Implement #[non_exhaustive] on variants.

This commit removes the check that disallows the `#[non_exhaustive]`
attribute from being placed on enum variants and removes the associated
tests.

Further, this commit lowers the visibility of enum variant constructors
when the variant is marked as non-exhaustive.

Author: davidtwco

src/librustc/ty/mod.rs

@@ -1869,6 +1869,11 @@ impl<'a, 'gcx, 'tcx> VariantDef {
         if adt_kind == AdtKind::Struct && tcx.has_attr(parent_did, "non_exhaustive") {
             debug!("found non-exhaustive field list for {:?}", parent_did);
             flags = flags | VariantFlags::IS_FIELD_LIST_NON_EXHAUSTIVE;
+        } else if let Some(variant_did) = variant_did {
+            if tcx.has_attr(variant_did, "non_exhaustive") {
+                debug!("found non-exhaustive field list for {:?}", variant_did);
+                flags = flags | VariantFlags::IS_FIELD_LIST_NON_EXHAUSTIVE;
+            }
         }
 
         VariantDef {
@@ -2935,8 +2940,8 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
-    // Returns `ty::VariantDef` if `def` refers to a struct,
-    // or variant or their constructors, panics otherwise.
+    /// Returns `ty::VariantDef` if `def` refers to a struct,
+    /// or variant or their constructors, panics otherwise.
     pub fn expect_variant_def(self, def: Def) -> &'tcx VariantDef {
         match def {
             Def::Variant(did) => {

src/librustc_metadata/encoder.rs

@@ -642,13 +642,18 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             }
         };
 
-        // Variant constructors have the same visibility as the parent enums.
+        // Variant constructors have the same visibility as the parent enums, unless marked as
+        // non-exhaustive, in which case they are lowered to `pub(crate)`.
         let enum_id = tcx.hir().as_local_hir_id(enum_did).unwrap();
         let enum_vis = &tcx.hir().expect_item_by_hir_id(enum_id).vis;
+        let mut ctor_vis = ty::Visibility::from_hir(enum_vis, enum_id, tcx);
+        if variant.is_field_list_non_exhaustive() && ctor_vis == ty::Visibility::Public {
+            ctor_vis = ty::Visibility::Restricted(DefId::local(CRATE_DEF_INDEX));
+        }
 
         Entry {
             kind: EntryKind::Variant(self.lazy(&data)),
-            visibility: self.lazy(&ty::Visibility::from_hir(enum_vis, enum_id, tcx)),
+            visibility: self.lazy(&ctor_vis),
             span: self.lazy(&tcx.def_span(def_id)),
             attributes: LazySeq::empty(),
             children: LazySeq::empty(),

src/librustc_passes/ast_validation.rs

@@ -192,14 +192,6 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn invalid_non_exhaustive_attribute(&self, variant: &Variant) {
-        let has_non_exhaustive = attr::contains_name(&variant.node.attrs, "non_exhaustive");
-        if has_non_exhaustive {
-            self.err_handler().span_err(variant.span,
-                                        "#[non_exhaustive] is not yet supported on variants");
-        }
-    }
-
     fn invalid_visibility(&self, vis: &Visibility, note: Option<&str>) {
         if let VisibilityKind::Inherited = vis.node {
             return
@@ -608,7 +600,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             }
             ItemKind::Enum(ref def, _) => {
                 for variant in &def.variants {
-                    self.invalid_non_exhaustive_attribute(variant);
                     for field in variant.node.data.fields() {
                         self.invalid_visibility(&field.vis, None);
                     }

