Implement empty structs with braces (RFC 218)

src/doc/reference.md

@@ -1178,11 +1178,22 @@ let px: i32 = match p { Point(x, _) => x };
 ```
 
 A _unit-like struct_ is a structure without any fields, defined by leaving off
-the list of fields entirely. Such types will have a single value. For example:
+the list of fields entirely. Such a structure implicitly defines a constant of
+its type with the same name. For example:
 
 ```
+# #![feature(braced_empty_structs)]
 struct Cookie;
-let c = [Cookie, Cookie, Cookie, Cookie];
+let c = [Cookie, Cookie {}, Cookie, Cookie {}];
+```
+
+is equivalent to
+
+```
+# #![feature(braced_empty_structs)]
+struct Cookie {}
+const Cookie: Cookie = Cookie {};
+let c = [Cookie, Cookie {}, Cookie, Cookie {}];
 ```
 
 The precise memory layout of a structure is not specified. One can specify a
@@ -2411,6 +2422,7 @@ The currently implemented features of the reference compiler are:
                               terms of encapsulation).
 * - `default_type_parameter_fallback` - Allows type parameter defaults to
                                         influence type inference.
+* - `braced_empty_structs` - Allows use of empty structs with braces.
 
 If a feature is promoted to a language feature, then all existing programs will
 start to receive compilation warnings about `#![feature]` directives which enabled

src/librustc_front/print/pprust.rs

@@ -1218,34 +1218,34 @@ impl<'a> State<'a> {
                          fields: &[hir::Field],
                          wth: &Option<P<hir::Expr>>) -> io::Result<()> {
         try!(self.print_path(path, true, 0));
-        if !(fields.is_empty() && wth.is_none()) {
-            try!(word(&mut self.s, "{"));
-            try!(self.commasep_cmnt(
-                Consistent,
-                &fields[..],
-                |s, field| {
-                    try!(s.ibox(indent_unit));
-                    try!(s.print_ident(field.ident.node));
-                    try!(s.word_space(":"));
-                    try!(s.print_expr(&*field.expr));
-                    s.end()
-                },
-                |f| f.span));
-            match *wth {
-                Some(ref expr) => {
-                    try!(self.ibox(indent_unit));
-                    if !fields.is_empty() {
-                        try!(word(&mut self.s, ","));
-                        try!(space(&mut self.s));
-                    }
-                    try!(word(&mut self.s, ".."));
-                    try!(self.print_expr(&**expr));
-                    try!(self.end());
+        try!(word(&mut self.s, "{"));
+        try!(self.commasep_cmnt(
+            Consistent,
+            &fields[..],
+            |s, field| {
+                try!(s.ibox(indent_unit));
+                try!(s.print_ident(field.ident.node));
+                try!(s.word_space(":"));
+                try!(s.print_expr(&*field.expr));
+                s.end()
+            },
+            |f| f.span));
+        match *wth {
+            Some(ref expr) => {
+                try!(self.ibox(indent_unit));
+                if !fields.is_empty() {
+                    try!(word(&mut self.s, ","));
+                    try!(space(&mut self.s));
                 }
-                _ => try!(word(&mut self.s, ",")),
+                try!(word(&mut self.s, ".."));
+                try!(self.print_expr(&**expr));
+                try!(self.end());
+            }
+            _ => if !fields.is_empty() {
+                try!(word(&mut self.s, ","))
             }
-            try!(word(&mut self.s, "}"));
         }
+        try!(word(&mut self.s, "}"));
         Ok(())
     }
 

src/librustc_typeck/check/mod.rs

@@ -1473,14 +1473,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => return None
         };
 
-        if let ty::VariantKind::Dict = variant.kind() {
+        let var_kind = variant.kind();
+        if var_kind == ty::VariantKind::Dict || var_kind == ty::VariantKind::Unit {
             Some((adt, variant))
         } else {
             None
         }
     }
 
-
     pub fn write_nil(&self, node_id: ast::NodeId) {
         self.write_ty(node_id, self.tcx().mk_nil());
     }

src/libsyntax/feature_gate.rs

@@ -191,6 +191,9 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Option<u32>, Status
 
     // allow `#[unwind]`
     ("unwind_attributes", "1.4.0", None, Active),
+
+    // allow empty structs/enum variants with braces
+    ("braced_empty_structs", "1.5.0", None, Active),
 ];
 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 
@@ -775,7 +778,7 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                 }
             }
 
-            ast::ItemStruct(..) => {
+            ast::ItemStruct(ref def, _) => {
