Auto merge of #45394 - davidtwco:rfc-2008, r=petrochenkov

RFC 2008: Future-proofing enums/structs with #[non_exhaustive] attribute

This work-in-progress pull request contains my changes to implement [RFC 2008](rust-lang/rfcs#2008). The related tracking issue is #44109.

As of writing, enum-related functionality is not included and there are some issues related to tuple/unit structs. Enum related tests are currently ignored.

WIP PR requested by @nikomatsakis [in Gitter](https://gitter.im/rust-impl-period/WG-compiler-middle?at=59e90e6297cedeb0482ade3e).

src/doc/unstable-book/src/language-features/non-exhaustive.md

@@ -0,0 +1,75 @@
+# `non_exhaustive`
+
+The tracking issue for this feature is: [#44109]
+
+[#44109]: https://github.com/rust-lang/rust/issues/44109
+
+------------------------
+
+The `non_exhaustive` gate allows you to use the `#[non_exhaustive]` attribute
+on structs and enums. When applied within a crate, users of the crate will need
+to use the `_` pattern when matching enums and use the `..` pattern when
+matching structs. Structs marked as `non_exhaustive` will not be able to be
+created normally outside of the defining crate. This is demonstrated below:
+
+```rust,ignore (pseudo-Rust)
+use std::error::Error as StdError;
+
+#[non_exhaustive]
+pub enum Error {
+    Message(String),
+    Other,
+}
+impl StdError for Error {
+    fn description(&self) -> &str {
+        // This will not error, despite being marked as non_exhaustive, as this
+        // enum is defined within the current crate, it can be matched
+        // exhaustively.
+        match *self {
+            Message(ref s) => s,
+            Other => "other or unknown error",
+        }
+    }
+}
+```
+
+```rust,ignore (pseudo-Rust)
+use mycrate::Error;
+
+// This will not error as the non_exhaustive Error enum has been matched with
+// a wildcard.
+match error {
+    Message(ref s) => ...,
+    Other => ...,
+    _ => ...,
+}
+```
+
+```rust,ignore (pseudo-Rust)
+#[non_exhaustive]
+pub struct Config {
+    pub window_width: u16,
+    pub window_height: u16,
+}
+
+// We can create structs as normal within the defining crate when marked as
+// non_exhaustive.
+let config = Config { window_width: 640, window_height: 480 };
+
+// We can match structs exhaustively when within the defining crate.
+if let Ok(Config { window_width, window_height }) = load_config() {
+    // ...
+}
+```
+
+```rust,ignore (pseudo-Rust)
+use mycrate::Config;
+
+// We cannot create a struct like normal if it has been marked as
+// non_exhaustive.
+let config = Config { window_width: 640, window_height: 480 };
+// By adding the `..` we can match the config as below outside of the crate
+// when marked non_exhaustive.
+let &Config { window_width, window_height, .. } = config;
+```
+

src/librustc/ty/mod.rs

@@ -1326,6 +1326,12 @@ bitflags! {
         const IS_FUNDAMENTAL      = 1 << 2;
         const IS_UNION            = 1 << 3;
         const IS_BOX              = 1 << 4;
+        /// Indicates whether this abstract data type will be expanded on in future (new
+        /// fields/variants) and as such, whether downstream crates must match exhaustively on the
+        /// fields/variants of this data type.
+        ///
+        /// See RFC 2008 (https://github.com/rust-lang/rfcs/pull/2008).
+        const IS_NON_EXHAUSTIVE   = 1 << 5;
     }
 }
 
@@ -1526,6 +1532,9 @@ impl<'a, 'gcx, 'tcx> AdtDef {
         if Some(did) == tcx.lang_items().owned_box() {
             flags = flags | AdtFlags::IS_BOX;
         }
+        if tcx.has_attr(did, "non_exhaustive") {
+            flags = flags | AdtFlags::IS_NON_EXHAUSTIVE;
+        }
         match kind {
             AdtKind::Enum => flags = flags | AdtFlags::IS_ENUM,
             AdtKind::Union => flags = flags | AdtFlags::IS_UNION,
@@ -1554,6 +1563,11 @@ impl<'a, 'gcx, 'tcx> AdtDef {
         self.flags.intersects(AdtFlags::IS_ENUM)
     }
 
+    #[inline]
+    pub fn is_non_exhaustive(&self) -> bool {
+        self.flags.intersects(AdtFlags::IS_NON_EXHAUSTIVE)
+    }
+
     /// Returns the kind of the ADT - Struct or Enum.
     #[inline]
     pub fn adt_kind(&self) -> AdtKind {

src/librustc_const_eval/_match.rs

@@ -208,6 +208,20 @@ impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
         }
     }
 
