librustc: Implement the syntax in the RFC for unboxed closure sugar.

Part of issue #16640. I am leaving this issue open to handle parsing of
higher-rank lifetimes in traits.

This change breaks code that used unboxed closures:

* Instead of `F:|&: int| -> int`, write `F:Fn(int) -> int`.

* Instead of `F:|&mut: int| -> int`, write `F:FnMut(int) -> int`.

* Instead of `F:|: int| -> int`, write `F:FnOnce(int) -> int`.

[breaking-change]

4 files changed, 77 insertions, 81 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index eac158e664c..e74222fac09 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -213,13 +213,20 @@ pub static DUMMY_NODE_ID: NodeId = -1;
 #[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
 pub enum TyParamBound {
     TraitTyParamBound(TraitRef),
-    UnboxedFnTyParamBound(UnboxedFnTy),
+    UnboxedFnTyParamBound(P<UnboxedFnBound>),
     RegionTyParamBound(Lifetime)
 }
 
 pub type TyParamBounds = OwnedSlice<TyParamBound>;
 
 #[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
+pub struct UnboxedFnBound {
+    pub path: Path,
+    pub decl: P<FnDecl>,
+    pub ref_id: NodeId,
+}
+
+#[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
 pub struct TyParam {
     pub ident: Ident,
     pub id: NodeId,
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 3beba5bcda4..1441f5beb6a 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -657,16 +657,26 @@ pub fn noop_fold_fn_decl<T: Folder>(decl: P<FnDecl>, fld: &mut T) -> P<FnDecl> {
     })
 }
 
-pub fn noop_fold_ty_param_bound<T: Folder>(tpb: TyParamBound, fld: &mut T)
-                                           -> TyParamBound {
+pub fn noop_fold_ty_param_bound<T>(tpb: TyParamBound, fld: &mut T)
+                                   -> TyParamBound
+                                   where T: Folder {
     match tpb {
         TraitTyParamBound(ty) => TraitTyParamBound(fld.fold_trait_ref(ty)),
         RegionTyParamBound(lifetime) => RegionTyParamBound(fld.fold_lifetime(lifetime)),
-        UnboxedFnTyParamBound(UnboxedFnTy {decl, kind}) => {
-            UnboxedFnTyParamBound(UnboxedFnTy {
-                decl: fld.fold_fn_decl(decl),
-                kind: kind,
-            })
+        UnboxedFnTyParamBound(bound) => {
+            match *bound {
+                UnboxedFnBound {
+                    ref path,
+                    ref decl,
+                    ref_id
+                } => {
+                    UnboxedFnTyParamBound(P(UnboxedFnBound {
+                        path: fld.fold_path(path.clone()),
+                        decl: fld.fold_fn_decl(decl.clone()),
+                        ref_id: fld.new_id(ref_id),
+                    }))
+                }
+            }
         }
     }
 }
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index ff4fd41fbd7..8a0027e5c06 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -55,7 +55,8 @@ use ast::{TyTypeof, TyInfer, TypeMethod};
 use ast::{TyNil, TyParam, TyParamBound, TyParen, TyPath, TyPtr, TyQPath};
 use ast::{TyRptr, TyTup, TyU32, TyUnboxedFn, TyUniq, TyVec, UnUniq};
 use ast::{TypeImplItem, TypeTraitItem, Typedef, UnboxedClosureKind};
-use ast::{UnboxedFnTy, UnboxedFnTyParamBound, UnnamedField, UnsafeBlock};
+use ast::{UnboxedFnBound, UnboxedFnTy, UnboxedFnTyParamBound};
+use ast::{UnnamedField, UnsafeBlock};
 use ast::{UnsafeFn, ViewItem, ViewItem_, ViewItemExternCrate, ViewItemUse};
 use ast::{ViewPath, ViewPathGlob, ViewPathList, ViewPathSimple};
 use ast::{Visibility, WhereClause, WherePredicate};
@@ -3666,39 +3667,6 @@ impl<'a> Parser<'a> {
         })
     }
 