                 if attr::contains_name(&i.attrs[..], "simd") {
                     self.gate_feature("simd", i.span,
                                       "SIMD types are experimental and possibly buggy");
@@ -794,6 +797,10 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                         }
                     }
                 }
+                if def.fields.is_empty() && def.ctor_id.is_none() {
+                    self.gate_feature("braced_empty_structs", i.span,
+                                      "empty structs with braces are unstable");
+                }
             }
 
             ast::ItemDefaultImpl(..) => {
@@ -843,6 +850,12 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                                   "box expression syntax is experimental; \
                                    you can call `Box::new` instead.");
             }
+            ast::ExprStruct(_, ref fields, ref expr) => {
+                if fields.is_empty() && expr.is_none() {
+                    self.gate_feature("braced_empty_structs", e.span,
+                                      "empty structs with braces are unstable");
+                }
+            }
             _ => {}
         }
         visit::walk_expr(self, e);
@@ -867,6 +880,12 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                                   pattern.span,
                                   "box pattern syntax is experimental");
             }
+            ast::PatStruct(_, ref fields, dotdot) => {
+                if fields.is_empty() && !dotdot {
+                    self.gate_feature("braced_empty_structs", pattern.span,
+                                      "empty structs with braces are unstable");
+                }
+            }
             _ => {}
         }
         visit::walk_pat(self, pattern)

src/libsyntax/parse/parser.rs

@@ -2231,14 +2231,6 @@ impl<'a> Parser<'a> {
                                                  &[token::CloseDelim(token::Brace)]));
                             }
 
-                            if fields.is_empty() && base.is_none() {
-                                let last_span = self.last_span;
-                                self.span_err(last_span,
-                                              "structure literal must either \
-                                              have at least one field or use \
-                                              structure update syntax");
-                            }
-
                             hi = self.span.hi;
                             try!(self.expect(&token::CloseDelim(token::Brace)));
                             ex = ExprStruct(pth, fields, base);
@@ -4713,14 +4705,14 @@ impl<'a> Parser<'a> {
                 (Vec::new(), Some(ast::DUMMY_NODE_ID))
             } else {
                 // If we see: `struct Foo<T> where T: Copy { ... }`
-                (try!(self.parse_record_struct_body(&class_name)), None)
+                (try!(self.parse_record_struct_body()), None)
             }
         // No `where` so: `struct Foo<T>;`
         } else if try!(self.eat(&token::Semi) ){
             (Vec::new(), Some(ast::DUMMY_NODE_ID))
         // Record-style struct definition
         } else if self.token == token::OpenDelim(token::Brace) {
-            let fields = try!(self.parse_record_struct_body(&class_name));
+            let fields = try!(self.parse_record_struct_body());
             (fields, None)
         // Tuple-style struct definition with optional where-clause.
         } else if self.token == token::OpenDelim(token::Paren) {
@@ -4740,20 +4732,13 @@ impl<'a> Parser<'a> {
          None))
     }
 
-    pub fn parse_record_struct_body(&mut self,
-                                    class_name: &ast::Ident) -> PResult<Vec<StructField>> {
+    pub fn parse_record_struct_body(&mut self) -> PResult<Vec<StructField>> {
         let mut fields = Vec::new();
         if try!(self.eat(&token::OpenDelim(token::Brace)) ){
             while self.token != token::CloseDelim(token::Brace) {
                 fields.push(try!(self.parse_struct_decl_field(true)));
             }
 
-            if fields.is_empty() {
-                return Err(self.fatal(&format!("unit-like struct definition should be \
-                    written as `struct {};`",
-                    class_name)));
-            }
-
             try!(self.bump());
         } else {
             let token_str = self.this_token_to_string();

src/libsyntax/print/pprust.rs

@@ -1855,34 +1855,34 @@ impl<'a> State<'a> {
                          fields: &[ast::Field],
                          wth: &Option<P<ast::Expr>>) -> io::Result<()> {
         try!(self.print_path(path, true, 0));