+    fn is_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool {
+        match ty.sty {
+            ty::TyAdt(adt_def, ..) => adt_def.is_enum() && adt_def.is_non_exhaustive(),
+            _ => false,
+        }
+    }
+
+    fn is_local(&self, ty: Ty<'tcx>) -> bool {
+        match ty.sty {
+            ty::TyAdt(adt_def, ..) => adt_def.did.is_local(),
+            _ => false,
+        }
+    }
+
     fn is_variant_uninhabited(&self,
                               variant: &'tcx ty::VariantDef,
                               substs: &'tcx ty::subst::Substs<'tcx>)
@@ -628,9 +642,16 @@ pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 
         let is_privately_empty =
             all_ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
-        debug!("missing_ctors={:?} is_privately_empty={:?}", missing_ctors,
-               is_privately_empty);
-        if missing_ctors.is_empty() && !is_privately_empty {
+        let is_declared_nonexhaustive =
+            cx.is_non_exhaustive_enum(pcx.ty) && !cx.is_local(pcx.ty);
+        debug!("missing_ctors={:?} is_privately_empty={:?} is_declared_nonexhaustive={:?}",
+               missing_ctors, is_privately_empty, is_declared_nonexhaustive);
+
+        // For privately empty and non-exhaustive enums, we work as if there were an "extra"
+        // `_` constructor for the type, so we can never match over all constructors.
+        let is_non_exhaustive = is_privately_empty || is_declared_nonexhaustive;
+
+        if missing_ctors.is_empty() && !is_non_exhaustive {
             all_ctors.into_iter().map(|c| {
                 is_useful_specialized(cx, matrix, v, c.clone(), pcx.ty, witness)
             }).find(|result| result.is_useful()).unwrap_or(NotUseful)
@@ -645,7 +666,51 @@ pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
             match is_useful(cx, &matrix, &v[1..], witness) {
                 UsefulWithWitness(pats) => {
                     let cx = &*cx;
-                    let new_witnesses = if used_ctors.is_empty() {
+                    // In this case, there's at least one "free"
+                    // constructor that is only matched against by
+                    // wildcard patterns.
+                    //
+                    // There are 2 ways we can report a witness here.
+                    // Commonly, we can report all the "free"
+                    // constructors as witnesses, e.g. if we have:
+                    //
+                    // ```
+                    //     enum Direction { N, S, E, W }
+                    //     let Direction::N = ...;
+                    // ```
+                    //
+                    // we can report 3 witnesses: `S`, `E`, and `W`.
+                    //
+                    // However, there are 2 cases where we don't want
+                    // to do this and instead report a single `_` witness:
+                    //
+                    // 1) If the user is matching against a non-exhaustive
+                    // enum, there is no point in enumerating all possible
+                    // variants, because the user can't actually match
+                    // against them himself, e.g. in an example like:
+                    // ```
+                    //     let err: io::ErrorKind = ...;
+                    //     match err {
+                    //         io::ErrorKind::NotFound => {},
+                    //     }
+                    // ```
+                    // we don't want to show every possible IO error,
+                    // but instead have `_` as the witness (this is
+                    // actually *required* if the user specified *all*
+                    // IO errors, but is probably what we want in every
+                    // case).
+                    //
+                    // 2) If the user didn't actually specify a constructor
+                    // in this arm, e.g. in
+                    // ```
+                    //     let x: (Direction, Direction, bool) = ...;
+                    //     let (_, _, false) = x;
+                    // ```
+                    // we don't want to show all 16 possible witnesses
+                    // `(<direction-1>, <direction-2>, true)` - we are
+                    // satisfied with `(_, _, true)`. In this case,
+                    // `used_ctors` is empty.
+                    let new_witnesses = if is_non_exhaustive || used_ctors.is_empty() {
                         // All constructors are unused. Add wild patterns
                         // rather than each individual constructor
                         pats.into_iter().map(|mut witness| {

src/librustc_metadata/encoder.rs

@@ -584,7 +584,8 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
     fn encode_struct_ctor(&mut self, (adt_def_id, def_id): (DefId, DefId)) -> Entry<'tcx> {
         debug!("IsolatedEncoder::encode_struct_ctor({:?})", def_id);
         let tcx = self.tcx;
-        let variant = tcx.adt_def(adt_def_id).struct_variant();
+        let adt_def = tcx.adt_def(adt_def_id);
+        let variant = adt_def.struct_variant();
 
         let data = VariantData {
             ctor_kind: variant.ctor_kind,
@@ -606,6 +607,12 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             }
         }
 
+        // If the structure is marked as non_exhaustive then lower the visibility
+        // to within the crate.
+        if adt_def.is_non_exhaustive() && ctor_vis == ty::Visibility::Public {
+            ctor_vis = ty::Visibility::Restricted(DefId::local(CRATE_DEF_INDEX));
+        }
+
         let repr_options = get_repr_options(&tcx, adt_def_id);
 
