Implement RFC 1506 "Clarify the relationships between various kinds of structs and variants"

src/librustc_passes/ast_validation.rs

@@ -197,27 +197,6 @@ impl<'a> Visitor for AstValidator<'a> {
         visit::walk_foreign_item(self, fi)
     }
 
-    fn visit_variant_data(&mut self,
-                          vdata: &VariantData,
-                          _: Ident,
-                          _: &Generics,
-                          _: NodeId,
-                          span: Span) {
-        if vdata.fields().is_empty() {
-            if vdata.is_tuple() {
-                self.err_handler()
-                    .struct_span_err(span,
-                                     "empty tuple structs and enum variants are not allowed, use \
-                                      unit structs and enum variants instead")
-                    .span_help(span,
-                               "remove trailing `()` to make a unit struct or unit enum variant")
-                    .emit();
-            }
-        }
-
-        visit::walk_struct_def(self, vdata)
-    }
-
     fn visit_vis(&mut self, vis: &Visibility) {
         match *vis {
             Visibility::Restricted { ref path, .. } => {

src/librustc_typeck/check/mod.rs

@@ -116,6 +116,7 @@ use syntax::ast;
 use syntax::attr;
 use syntax::attr::AttrMetaMethods;
 use syntax::codemap::{self, Spanned};
+use syntax::feature_gate::{GateIssue, emit_feature_err};
 use syntax::parse::token::{self, InternedString, keywords};
 use syntax::ptr::P;
 use syntax::util::lev_distance::find_best_match_for_name;
@@ -1700,7 +1701,12 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                                  node_id: ast::NodeId)
                                  -> Ty<'tcx> {
         debug!("instantiate_type_path(did={:?}, path={:?})", did, path);
-        let type_scheme = self.tcx.lookup_item_type(did);
+        let mut type_scheme = self.tcx.lookup_item_type(did);
+        if type_scheme.ty.is_fn() {
+            // Tuple variants have fn type even in type namespace, extract true variant type from it
+            let fn_ret = self.tcx.no_late_bound_regions(&type_scheme.ty.fn_ret()).unwrap().unwrap();
+            type_scheme = ty::TypeScheme { ty: fn_ret, generics: type_scheme.generics }
+        }
         let type_predicates = self.tcx.lookup_predicates(did);
         let substs = AstConv::ast_path_substs_for_ty(self, self,
                                                      path.span,
@@ -3244,19 +3250,24 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             }
             _ => None
         };
-        if variant.is_none() || variant.unwrap().kind == ty::VariantKind::Tuple {
-            // Reject tuple structs for now, braced and unit structs are allowed.
+
+        if let Some(variant) = variant {
+            if variant.kind == ty::VariantKind::Tuple &&
+                    !self.tcx.sess.features.borrow().relaxed_adts {
+                emit_feature_err(&self.tcx.sess.parse_sess.span_diagnostic,
+                                 "relaxed_adts", span, GateIssue::Language,
+                                 "tuple structs and variants in struct patterns are unstable");
+            }
+            let ty = self.instantiate_type_path(def.def_id(), path, node_id);
+            Some((variant, ty))
+        } else {
             struct_span_err!(self.tcx.sess, path.span, E0071,
                              "`{}` does not name a struct or a struct variant",
                              pprust::path_to_string(path))
                 .span_label(path.span, &format!("not a struct"))
                 .emit();
-
-            return None;
+            None
         }
-
-        let ty = self.instantiate_type_path(def.def_id(), path, node_id);
-        Some((variant.unwrap(), ty))
     }
 
     fn check_expr_struct(&self,

src/libsyntax/feature_gate.rs

@@ -280,7 +280,11 @@ declare_features! (
     (active, dotdot_in_tuple_patterns, "1.10.0", Some(33627)),
 
     // Allows `impl Trait` in function return types.
-    (active, conservative_impl_trait, "1.12.0", Some(34511))
+    (active, conservative_impl_trait, "1.12.0", Some(34511)),
+
+    // Allows tuple structs and variants in more contexts,
+    // Permits numeric fields in struct expressions and patterns.
+    (active, relaxed_adts, "1.12.0", Some(35626))
 );
 
 declare_features! (
@@ -1022,9 +1026,8 @@ impl<'a> Visitor for PostExpansionVisitor<'a> {
             }
             PatKind::TupleStruct(_, ref fields, ddpos)
                     if ddpos.is_none() && fields.is_empty() => {
-                self.context.span_handler.struct_span_err(pattern.span,
-                                                          "nullary enum variants are written with \
-                                                           no trailing `( )`").emit();
+                gate_feature_post!(&self, relaxed_adts, pattern.span,
+                                   "empty tuple structs patterns are unstable");
             }
             _ => {}
         }
@@ -1107,6 +1110,19 @@ impl<'a> Visitor for PostExpansionVisitor<'a> {
         visit::walk_impl_item(self, ii);
     }
 
