

r? `@matthewjasper`

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -22,8 +22,6 @@ use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty};
 use rustc_span::symbol::Symbol;
 use rustc_span::{BytePos, Pos, Span};
 use rustc_target::abi::VariantIdx;
-use smallvec::{smallvec, SmallVec};
-
 // helper functions, broken out by category:
 mod simplify;
 mod test;
@@ -949,12 +947,16 @@ struct Candidate<'pat, 'tcx> {
     has_guard: bool,
 
     /// All of these must be satisfied...
-    match_pairs: SmallVec<[MatchPair<'pat, 'tcx>; 1]>,
+    // Invariant: all the `MatchPair`s are recursively simplified.
+    // Invariant: or-patterns must be sorted at the end.
+    match_pairs: Vec<MatchPair<'pat, 'tcx>>,
 
     /// ...these bindings established...
+    // Invariant: not mutated outside `Candidate::new()`.
     bindings: Vec<Binding<'tcx>>,
 
     /// ...and these types asserted...
+    // Invariant: not mutated outside `Candidate::new()`.
     ascriptions: Vec<Ascription<'tcx>>,
 
     /// ...and if this is non-empty, one of these subcandidates also has to match...
@@ -974,19 +976,27 @@ impl<'tcx, 'pat> Candidate<'pat, 'tcx> {
         place: PlaceBuilder<'tcx>,
         pattern: &'pat Pat<'tcx>,
         has_guard: bool,
-        cx: &Builder<'_, 'tcx>,
+        cx: &mut Builder<'_, 'tcx>,
     ) -> Self {
-        Candidate {
+        let mut candidate = Candidate {
             span: pattern.span,
             has_guard,
-            match_pairs: smallvec![MatchPair::new(place, pattern, cx)],
+            match_pairs: vec![MatchPair::new(place, pattern, cx)],
             bindings: Vec::new(),
             ascriptions: Vec::new(),
             subcandidates: Vec::new(),
             otherwise_block: None,
             pre_binding_block: None,
             next_candidate_pre_binding_block: None,
-        }
+        };
+
+        cx.simplify_match_pairs(
+            &mut candidate.match_pairs,
+            &mut candidate.bindings,
+            &mut candidate.ascriptions,
+        );
+
+        candidate
     }
 
     /// Visit the leaf candidates (those with no subcandidates) contained in
@@ -1042,13 +1052,18 @@ struct Ascription<'tcx> {
     variance: ty::Variance,
 }
 
-#[derive(Clone, Debug)]
+#[derive(Debug)]
 pub(crate) struct MatchPair<'pat, 'tcx> {
-    // this place...
+    // This place...
     place: PlaceBuilder<'tcx>,
 
     // ... must match this pattern.
+    // Invariant: after creation and simplification in `Candidate::new()`, all match pairs must be
+    // simplified, i.e. require a test.
     pattern: &'pat Pat<'tcx>,
+
+    /// Precomputed sub-match pairs of `pattern`.
+    subpairs: Vec<Self>,
 }
 
 /// See [`Test`] for more.
@@ -1165,12 +1180,25 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         candidates: &mut [&mut Candidate<'pat, 'tcx>],
         fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
     ) {
-        // Start by simplifying candidates. Once this process is complete, all
-        // the match pairs which remain require some form of test, whether it
-        // be a switch or pattern comparison.
         let mut split_or_candidate = false;
         for candidate in &mut *candidates {
-            split_or_candidate |= self.simplify_candidate(candidate);
+            if let [MatchPair { pattern: Pat { kind: PatKind::Or { pats }, .. }, place, .. }] =
+                &*candidate.match_pairs
+            {
+                // Split a candidate in which the only match-pair is an or-pattern into multiple
+                // candidates. This is so that
+                //
+                // match x {
+                //     0 | 1 => { ... },
+                //     2 | 3 => { ... },
+                // }
+                //
+                // only generates a single switch.
+                candidate.subcandidates =
+                    self.create_or_subcandidates(place, pats, candidate.has_guard);
+                candidate.match_pairs.pop();
+                split_or_candidate = true;
+            }
         }
 
         ensure_sufficient_stack(|| {