Auto merge of #119031 - Nadrieril:two-phase-match-lowering, r=<try>

[Experiment] Play with match lowering

Match lowering to MIR has the reputation of being the most intricate piece of the compiler, and after banging my head on it for a bit I sure hope there isn't anything more intricate somewhere else. It's good quality code but I hope to unentangle it. This PR is me wrestling with it and asking `rustc-timer` for its opinion.

r? `@ghost`

Author: bors

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;
@@ -951,12 +949,15 @@ 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.
+    match_pairs: Vec<MatchPair<'pat, 'tcx>>,
 
     /// ...these bindings established...
+    // Invariant: not mutated during match tree construction.
     bindings: Vec<Binding<'tcx>>,
 
     /// ...and these types asserted...
+    // Invariant: not mutated during match tree construction.
     ascriptions: Vec<Ascription<'tcx>>,
 
     /// ...and if this is non-empty, one of these subcandidates also has to match...
@@ -976,19 +977,27 @@ 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_candidate(
+            &mut candidate.bindings,
+            &mut candidate.ascriptions,
+            &mut candidate.match_pairs,
+        );
+
+        candidate
     }
 
     /// Visit the leaf candidates (those with no subcandidates) contained in
@@ -1044,13 +1053,17 @@ 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: this pattern must be simplified, i.e. requires a test.
     pattern: &'pat Pat<'tcx>,
+
+    /// Precomputed sub-match pairs.
+    subpairs: Vec<Self>,
 }
 
 /// See [`Test`] for more.
@@ -1172,7 +1185,27 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         // be a switch or pattern comparison.
         let mut split_or_candidate = false;
         for candidate in &mut *candidates {
-            split_or_candidate |= self.simplify_candidate(candidate);
+            split_or_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();
+                    true
+                } else {
+                    false
+                }
+            };
         }
 
         ensure_sufficient_stack(|| {

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

@@ -15,31 +15,20 @@
 use crate::build::expr::as_place::PlaceBuilder;
 use crate::build::matches::{Ascription, Binding, Candidate, MatchPair};
 use crate::build::Builder;
+use rustc_middle::mir::PlaceElem;
 use rustc_middle::thir::{self, *};
-
 use std::mem;
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Simplify a candidate so that all match pairs require a test.
-    ///
-    /// This method will also 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. If this happens this method returns
-    /// `true`.
-    #[instrument(skip(self, candidate), level = "debug")]
+    #[instrument(skip(self), level = "debug")]
     pub(super) fn simplify_candidate<'pat>(
         &mut self,
-        candidate: &mut Candidate<'pat, 'tcx>,
-    ) -> bool {
+        candidate_bindings: &mut Vec<Binding<'tcx>>,
+        candidate_ascriptions: &mut Vec<Ascription<'tcx>>,
+        candidate_match_pairs: &mut Vec<MatchPair<'pat, 'tcx>>,
+    ) {
         // repeatedly simplify match pairs until fixed point is reached
-        debug!("{candidate:#?}");
 
         // existing_bindings and new_bindings exists to keep the semantics in order.
         // Reversing the binding order for bindings after `@` changes the binding order in places
@@ -59,28 +48,32 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         // binding in iter 2: [6, 7]
         //
         // final binding: [1, 2, 3, 6, 7, 4, 5]
-        let mut existing_bindings = mem::take(&mut candidate.bindings);
+        let mut existing_bindings = mem::take(candidate_bindings);
         let mut new_bindings = Vec::new();
         loop {
-            let match_pairs = mem::take(&mut candidate.match_pairs);
+            let mut match_pairs = mem::take(candidate_match_pairs);
 