+    fn visit_variant_data(&mut self, vdata: &ast::VariantData, _: ast::Ident,
+                          _: &ast::Generics, _: NodeId, span: Span) {
+        if vdata.fields().is_empty() {
+            if vdata.is_tuple() {
+                gate_feature_post!(&self, relaxed_adts, span,
+                                   "empty tuple structs and enum variants are unstable, \
+                                    use unit structs and enum variants instead");
+            }
+        }
+
+        visit::walk_struct_def(self, vdata)
+    }
+
     fn visit_vis(&mut self, vis: &ast::Visibility) {
         let span = match *vis {
             ast::Visibility::Crate(span) => span,

src/libsyntax/parse/parser.rs

@@ -2009,10 +2009,19 @@ impl<'a> Parser<'a> {
         }
     }
 
+    pub fn parse_field_name(&mut self) -> PResult<'a, Ident> {
+        if let token::Literal(token::Integer(name), None) = self.token {
+            self.bump();
+            Ok(Ident::with_empty_ctxt(name))
+        } else {
+            self.parse_ident()
+        }
+    }
+
     /// Parse ident COLON expr
     pub fn parse_field(&mut self) -> PResult<'a, Field> {
         let lo = self.span.lo;
-        let i = self.parse_ident()?;
+        let i = self.parse_field_name()?;
         let hi = self.last_span.hi;
         self.expect(&token::Colon)?;
         let e = self.parse_expr()?;
@@ -3508,7 +3517,7 @@ impl<'a> Parser<'a> {
             // Check if a colon exists one ahead. This means we're parsing a fieldname.
             let (subpat, fieldname, is_shorthand) = if self.look_ahead(1, |t| t == &token::Colon) {
                 // Parsing a pattern of the form "fieldname: pat"
-                let fieldname = self.parse_ident()?;
+                let fieldname = self.parse_field_name()?;
                 self.bump();
                 let pat = self.parse_pat()?;
                 hi = pat.span.hi;

src/test/compile-fail/E0071.rs

@@ -8,11 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-enum Foo { FirstValue(i32) }
+enum Foo {}
 
 fn main() {
-    let u = Foo::FirstValue { value: 0 };
-    //~^ ERROR `Foo::FirstValue` does not name a struct or a struct variant [E0071]
+    let u = Foo { value: 0 };
+    //~^ ERROR `Foo` does not name a struct or a struct variant [E0071]
     //~| NOTE not a struct
 
     let t = u32 { value: 4 };

src/test/compile-fail/auxiliary/empty-struct.rs

@@ -8,10 +8,14 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(relaxed_adts)]
+
 pub struct XEmpty1 {}
 pub struct XEmpty2;
+pub struct XEmpty6();
 
 pub enum XE {
     XEmpty3 {},
     XEmpty4,
+    XEmpty5(),
 }

src/test/compile-fail/empty-struct-braces-pat-2.rs

@@ -12,6 +12,8 @@
 
 // aux-build:empty-struct.rs
 
+#![feature(relaxed_adts)]
+
 extern crate empty_struct;
 use empty_struct::*;
 
@@ -21,13 +23,12 @@ fn main() {
     let e1 = Empty1 {};
     let xe1 = XEmpty1 {};
 
-    // Rejected by parser as yet
-    // match e1 {
-    //     Empty1() => () // ERROR unresolved enum variant, struct or const `Empty1`
-    // }
-    // match xe1 {
-    //     XEmpty1() => () // ERROR unresolved enum variant, struct or const `XEmpty1`
-    // }
+    match e1 {
+        Empty1() => () //~ ERROR unresolved variant or struct `Empty1`
+    }
+    match xe1 {
+        XEmpty1() => () //~ ERROR unresolved variant or struct `XEmpty1`
+    }
     match e1 {
         Empty1(..) => () //~ ERROR unresolved variant or struct `Empty1`
     }

src/test/compile-fail/empty-struct-braces-pat-3.rs

@@ -12,6 +12,8 @@
 
 // aux-build:empty-struct.rs
 
+#![feature(relaxed_adts)]
+
 extern crate empty_struct;
 use empty_struct::*;
 
@@ -23,13 +25,12 @@ fn main() {
     let e3 = E::Empty3 {};
     let xe3 = XE::XEmpty3 {};
 
