Rollup merge of #154943 - Zalathar:for-each-subpat, r=Nadrieril

Move recursion out of `MatchPairTree::for_pattern` helpers

The helper functions now just iterate over the relevant subpatterns, while leaving recursion up to the main function.

This avoids passing parameters that were only used for recursive plumbing, and consolidates all recursive calls into `for_pattern` itself, which should make it easier to experiment with changes to the recursive structure.

There should be no change to compiler behaviour.

Author: jhpratt

compiler/rustc_mir_build/src/builder/matches/match_pair.rs

@@ -12,77 +12,58 @@ use crate::builder::matches::{
     FlatPat, MatchPairTree, PatConstKind, PatternExtraData, SliceLenOp, TestableCase,
 };
 
-impl<'a, 'tcx> Builder<'a, 'tcx> {
-    /// Builds and pushes [`MatchPairTree`] subtrees, one for each pattern in
-    /// `subpatterns`, representing the fields of a [`PatKind::Variant`] or
-    /// [`PatKind::Leaf`].
-    ///
-    /// Used internally by [`MatchPairTree::for_pattern`].
-    fn field_match_pairs(
-        &mut self,
-        match_pairs: &mut Vec<MatchPairTree<'tcx>>,
-        extra_data: &mut PatternExtraData<'tcx>,
-        place: PlaceBuilder<'tcx>,
-        subpatterns: &[FieldPat<'tcx>],
-    ) {
-        for fieldpat in subpatterns {
-            let place = place.clone_project(PlaceElem::Field(fieldpat.field, fieldpat.pattern.ty));
-            MatchPairTree::for_pattern(place, &fieldpat.pattern, self, match_pairs, extra_data);
-        }
+/// For an array or slice pattern's subpatterns (prefix/slice/suffix), returns a list
+/// of those subpatterns, each paired with a suitably-projected [`PlaceBuilder`].
+fn prefix_slice_suffix<'a, 'tcx>(
+    place: &PlaceBuilder<'tcx>,
+    array_len: Option<u64>, // Some for array patterns; None for slice patterns
+    prefix: &'a [Pat<'tcx>],
+    opt_slice: &'a Option<Box<Pat<'tcx>>>,
+    suffix: &'a [Pat<'tcx>],
+) -> Vec<(PlaceBuilder<'tcx>, &'a Pat<'tcx>)> {
+    let prefix_len = u64::try_from(prefix.len()).unwrap();
+    let suffix_len = u64::try_from(suffix.len()).unwrap();
+
+    let mut output_pairs =
+        Vec::with_capacity(prefix.len() + usize::from(opt_slice.is_some()) + suffix.len());
+
+    // For slice patterns with a `..` followed by 0 or more suffix subpatterns,
+    // the actual slice index of those subpatterns isn't statically known, so
+    // we have to index them relative to the end of the slice.
+    //
+    // For array patterns, all subpatterns are indexed relative to the start.
+    let (min_length, is_array) = match array_len {
+        Some(len) => (len, true),
+        None => (prefix_len + suffix_len, false),
+    };
+
+    for (offset, prefix_subpat) in (0u64..).zip(prefix) {
+        let elem = ProjectionElem::ConstantIndex { offset, min_length, from_end: false };
+        let subplace = place.clone_project(elem);
+        output_pairs.push((subplace, prefix_subpat));
     }
 
-    /// Builds [`MatchPairTree`] subtrees for the prefix/middle/suffix parts of an
-    /// array pattern or slice pattern, and adds those trees to `match_pairs`.
-    ///
-    /// Used internally by [`MatchPairTree::for_pattern`].
-    fn prefix_slice_suffix(
-        &mut self,
-        match_pairs: &mut Vec<MatchPairTree<'tcx>>,
-        extra_data: &mut PatternExtraData<'tcx>,
-        place: &PlaceBuilder<'tcx>,
-        array_len: Option<u64>,
-        prefix: &[Pat<'tcx>],
-        opt_slice: &Option<Box<Pat<'tcx>>>,
-        suffix: &[Pat<'tcx>],
-    ) {
-        let prefix_len = u64::try_from(prefix.len()).unwrap();
-        let suffix_len = u64::try_from(suffix.len()).unwrap();
-
-        // For slice patterns with a `..` followed by 0 or more suffix subpatterns,
-        // the actual slice index of those subpatterns isn't statically known, so
-        // we have to index them relative to the end of the slice.
-        //
-        // For array patterns, all subpatterns are indexed relative to the start.
-        let (min_length, is_array) = match array_len {
-            Some(len) => (len, true),
-            None => (prefix_len + suffix_len, false),
+    if let Some(slice_subpat) = opt_slice {
+        let elem = PlaceElem::Subslice {
+            from: prefix_len,
+            to: if is_array { min_length - suffix_len } else { suffix_len },
+            from_end: !is_array,
         };
+        let subplace = place.clone_project(elem);
+        output_pairs.push((subplace, slice_subpat));
+    }
 