         Entry {

src/librustc_passes/ast_validation.rs

@@ -42,6 +42,15 @@ impl<'a> AstValidator<'a> {
         }
     }
 
+    fn invalid_non_exhaustive_attribute(&self, variant: &Variant) {
+        let has_non_exhaustive = variant.node.attrs.iter()
+            .any(|attr| attr.check_name("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, span: Span, note: Option<&str>) {
         if vis != &Visibility::Inherited {
             let mut err = struct_span_err!(self.session,
@@ -224,6 +233,7 @@ 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, field.span, None);
                     }

src/librustc_privacy/lib.rs

@@ -627,6 +627,16 @@ impl<'a, 'tcx> TypePrivacyVisitor<'a, 'tcx> {
                                 ctor_vis = field_vis;
                             }
                         }
+
+                        // If the structure is marked as non_exhaustive then lower the
+                        // visibility to within the crate.
+                        let struct_def_id = self.tcx.hir.get_parent_did(node_id);
+                        let adt_def = self.tcx.adt_def(struct_def_id);
+                        if adt_def.is_non_exhaustive() && ctor_vis == ty::Visibility::Public {
+                            ctor_vis = ty::Visibility::Restricted(
+                                DefId::local(CRATE_DEF_INDEX));
+                        }
+
                         return ctor_vis;
                     }
                     node => bug!("unexpected node kind: {:?}", node)

src/librustc_resolve/build_reduced_graph.rs

@@ -338,11 +338,22 @@ impl<'a> Resolver<'a> {
             // These items live in both the type and value namespaces.
             ItemKind::Struct(ref struct_def, _) => {
                 // Define a name in the type namespace.
-                let def = Def::Struct(self.definitions.local_def_id(item.id));
+                let def_id = self.definitions.local_def_id(item.id);
+                let def = Def::Struct(def_id);
                 self.define(parent, ident, TypeNS, (def, vis, sp, expansion));
 
-                // Record field names for error reporting.
                 let mut ctor_vis = vis;
+
+                let has_non_exhaustive = item.attrs.iter()
+                    .any(|item| item.check_name("non_exhaustive"));
+
+                // If the structure is marked as non_exhaustive then lower the visibility
+                // to within the crate.
+                if has_non_exhaustive && vis == ty::Visibility::Public {
+                    ctor_vis = ty::Visibility::Restricted(DefId::local(CRATE_DEF_INDEX));
+                }
+
+                // Record field names for error reporting.
                 let field_names = struct_def.fields().iter().filter_map(|field| {
                     let field_vis = self.resolve_visibility(&field.vis);
                     if ctor_vis.is_at_least(field_vis, &*self) {
@@ -414,6 +425,7 @@ impl<'a> Resolver<'a> {
         // value namespace, they are reserved for possible future use.
         let ctor_kind = CtorKind::from_ast(&variant.node.data);
         let ctor_def = Def::VariantCtor(def_id, ctor_kind);
+
         self.define(parent, ident, ValueNS, (ctor_def, vis, variant.span, expansion));
     }
 

src/librustc_typeck/check/_match.rs

@@ -825,10 +825,11 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                                def_bm: ty::BindingMode) {
         let tcx = self.tcx;
 
-        let (substs, kind_name) = match adt_ty.sty {
-            ty::TyAdt(adt, substs) => (substs, adt.variant_descr()),
+        let (substs, adt) = match adt_ty.sty {
+            ty::TyAdt(adt, substs) => (substs, adt),
             _ => span_bug!(span, "struct pattern is not an ADT")
         };
+        let kind_name = adt.variant_descr();
 
         // Index the struct fields' types.
         let field_map = variant.fields
@@ -882,6 +883,13 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             self.check_pat_walk(&field.pat, field_ty, def_bm, true);
         }
 
+        // Require `..` if struct has non_exhaustive attribute.
+        if adt.is_struct() && adt.is_non_exhaustive() && !adt.did.is_local() && !etc {
+            span_err!(tcx.sess, span, E0638,
+                      "`..` required with {} marked as non-exhaustive",
+                      kind_name);
+        }
+
         // Report an error if incorrect number of the fields were specified.
         if kind_name == "union" {
             if fields.len() != 1 {

src/librustc_typeck/check/mod.rs

@@ -3448,6 +3448,15 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             hir::QPath::TypeRelative(ref qself, _) => qself.span
         };
 
+        // Prohibit struct expressions when non exhaustive flag is set.
+        if let ty::TyAdt(adt, _) = struct_ty.sty {
+            if !adt.did.is_local() && adt.is_non_exhaustive() {
+                span_err!(self.tcx.sess, expr.span, E0639,
+                          "cannot create non-exhaustive {} using struct expression",
+                          adt.variant_descr());
+            }
+        }
+
         self.check_expr_struct_fields(struct_ty, expected, expr.id, path_span, variant, fields,
                                       base_expr.is_none());
         if let &Some(ref base_expr) = base_expr {