-        if !(fields.is_empty() && wth.is_none()) {
-            try!(word(&mut self.s, "{"));
-            try!(self.commasep_cmnt(
-                Consistent,
-                &fields[..],
-                |s, field| {
-                    try!(s.ibox(indent_unit));
-                    try!(s.print_ident(field.ident.node));
-                    try!(s.word_space(":"));
-                    try!(s.print_expr(&*field.expr));
-                    s.end()
-                },
-                |f| f.span));
-            match *wth {
-                Some(ref expr) => {
-                    try!(self.ibox(indent_unit));
-                    if !fields.is_empty() {
-                        try!(word(&mut self.s, ","));
-                        try!(space(&mut self.s));
-                    }
-                    try!(word(&mut self.s, ".."));
-                    try!(self.print_expr(&**expr));
-                    try!(self.end());
+        try!(word(&mut self.s, "{"));
+        try!(self.commasep_cmnt(
+            Consistent,
+            &fields[..],
+            |s, field| {
+                try!(s.ibox(indent_unit));
+                try!(s.print_ident(field.ident.node));
+                try!(s.word_space(":"));
+                try!(s.print_expr(&*field.expr));
+                s.end()
+            },
+            |f| f.span));
+        match *wth {
+            Some(ref expr) => {
+                try!(self.ibox(indent_unit));
+                if !fields.is_empty() {
+                    try!(word(&mut self.s, ","));
+                    try!(space(&mut self.s));
                 }
-                _ => try!(word(&mut self.s, ",")),
+                try!(word(&mut self.s, ".."));
+                try!(self.print_expr(&**expr));
+                try!(self.end());
+            }
+            _ => if !fields.is_empty() {
+                try!(word(&mut self.s, ","))
             }
-            try!(word(&mut self.s, "}"));
         }
+        try!(word(&mut self.s, "}"));
         Ok(())
     }
 

src/test/parse-fail/struct-no-fields-2.rs → src/test/compile-fail/empty-struct-with-braces-1.rs

@@ -1,4 +1,4 @@
-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// 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.
 //
@@ -8,13 +8,12 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -Z parse-only
+// Empty struct defined with braces shouldn't add names into value namespace
 
-struct Foo;
+#![feature(braced_empty_structs)]
 
-fn f2() {
-    let _end_stmt     = Foo { };
-    //~^ ERROR: structure literal must either have at least one field
-}
+struct Empty {}
 
-fn main() {}
+fn main() {
+    let e = Empty; //~ ERROR `Empty` is the name of a struct or struct variant
+}

src/test/parse-fail/struct-no-fields-4.rs → src/test/compile-fail/empty-struct-with-braces-2.rs

@@ -1,4 +1,4 @@
-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// 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.
 //
@@ -8,13 +8,18 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -Z parse-only
+// Empty struct defined with braces shouldn't add names into value namespace
 
-struct Foo;
+#![feature(braced_empty_structs)]
+#![deny(warnings)]
 
-fn h4() {
-    let _end_of_tuple = (3, Foo { });
-    //~^ ERROR: structure literal must either have at least one field
-}
+struct Empty {}
+
+fn main() {
+    let e = Empty {};
 
-fn main() {}
+    match e {
+        Empty => () //~ ERROR unused variable: `Empty`
+        //~^ ERROR variable `Empty` should have a snake case name such as `empty`
+    }
+}

src/test/parse-fail/struct-no-fields-3.rs → src/test/compile-fail/empty-struct-with-braces-3.rs

@@ -1,4 +1,4 @@
-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// 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.
 //
@@ -8,13 +8,14 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -Z parse-only
+// Feature gate test for empty struct with braces
 
-struct Foo;
+struct Empty {} //~ ERROR empty structs with braces are unstable
 
-fn g3() {
-    let _mid_tuple    = (Foo { }, 2);
-    //~^ ERROR: structure literal must either have at least one field
-}
+fn main() {
+    let e = Empty {}; //~ ERROR empty structs with braces are unstable
 
-fn main() {}
+    match e {
+        Empty {} => {} //~ ERROR empty structs with braces are unstable
+    }
+}

src/test/compile-fail/issue-27831.rs

@@ -22,8 +22,8 @@ fn main() {
     let Foo { .. } = x; //~ ERROR `Foo` does not name a struct
 
     let x = Bar;
-    Bar { ..x }; //~ ERROR `Bar` does not name a structure
-    let Bar { .. } = x; //~ ERROR `Bar` does not name a struct
+    Bar { ..x };
+    let Bar { .. } = x;
 
     match Enum::Bar {
         Enum::Bar { .. } //~ ERROR `Enum::Bar` does not name a struct