-        for (offset, subpattern) in (0u64..).zip(prefix) {
-            let elem = ProjectionElem::ConstantIndex { offset, min_length, from_end: false };
-            let place = place.clone_project(elem);
-            MatchPairTree::for_pattern(place, subpattern, self, match_pairs, extra_data)
-        }
-
-        if let Some(subslice_pat) = opt_slice {
-            let subslice = place.clone_project(PlaceElem::Subslice {
-                from: prefix_len,
-                to: if is_array { min_length - suffix_len } else { suffix_len },
-                from_end: !is_array,
-            });
-            MatchPairTree::for_pattern(subslice, subslice_pat, self, match_pairs, extra_data);
-        }
-
-        for (end_offset, subpattern) in (1u64..).zip(suffix.iter().rev()) {
-            let elem = ProjectionElem::ConstantIndex {
-                offset: if is_array { min_length - end_offset } else { end_offset },
-                min_length,
-                from_end: !is_array,
-            };
-            let place = place.clone_project(elem);
-            MatchPairTree::for_pattern(place, subpattern, self, match_pairs, extra_data)
-        }
+    for (offset_from_end, suffix_subpat) in (1u64..).zip(suffix.iter().rev()) {
+        let elem = ProjectionElem::ConstantIndex {
+            offset: if is_array { min_length - offset_from_end } else { offset_from_end },
+            min_length,
+            from_end: !is_array,
+        };
+        let subplace = place.clone_project(elem);
+        output_pairs.push((subplace, suffix_subpat));
     }
+
+    output_pairs
 }
 
 impl<'tcx> MatchPairTree<'tcx> {
@@ -239,15 +220,11 @@ impl<'tcx> MatchPairTree<'tcx> {
                     _ => None,
                 };
                 if let Some(array_len) = array_len {
-                    cx.prefix_slice_suffix(
-                        &mut subpairs,
-                        extra_data,
-                        &place_builder,
-                        Some(array_len),
-                        prefix,
-                        slice,
-                        suffix,
-                    );
+                    for (subplace, subpat) in
+                        prefix_slice_suffix(&place_builder, Some(array_len), prefix, slice, suffix)
+                    {
+                        MatchPairTree::for_pattern(subplace, subpat, cx, &mut subpairs, extra_data);
+                    }
                 } else {
                     // If the array length couldn't be determined, ignore the
                     // subpatterns and delayed-assert that compilation will fail.
@@ -263,15 +240,11 @@ impl<'tcx> MatchPairTree<'tcx> {
                 None
             }
             PatKind::Slice { ref prefix, ref slice, ref suffix } => {
-                cx.prefix_slice_suffix(
-                    &mut subpairs,
-                    extra_data,
-                    &place_builder,
-                    None,
-                    prefix,
-                    slice,
-                    suffix,
-                );
+                for (subplace, subpat) in
+                    prefix_slice_suffix(&place_builder, None, prefix, slice, suffix)
+                {
+                    MatchPairTree::for_pattern(subplace, subpat, cx, &mut subpairs, extra_data);
+                }
 
                 if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
                     // This pattern is shaped like `[..]`. It can match a slice
@@ -294,7 +267,10 @@ impl<'tcx> MatchPairTree<'tcx> {
 
             PatKind::Variant { adt_def, variant_index, args: _, ref subpatterns } => {
                 let downcast_place = place_builder.downcast(adt_def, variant_index); // `(x as Variant)`
-                cx.field_match_pairs(&mut subpairs, extra_data, downcast_place, subpatterns);
+                for &FieldPat { field, pattern: ref subpat } in subpatterns {
+                    let subplace = downcast_place.clone_project(PlaceElem::Field(field, subpat.ty));
+                    MatchPairTree::for_pattern(subplace, subpat, cx, &mut subpairs, extra_data);
+                }
 
                 // We treat non-exhaustive enums the same independent of the crate they are
                 // defined in, to avoid differences in the operational semantics between crates.
@@ -308,7 +284,10 @@ impl<'tcx> MatchPairTree<'tcx> {
             }
 
             PatKind::Leaf { ref subpatterns } => {
-                cx.field_match_pairs(&mut subpairs, extra_data, place_builder, subpatterns);
+                for &FieldPat { field, pattern: ref subpat } in subpatterns {
+                    let subplace = place_builder.clone_project(PlaceElem::Field(field, subpat.ty));
+                    MatchPairTree::for_pattern(subplace, subpat, cx, &mut subpairs, extra_data);
+                }
                 None
             }