Split out hoisting/printing of box patterns

Author: Zalathar

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -833,6 +833,11 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
         let kind = match pat.ctor() {
             Bool(b) => PatKind::Constant { value: mir::Const::from_bool(cx.tcx, *b) },
             IntRange(range) => return self.hoist_pat_range(range, *pat.ty()),
+            Struct if pat.ty().is_box() => {
+                // Outside of the `alloc` crate, the only way to create a struct pattern
+                // of type `Box` is to use a `box` pattern via #[feature(box_patterns)].
+                PatKind::Box { subpattern: hoist(&pat.fields[0]) }
+            }
             Struct | Variant(_) | UnionField => match pat.ty().kind() {
                 ty::Tuple(..) => PatKind::StructLike {
                     enum_info: EnumInfo::NotEnum,
@@ -841,12 +846,6 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
                         .map(|(i, pattern)| FieldPat { field: FieldIdx::new(i), pattern })
                         .collect(),
                 },
-                ty::Adt(adt_def, _) if adt_def.is_box() => {
-                    // Without `box_patterns`, the only legal pattern of type `Box` is `_` (outside
-                    // of `std`). So this branch is only reachable when the feature is enabled and
-                    // the pattern is a box pattern.
-                    PatKind::Deref { subpattern: subpatterns.next().unwrap() }
-                }
                 &ty::Adt(adt_def, _) => {
                     let variant_index = RustcPatCtxt::variant_index_for_adt(&pat.ctor(), adt_def);
                     let subpatterns = subpatterns

compiler/rustc_pattern_analysis/src/rustc/print.rs

@@ -38,6 +38,10 @@ pub(crate) enum PatKind<'tcx> {
         subpatterns: Vec<FieldPat<'tcx>>,
     },
 
+    Box {
+        subpattern: Box<Pat<'tcx>>,
+    },
+
     Deref {
         subpattern: Box<Pat<'tcx>>,
     },
@@ -64,6 +68,7 @@ impl<'tcx> fmt::Display for Pat<'tcx> {
         match self.kind {
             PatKind::Wild => write!(f, "_"),
             PatKind::Never => write!(f, "!"),
+            PatKind::Box { ref subpattern } => write!(f, "box {subpattern}"),
             PatKind::StructLike { ref enum_info, ref subpatterns } => {
                 write_struct_like(f, self.ty, enum_info, subpatterns)
             }
@@ -184,7 +189,6 @@ fn write_ref_like<'tcx>(
     subpattern: &Pat<'tcx>,
 ) -> fmt::Result {
     match ty.kind() {
-        ty::Adt(def, _) if def.is_box() => write!(f, "box ")?,
         ty::Ref(_, _, mutbl) => {
             write!(f, "&{}", mutbl.prefix_str())?;
         }