Move FlatPat::new into match_pair.rs

This makes it easier to perform coordinated modifications to `FlatPat::new` and
`MatchPairTree::from_pattern`, because the two functions are heavily coupled.

Author: Zalathar

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

@@ -66,10 +66,32 @@ fn prefix_slice_suffix<'a, 'tcx>(
     output_pairs
 }
 
+impl<'tcx> FlatPat<'tcx> {
+    /// Creates a `FlatPat` containing a simplified [`MatchPairTree`] list/forest
+    /// for the given pattern.
+    pub(crate) fn new(
+        place: PlaceBuilder<'tcx>,
+        pattern: &Pat<'tcx>,
+        cx: &mut Builder<'_, 'tcx>,
+    ) -> Self {
+        // Recursively build a tree of match pairs for the given pattern.
+        let mut match_pairs = vec![];
+        let mut extra_data = PatternExtraData {
+            span: pattern.span,
+            bindings: vec![],
+            ascriptions: vec![],
+            is_never: pattern.is_never_pattern(),
+        };
+        MatchPairTree::for_pattern(place, pattern, cx, &mut match_pairs, &mut extra_data);
+
+        FlatPat { match_pairs, extra_data }
+    }
+}
+
 impl<'tcx> MatchPairTree<'tcx> {
     /// Recursively builds a match pair tree for the given pattern and its
     /// subpatterns.
-    pub(super) fn for_pattern(
+    fn for_pattern(
         mut place_builder: PlaceBuilder<'tcx>,
         pattern: &Pat<'tcx>,
         cx: &mut Builder<'_, 'tcx>,

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

@@ -999,24 +999,6 @@ struct FlatPat<'tcx> {
     extra_data: PatternExtraData<'tcx>,
 }
 
-impl<'tcx> FlatPat<'tcx> {
-    /// Creates a `FlatPat` containing a simplified [`MatchPairTree`] list/forest
-    /// for the given pattern.
-    fn new(place: PlaceBuilder<'tcx>, pattern: &Pat<'tcx>, cx: &mut Builder<'_, 'tcx>) -> Self {
-        // Recursively build a tree of match pairs for the given pattern.
-        let mut match_pairs = vec![];
-        let mut extra_data = PatternExtraData {
-            span: pattern.span,
-            bindings: Vec::new(),
-            ascriptions: Vec::new(),
-            is_never: pattern.is_never_pattern(),
-        };
-        MatchPairTree::for_pattern(place, pattern, cx, &mut match_pairs, &mut extra_data);
-
-        Self { match_pairs, extra_data }
-    }
-}
-
 /// Candidates are a generalization of (a) top-level match arms, and
 /// (b) sub-branches of or-patterns, allowing the match-lowering process to handle
 /// them both in a mostly-uniform way. For example, the list of candidates passed