-            if let [MatchPair { pattern: Pat { kind: PatKind::Or { pats }, .. }, place }] =
-                &*match_pairs
+            if let [MatchPair { pattern: Pat { kind: PatKind::Or { .. }, .. }, .. }] = &*match_pairs
             {
                 existing_bindings.extend_from_slice(&new_bindings);
-                mem::swap(&mut candidate.bindings, &mut existing_bindings);
-                candidate.subcandidates = self.create_or_subcandidates(candidate, place, pats);
-                return true;
+                mem::swap(candidate_bindings, &mut existing_bindings);
+                mem::swap(candidate_match_pairs, &mut match_pairs);
+                return;
             }
 
             let mut changed = false;
             for match_pair in match_pairs {
-                match self.simplify_match_pair(match_pair, candidate) {
+                match self.simplify_match_pair(
+                    match_pair,
+                    candidate_bindings,
+                    candidate_ascriptions,
+                    candidate_match_pairs,
+                ) {
                     Ok(()) => {
                         changed = true;
                     }
                     Err(match_pair) => {
-                        candidate.match_pairs.push(match_pair);
+                        candidate_match_pairs.push(match_pair);
                     }
                 }
             }
@@ -97,38 +90,43 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             //     let z = x.copy_field;
             //     let y = x;
             // }
-            candidate.bindings.extend_from_slice(&new_bindings);
-            mem::swap(&mut candidate.bindings, &mut new_bindings);
-            candidate.bindings.clear();
+            candidate_bindings.extend_from_slice(&new_bindings);
+            mem::swap(candidate_bindings, &mut new_bindings);
+            candidate_bindings.clear();
 
             if !changed {
                 existing_bindings.extend_from_slice(&new_bindings);
-                mem::swap(&mut candidate.bindings, &mut existing_bindings);
+                mem::swap(candidate_bindings, &mut existing_bindings);
                 // Move or-patterns to the end, because they can result in us
                 // creating additional candidates, so we want to test them as
                 // late as possible.
-                candidate
-                    .match_pairs
+                candidate_match_pairs
                     .sort_by_key(|pair| matches!(pair.pattern.kind, PatKind::Or { .. }));
-                debug!(simplified = ?candidate, "simplify_candidate");
-                return false; // if we were not able to simplify any, done.
+                debug!(simplified = ?candidate_match_pairs, "simplify_candidate");
+                return; // if we were not able to simplify any, done.
             }
         }
     }
 
     /// Given `candidate` that has a single or-pattern for its match-pairs,
     /// creates a fresh candidate for each of its input subpatterns passed via
     /// `pats`.
-    fn create_or_subcandidates<'pat>(
+    pub(super) fn create_or_subcandidates<'pat>(
         &mut self,
-        candidate: &Candidate<'pat, 'tcx>,
         place: &PlaceBuilder<'tcx>,
         pats: &'pat [Box<Pat<'tcx>>],
+        has_guard: bool,
     ) -> Vec<Candidate<'pat, 'tcx>> {
         pats.iter()
             .map(|box pat| {
-                let mut candidate = Candidate::new(place.clone(), pat, candidate.has_guard, self);
-                self.simplify_candidate(&mut candidate);
+                let mut candidate = Candidate::new(place.clone(), pat, has_guard, self);
+                if let [MatchPair { pattern: Pat { kind: PatKind::Or { pats }, .. }, place, .. }] =
+                    &*candidate.match_pairs
+                {
+                    candidate.subcandidates =
+                        self.create_or_subcandidates(place, pats, candidate.has_guard);
+                    candidate.match_pairs.pop();
+                }
                 candidate
             })
             .collect()
@@ -141,8 +139,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// candidate.
     fn simplify_match_pair<'pat>(
         &mut self,
-        match_pair: MatchPair<'pat, 'tcx>,
-        candidate: &mut Candidate<'pat, 'tcx>,
+        mut match_pair: MatchPair<'pat, 'tcx>,
+        bindings: &mut Vec<Binding<'tcx>>,
+        ascriptions: &mut Vec<Ascription<'tcx>>,
+        match_pairs: &mut Vec<MatchPair<'pat, 'tcx>>,
     ) -> Result<(), MatchPair<'pat, 'tcx>> {
         match match_pair.pattern.kind {
             PatKind::AscribeUserType {
@@ -151,14 +151,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             } => {
                 // Apply the type ascription to the value at `match_pair.place`, which is the
                 if let Some(source) = match_pair.place.try_to_place(self) {
-                    candidate.ascriptions.push(Ascription {
+                    ascriptions.push(Ascription {
                         annotation: annotation.clone(),
                         source,
                         variance,
                     });
                 }
 
