Add special Skip constructor

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -688,6 +688,10 @@ pub enum Constructor<Cx: TypeCx> {
     /// Fake extra constructor for constructors that are not seen in the matrix, as explained at the
     /// top of the file.
     Missing,
+    /// Fake extra constructor that indicates that we should skip the column entirely. This is used
+    /// when a private field is empty, so that we don't observe its emptiness. Only used for
+    /// specialization.
+    Skip,
 }
 
 impl<Cx: TypeCx> Clone for Constructor<Cx> {
@@ -709,6 +713,7 @@ fn clone(&self) -> Self {
             Constructor::NonExhaustive => Constructor::NonExhaustive,
             Constructor::Hidden => Constructor::Hidden,
             Constructor::Missing => Constructor::Missing,
+            Constructor::Skip => Constructor::Skip,
         }
     }
 }
@@ -763,6 +768,8 @@ pub(crate) fn is_covered_by(&self, cx: &Cx, other: &Self) -> Result<bool, Cx::Er
             }
             // Wildcards cover anything
             (_, Wildcard) => true,
+            // `Skip` skips everything.
+            (Skip, _) => true,
             // Only a wildcard pattern can match these special constructors.
             (Missing { .. } | NonExhaustive | Hidden, _) => false,
 

compiler/rustc_pattern_analysis/src/pat.rs

@@ -84,6 +84,8 @@ pub(crate) fn specialize<'a>(
         match (&self.ctor, other_ctor) {
             // Return a wildcard for each field of `other_ctor`.
             (Wildcard, _) => wildcard_sub_tys(),
+            // Skip this column.
+            (_, Skip) => smallvec![],
             // The only non-trivial case: two slices of different arity. `other_slice` is
             // guaranteed to have a larger arity, so we fill the middle part with enough
             // wildcards to reach the length of the new, larger slice.
@@ -192,7 +194,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                 }
                 Ok(())
             }
-            Wildcard | Missing { .. } | NonExhaustive | Hidden => write!(f, "_ : {:?}", pat.ty()),
+            Wildcard | Missing | NonExhaustive | Hidden | Skip => write!(f, "_ : {:?}", pat.ty()),
         }
     }
 }

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -249,16 +249,8 @@ fn reveal_and_alloc<'a, 'tcx>(
                 }
                 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty),
             },
-            Bool(..)
-            | IntRange(..)
-            | F32Range(..)
-            | F64Range(..)
-            | Str(..)
-            | Opaque(..)
-            | NonExhaustive
-            | Hidden
-            | Missing { .. }
-            | Wildcard => &[],
+            Bool(..) | IntRange(..) | F32Range(..) | F64Range(..) | Str(..) | Opaque(..)
+            | NonExhaustive | Hidden | Missing | Skip | Wildcard => &[],
             Or => {
                 bug!("called `Fields::wildcards` on an `Or` ctor")
             }
@@ -288,16 +280,8 @@ pub(crate) fn ctor_arity(&self, ctor: &Constructor<'p, 'tcx>, ty: RevealedTy<'tc
             },
             Ref => 1,
             Slice(slice) => slice.arity(),
-            Bool(..)
-            | IntRange(..)
-            | F32Range(..)
-            | F64Range(..)
-            | Str(..)
-            | Opaque(..)
-            | NonExhaustive
-            | Hidden
-            | Missing { .. }
-            | Wildcard => 0,
+            Bool(..) | IntRange(..) | F32Range(..) | F64Range(..) | Str(..) | Opaque(..)
+            | NonExhaustive | Hidden | Missing | Skip | Wildcard => 0,
             Or => bug!("The `Or` constructor doesn't have a fixed arity"),
         }
     }
@@ -838,7 +822,7 @@ pub fn hoist_witness_pat(&self, pat: &WitnessPat<'p, 'tcx>) -> Pat<'tcx> {
                 }
             }
             &Str(value) => PatKind::Constant { value },
-            Wildcard | NonExhaustive | Hidden => PatKind::Wild,
+            Wildcard | NonExhaustive | Hidden | Skip => PatKind::Wild,
             Missing { .. } => bug!(
                 "trying to convert a `Missing` constructor into a `Pat`; this is probably a bug,
                 `Missing` should have been processed in `apply_constructors`"