-    fn parse_unboxed_function_type(&mut self) -> UnboxedFnTy {
-        let (optional_unboxed_closure_kind, inputs) =
-            if self.eat(&token::OROR) {
-                (None, Vec::new())
-            } else {
-                self.expect_or();
-
-                let optional_unboxed_closure_kind =
-                    self.parse_optional_unboxed_closure_kind();
-
-                let inputs = self.parse_seq_to_before_or(&token::COMMA,
-                                                         |p| {
-                    p.parse_arg_general(false)
-                });
-                self.expect_or();
-                (optional_unboxed_closure_kind, inputs)
-            };
-
-        let (return_style, output) = self.parse_ret_ty();
-        UnboxedFnTy {
-            decl: P(FnDecl {
-                inputs: inputs,
-                output: output,
-                cf: return_style,
-                variadic: false,
-            }),
-            kind: match optional_unboxed_closure_kind {
-                Some(kind) => kind,
-                None => FnMutUnboxedClosureKind,
-            },
-        }
-    }
-
     // Parses a sequence of bounds if a `:` is found,
     // otherwise returns empty list.
     fn parse_colon_then_ty_param_bounds(&mut self)
@@ -3730,13 +3698,31 @@ impl<'a> Parser<'a> {
                     self.bump();
                 }
                 token::MOD_SEP | token::IDENT(..) => {
-                    let tref = self.parse_trait_ref();
-                    result.push(TraitTyParamBound(tref));
-                }
-                token::BINOP(token::OR) | token::OROR => {
-                    let unboxed_function_type =
-                        self.parse_unboxed_function_type();
-                    result.push(UnboxedFnTyParamBound(unboxed_function_type));
+                    let path =
+                        self.parse_path(LifetimeAndTypesWithoutColons).path;
+                    if self.token == token::LPAREN {
+                        self.bump();
+                        let inputs = self.parse_seq_to_end(
+                            &token::RPAREN,
+                            seq_sep_trailing_allowed(token::COMMA),
+                            |p| p.parse_arg_general(false));
+                        let (return_style, output) = self.parse_ret_ty();
+                        result.push(UnboxedFnTyParamBound(P(UnboxedFnBound {
+                            path: path,
+                            decl: P(FnDecl {
+                                inputs: inputs,
+                                output: output,
+                                cf: return_style,
+                                variadic: false,
+                            }),
+                            ref_id: ast::DUMMY_NODE_ID,
+                        })));
+                    } else {
+                        result.push(TraitTyParamBound(ast::TraitRef {
+                            path: path,
+                            ref_id: ast::DUMMY_NODE_ID,
+                        }))
+                    }
                 }
                 _ => break,
             }
@@ -4423,14 +4409,6 @@ impl<'a> Parser<'a> {
          Some(attrs))
     }
 
-    /// Parse a::B<String,int>
-    fn parse_trait_ref(&mut self) -> TraitRef {
-        ast::TraitRef {
-            path: self.parse_path(LifetimeAndTypesWithoutColons).path,
-            ref_id: ast::DUMMY_NODE_ID,
-        }
-    }
-
     /// Parse struct Foo { ... }
     fn parse_item_struct(&mut self, is_virtual: bool) -> ItemInfo {
         let class_name = self.parse_ident();
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 0ae5303641b..d7dd87a096e 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -2190,16 +2190,13 @@ impl<'a> State<'a> {
                         self.print_lifetime(lt)
                     }
                     UnboxedFnTyParamBound(ref unboxed_function_type) => {
-                        self.print_ty_fn(None,
-                                         None,
-                                         ast::NormalFn,
-                                         ast::Many,
-                                         &*unboxed_function_type.decl,
-                                         None,
-                                         &OwnedSlice::empty(),
-                                         None,
-                                         None,
-                                         Some(unboxed_function_type.kind))
+                        try!(self.print_path(&unboxed_function_type.path,
+                                             false));
+                        try!(self.popen());
+                        try!(self.print_fn_args(&*unboxed_function_type.decl,
+                                                None));
+                        try!(self.pclose());
+                        self.print_fn_output(&*unboxed_function_type.decl)
                     }
                 })
             }
@@ -2430,6 +2427,23 @@ impl<'a> State<'a> {
         self.end()
     }
 
+    pub fn print_fn_output(&mut self, decl: &ast::FnDecl) -> IoResult<()> {
+        match decl.output.node {
+            ast::TyNil => Ok(()),
+            _ => {
+                try!(self.space_if_not_bol());
+                try!(self.ibox(indent_unit));
+                try!(self.word_space("->"));
+                if decl.cf == ast::NoReturn {
+                    try!(self.word_nbsp("!"));
+                } else {
+                    try!(self.print_type(&*decl.output));
+                }
+                self.end()
+            }
+        }
+    }
+
     pub fn print_ty_fn(&mut self,
                        opt_abi: Option<abi::Abi>,
                        opt_sigil: Option<char>,
@@ -2510,20 +2524,7 @@ impl<'a> State<'a> {
 
         try!(self.maybe_print_comment(decl.output.span.lo));
 
-        match decl.output.node {
-            ast::TyNil => {}
-            _ => {
-                try!(self.space_if_not_bol());
-                try!(self.ibox(indent_unit));
-                try!(self.word_space("->"));
-                if decl.cf == ast::NoReturn {
-                    try!(self.word_nbsp("!"));
-                } else {
-                    try!(self.print_type(&*decl.output));
-                }
-                try!(self.end());
-            }
-        }
+        try!(self.print_fn_output(decl));
 
         match generics {
             Some(generics) => try!(self.print_where_clause(generics)),