-                candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern, self));
+                match_pairs.push(MatchPair::new(match_pair.place, subpattern, self));
 
                 Ok(())
             }
@@ -178,7 +178,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 is_primary: _,
             } => {
                 if let Some(source) = match_pair.place.try_to_place(self) {
-                    candidate.bindings.push(Binding {
+                    bindings.push(Binding {
                         span: match_pair.pattern.span,
                         source,
                         var_id: var,
@@ -188,7 +188,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
                 if let Some(subpattern) = subpattern.as_ref() {
                     // this is the `x @ P` case; have to keep matching against `P` now
-                    candidate.match_pairs.push(MatchPair::new(match_pair.place, subpattern, self));
+                    match_pairs.push(MatchPair::new(match_pair.place, subpattern, self));
                 }
 
                 Ok(())
@@ -206,7 +206,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
 
             PatKind::InlineConstant { subpattern: ref pattern, def: _ } => {
-                candidate.match_pairs.push(MatchPair::new(match_pair.place, pattern, self));
+                match_pairs.push(MatchPair::new(match_pair.place, pattern, self));
 
                 Ok(())
             }
@@ -222,15 +222,17 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             PatKind::Slice { ref prefix, ref slice, ref suffix } => {
                 if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
                     // irrefutable
+                    self.prefix_slice_suffix(match_pairs, &match_pair.place, prefix, slice, suffix);
+                    Ok(())
+                } else {
                     self.prefix_slice_suffix(
-                        &mut candidate.match_pairs,
+                        &mut match_pair.subpairs,
                         &match_pair.place,
                         prefix,
                         slice,
                         suffix,
                     );
-                    Ok(())
-                } else {
+                    self.simplify_candidate(bindings, ascriptions, &mut match_pair.subpairs);
                     Err(match_pair)
                 }
             }
@@ -248,35 +250,43 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     || !adt_def.is_variant_list_non_exhaustive());
                 if irrefutable {
                     let place_builder = match_pair.place.downcast(adt_def, variant_index);
-                    candidate
-                        .match_pairs
-                        .extend(self.field_match_pairs(place_builder, subpatterns));
+                    match_pairs.extend(self.field_match_pairs(place_builder, subpatterns));
                     Ok(())
                 } else {
+                    // If we have a match-pattern like `x @ Enum::Variant(P1, P2)`,
+                    // we want to create a set of derived match-patterns like
+                    // `(x as Variant).0 @ P1` and `(x as Variant).1 @ P1`.
+                    let downcast_place = match_pair.place.clone().downcast(adt_def, variant_index); // `(x as Variant)`
+                    let consequent_match_pairs = subpatterns.iter().map(|subpattern| {
+                        // e.g., `(x as Variant).0`
+                        let place = downcast_place.clone_project(PlaceElem::Field(
+                            subpattern.field,
+                            subpattern.pattern.ty,
+                        ));
+                        // e.g., `(x as Variant).0 @ P1`
+                        MatchPair::new(place, &subpattern.pattern, self)
+                    });
+
+                    match_pair.subpairs.extend(consequent_match_pairs);
+                    self.simplify_candidate(bindings, ascriptions, &mut match_pair.subpairs);
                     Err(match_pair)
                 }
             }
 
             PatKind::Array { ref prefix, ref slice, ref suffix } => {
-                self.prefix_slice_suffix(
-                    &mut candidate.match_pairs,
-                    &match_pair.place,
-                    prefix,
-                    slice,
-                    suffix,
-                );
+                self.prefix_slice_suffix(match_pairs, &match_pair.place, prefix, slice, suffix);
                 Ok(())
             }
 
             PatKind::Leaf { ref subpatterns } => {
                 // tuple struct, match subpats (if any)
-                candidate.match_pairs.extend(self.field_match_pairs(match_pair.place, subpatterns));
+                match_pairs.extend(self.field_match_pairs(match_pair.place, subpatterns));
                 Ok(())
             }
 
             PatKind::Deref { ref subpattern } => {
                 let place_builder = match_pair.place.deref();
-                candidate.match_pairs.push(MatchPair::new(place_builder, subpattern, self));
+                match_pairs.push(MatchPair::new(place_builder, subpattern, self));
                 Ok(())
             }