src/librustc_privacy/lib.rs

@@ -244,7 +244,26 @@ fn def_id_visibility<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId)
                     match tcx.hir().get_by_hir_id(parent_hir_id) {
                         Node::Variant(..) => {
                             let parent_did = tcx.hir().local_def_id_from_hir_id(parent_hir_id);
-                            return def_id_visibility(tcx, parent_did);
+                            let (mut ctor_vis, mut span, mut descr) = def_id_visibility(
+                                tcx, parent_did,
+                            );
+
+                            let adt_def = tcx.adt_def(tcx.hir().get_parent_did_by_hir_id(hir_id));
+                            let ctor_did = tcx.hir().local_def_id_from_hir_id(
+                                vdata.ctor_hir_id().unwrap());
+                            let variant = adt_def.variant_with_ctor_id(ctor_did);
+
+                            if variant.is_field_list_non_exhaustive() &&
+                                ctor_vis == ty::Visibility::Public
+                            {
+                                ctor_vis = ty::Visibility::Restricted(
+                                    DefId::local(CRATE_DEF_INDEX));
+                                let attrs = tcx.get_attrs(variant.def_id);
+                                span = attr::find_by_name(&attrs, "non_exhaustive").unwrap().span;
+                                descr = "crate-visible";
+                            }
+
+                            return (ctor_vis, span, descr);
                         }
                         Node::Item(..) => {
                             let item = match tcx.hir().get_by_hir_id(parent_hir_id) {

src/librustc_resolve/build_reduced_graph.rs

@@ -588,6 +588,14 @@ impl<'a> Resolver<'a> {
         let def = Def::Variant(def_id);
         self.define(parent, ident, TypeNS, (def, vis, variant.span, expansion));
 
+        // If the variant is marked as non_exhaustive then lower the visibility to within the
+        // crate.
+        let mut ctor_vis = vis;
+        let has_non_exhaustive = attr::contains_name(&variant.node.attrs, "non_exhaustive");
+        if has_non_exhaustive && vis == ty::Visibility::Public {
+            ctor_vis = ty::Visibility::Restricted(DefId::local(CRATE_DEF_INDEX));
+        }
+
         // Define a constructor name in the value namespace.
         // Braced variants, unlike structs, generate unusable names in
         // value namespace, they are reserved for possible future use.
@@ -597,7 +605,7 @@ impl<'a> Resolver<'a> {
         let ctor_def_id = self.definitions.local_def_id(ctor_node_id);
         let ctor_kind = CtorKind::from_ast(&variant.node.data);
         let ctor_def = Def::Ctor(ctor_def_id, CtorOf::Variant, ctor_kind);
-        self.define(parent, ident, ValueNS, (ctor_def, vis, variant.span, expansion));
+        self.define(parent, ident, ValueNS, (ctor_def, ctor_vis, variant.span, expansion));
     }
 
     /// Constructs the reduced graph for one foreign item.

src/test/run-pass/rfcs/rfc-2008-non-exhaustive/auxiliary/enums.rs

@@ -12,4 +12,48 @@ fn main() {
         NonExhaustiveEnum::Tuple(_) => "second",
         NonExhaustiveEnum::Struct { .. } => "third"
     };
+
+    // Everything below this is expected to compile successfully.
+
+    let enum_unit = NonExhaustiveEnum::Unit;
+
+    match enum_unit {
+        NonExhaustiveEnum::Unit => 1,
+        NonExhaustiveEnum::Tuple(_) => 2,
+        // This particular arm tests that a enum marked as non-exhaustive
+        // will not error if its variants are matched exhaustively.
+        NonExhaustiveEnum::Struct { field } => field,
+        _ => 0 // no error with wildcard
+    };
+
+    match enum_unit {
+        _ => "no error with only wildcard"
+    };
+
+    // #53549: Check that variant constructors can still be called normally.
+    match NonExhaustiveEnum::Unit {
+        NonExhaustiveEnum::Unit => {},
+        _ => {}
+    };
+
+    match NonExhaustiveEnum::Tuple(2) {
+        NonExhaustiveEnum::Tuple(2) => {},
+        _ => {}
+    };
+
+    match (NonExhaustiveEnum::Unit {}) {
+        NonExhaustiveEnum::Unit {} => {},
+        _ => {}
+    };
+
+    match (NonExhaustiveEnum::Tuple { 0: 2 }) {
+        NonExhaustiveEnum::Tuple { 0: 2 } => {},
+        _ => {}
+    };
+
+    match (NonExhaustiveEnum::Struct { field: 2 }) {
+        NonExhaustiveEnum::Struct { field: 2 } => {},
+        _ => {}
+    };
+
 }

src/test/run-pass/rfcs/rfc-2008-non-exhaustive/auxiliary/structs.rs

@@ -1,4 +1,5 @@
 // run-pass
+
 #![feature(non_exhaustive)]
 