-    // Rejected by parser as yet
-    // match e3 {
-    //     E::Empty3() => () // ERROR `E::Empty3` does not name a tuple variant or a tuple struct
-    // }
-    // match xe3 {
-    //     E::Empty3() => () // ERROR `XE::XEmpty3` does not name a tuple variant or a tuple struct
-    // }
+    match e3 {
+        E::Empty3() => () //~ ERROR `E::Empty3` does not name a tuple variant or a tuple struct
+    }
+    match xe3 {
+        XE::XEmpty3() => () //~ ERROR `XE::XEmpty3` does not name a tuple variant or a tuple struct
+    }
     match e3 {
         E::Empty3(..) => () //~ ERROR `E::Empty3` does not name a tuple variant or a tuple struct
     }

src/test/compile-fail/empty-struct-tuple-pat.rs

@@ -0,0 +1,47 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Can't use unit struct as enum pattern
+
+// aux-build:empty-struct.rs
+
+#![feature(relaxed_adts)]
+
+extern crate empty_struct;
+use empty_struct::*;
+
+struct Empty2();
+
+enum E {
+    Empty4()
+}
+
+// remove attribute after warning cycle and promoting warnings to errors
+fn main() {
+    let e2 = Empty2();
+    let e4 = E::Empty4();
+    let xe6 = XEmpty6();
+    let xe5 = XE::XEmpty5();
+
+    match e2 {
+        Empty2 => () //~ ERROR `Empty2` does not name a unit variant, unit struct or a constant
+    }
+    match xe6 {
+        XEmpty6 => () //~ ERROR `XEmpty6` does not name a unit variant, unit struct or a constant
+    }
+
+    match e4 {
+        E::Empty4 => () //~ ERROR `E::Empty4` does not name a unit variant, unit struct or a
+    }
+    match xe5 {
+        XE::XEmpty5 => (), //~ ERROR `XE::XEmpty5` does not name a unit variant, unit struct or a
+        _ => {},
+    }
+}

src/test/compile-fail/empty-struct-unit-pat.rs → src/test/compile-fail/empty-struct-unit-pat-1.rs

@@ -12,6 +12,8 @@
 
 // aux-build:empty-struct.rs
 
+#![feature(relaxed_adts)]
+
 extern crate empty_struct;
 use empty_struct::*;
 
@@ -28,13 +30,6 @@ fn main() {
     let xe2 = XEmpty2;
     let xe4 = XE::XEmpty4;
 
-    // Rejected by parser as yet
-    // match e2 {
-    //     Empty2() => () // ERROR `Empty2` does not name a tuple variant or a tuple struct
-    // }
-    // match xe2 {
-    //     XEmpty2() => () // ERROR `XEmpty2` does not name a tuple variant or a tuple struct
-    // }
     match e2 {
         Empty2(..) => () //~ ERROR `Empty2` does not name a tuple variant or a tuple struct
             //~^ WARNING hard error
@@ -43,14 +38,7 @@ fn main() {
         XEmpty2(..) => () //~ ERROR `XEmpty2` does not name a tuple variant or a tuple struct
             //~^ WARNING hard error
     }
-    // Rejected by parser as yet
-    // match e4 {
-    //     E::Empty4() => () // ERROR `E::Empty4` does not name a tuple variant or a tuple struct
-    // }
-    // match xe4 {
-    //     XE::XEmpty4() => (), // ERROR `XE::XEmpty4` does not name a tuple variant or a tuple
-    //     _ => {},
-    // }
+
     match e4 {
         E::Empty4(..) => () //~ ERROR `E::Empty4` does not name a tuple variant or a tuple struct
             //~^ WARNING hard error

src/test/compile-fail/empty-struct-unit-pat-2.rs

@@ -0,0 +1,47 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Can't use unit struct as enum pattern
+
+// aux-build:empty-struct.rs
+
+#![feature(relaxed_adts)]
+
+extern crate empty_struct;
+use empty_struct::*;
+
+struct Empty2;
+
+enum E {
+    Empty4
+}
+
+// remove attribute after warning cycle and promoting warnings to errors
+fn main() {
+    let e2 = Empty2;
+    let e4 = E::Empty4;
+    let xe2 = XEmpty2;
+    let xe4 = XE::XEmpty4;
+
+    match e2 {
+        Empty2() => () //~ ERROR `Empty2` does not name a tuple variant or a tuple struct
+    }
+    match xe2 {
+        XEmpty2() => () //~ ERROR `XEmpty2` does not name a tuple variant or a tuple struct
+    }
+
+    match e4 {
+        E::Empty4() => () //~ ERROR `E::Empty4` does not name a tuple variant or a tuple struct
+    }
+    match xe4 {
+        XE::XEmpty4() => (), //~ ERROR `XE::XEmpty4` does not name a tuple variant or a tuple
+        _ => {},
+    }
+}