Rollup merge of #121000 - Nadrieril:keep_all_fields, r=compiler-errors

pattern_analysis: rework how we hide empty private fields

Consider this:
```rust
mod foo {
  pub struct Bar {
    pub a: bool,
    b: !,
  }
}

fn match_a_bar(bar: foo::Bar) -> bool {
  match bar {
    Bar { a, .. } => a,
  }
}
```

Because the field `b` is private, matches outside the module are not allowed to observe the fact that `Bar` is empty. In particular `match bar {}` is valid within the module `foo` but an error outside (assuming `exhaustive_patterns`).

We currently handle this by hiding the field `b` when it's both private and empty. This means that the pattern `Bar { a, .. }` is lowered to `Bar(a, _)` if we're inside of `foo` and to `Bar(a)` outside. This involves a bit of a dance to keep field indices straight. But most importantly this makes pattern lowering depend on the module.

In this PR, I instead do nothing special when lowering. Only during analysis do we track whether a place must be skipped.

r? `@compiler-errors`

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -688,6 +688,9 @@ 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 and empty field that is private. When we encounter one
+    /// we skip the column entirely so we don't observe its emptiness. Only used for specialization.
+    PrivateUninhabited,
 }
 
 impl<Cx: TypeCx> Clone for Constructor<Cx> {
@@ -709,6 +712,7 @@ impl<Cx: TypeCx> Clone for Constructor<Cx> {
             Constructor::NonExhaustive => Constructor::NonExhaustive,
             Constructor::Hidden => Constructor::Hidden,
             Constructor::Missing => Constructor::Missing,
+            Constructor::PrivateUninhabited => Constructor::PrivateUninhabited,
         }
     }
 }
@@ -763,6 +767,8 @@ impl<Cx: TypeCx> Constructor<Cx> {
             }
             // Wildcards cover anything
             (_, Wildcard) => true,
+            // `PrivateUninhabited` skips everything.
+            (PrivateUninhabited, _) => true,
             // Only a wildcard pattern can match these special constructors.
             (Missing { .. } | NonExhaustive | Hidden, _) => false,