 #[non_exhaustive]

src/test/run-pass/rfcs/rfc-2008-non-exhaustive/auxiliary/variants.rs

@@ -35,3 +35,15 @@ fn main() {
     let UnitStruct { } = us;
     //~^ ERROR `..` required with struct marked as non-exhaustive
 }
+
+// Everything below this is expected to compile successfully.
+
+// We only test matching here as we cannot create non-exhaustive
+// structs from another crate. ie. they'll never pass in run-pass tests.
+fn match_structs(ns: NormalStruct, ts: TupleStruct, us: UnitStruct) {
+    let NormalStruct { first_field, second_field, .. } = ns;
+
+    let TupleStruct { 0: first, 1: second, .. } = ts;
+
+    let UnitStruct { .. } = us;
+}

src/test/run-pass/rfcs/rfc-2008-non-exhaustive/enums.rs

@@ -1,29 +1,29 @@
 error[E0423]: expected function, found struct `TupleStruct`
-  --> $DIR/structs.rs:20:14
+  --> $DIR/struct.rs:20:14
    |
 LL |     let ts = TupleStruct(640, 480);
    |              ^^^^^^^^^^^ constructor is not visible here due to private fields
 
 error[E0423]: expected value, found struct `UnitStruct`
-  --> $DIR/structs.rs:29:14
+  --> $DIR/struct.rs:29:14
    |
 LL |     let us = UnitStruct;
    |              ^^^^^^^^^^ constructor is not visible here due to private fields
 
 error[E0603]: tuple struct `TupleStruct` is private
-  --> $DIR/structs.rs:23:32
+  --> $DIR/struct.rs:23:32
    |
 LL |     let ts_explicit = structs::TupleStruct(640, 480);
    |                                ^^^^^^^^^^^
 
 error[E0603]: unit struct `UnitStruct` is private
-  --> $DIR/structs.rs:32:32
+  --> $DIR/struct.rs:32:32
    |
 LL |     let us_explicit = structs::UnitStruct;
    |                                ^^^^^^^^^^
 
 error[E0639]: cannot create non-exhaustive struct using struct expression
-  --> $DIR/structs.rs:7:14
+  --> $DIR/struct.rs:7:14
    |
 LL |       let fr = FunctionalRecord {
    |  ______________^
@@ -35,25 +35,25 @@ LL | |     };
    | |_____^
 
 error[E0639]: cannot create non-exhaustive struct using struct expression
-  --> $DIR/structs.rs:14:14
+  --> $DIR/struct.rs:14:14
    |
 LL |     let ns = NormalStruct { first_field: 640, second_field: 480 };
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0638]: `..` required with struct marked as non-exhaustive
-  --> $DIR/structs.rs:17:9
+  --> $DIR/struct.rs:17:9
    |
 LL |     let NormalStruct { first_field, second_field } = ns;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0638]: `..` required with struct marked as non-exhaustive
-  --> $DIR/structs.rs:26:9
+  --> $DIR/struct.rs:26:9
    |
 LL |     let TupleStruct { 0: first_field, 1: second_field } = ts;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0638]: `..` required with struct marked as non-exhaustive
-  --> $DIR/structs.rs:35:9
+  --> $DIR/struct.rs:35:9
    |
 LL |     let UnitStruct { } = us;
    |         ^^^^^^^^^^^^^^

src/test/run-pass/rfcs/rfc-2008-non-exhaustive/structs.rs

@@ -1,4 +1,5 @@
 // run-pass
+
 #![allow(unused_variables)]
 #![feature(non_exhaustive)]