Generalise associative operator parsing

This commit generalises parsing of associative operators from left-associative
only (with some ugly hacks to support right-associative assignment) to properly
left/right-associative operators.

Parsing still is not general enough to handle non-associative,
non-highest-precedence prefix or non-highest-precedence postfix operators (e.g.
`..` range syntax), though. That should be fixed in the future.

Lastly, this commit adds support for parsing right-associative `<-` (left arrow)
operator with precedence higher than assignment as the operator for placement-in
feature.

src/libsyntax/ast_util.rs

@@ -242,26 +242,6 @@ pub fn struct_field_visibility(field: ast::StructField) -> Visibility {
     }
 }
 
-/// Maps a binary operator to its precedence
-pub fn operator_prec(op: ast::BinOp_) -> usize {
-  match op {
-      // 'as' sits here with 12
-      BiMul | BiDiv | BiRem     => 11,
-      BiAdd | BiSub             => 10,
-      BiShl | BiShr             =>  9,
-      BiBitAnd                  =>  8,
-      BiBitXor                  =>  7,
-      BiBitOr                   =>  6,
-      BiLt | BiLe | BiGe | BiGt | BiEq | BiNe => 3,
-      BiAnd                     =>  2,
-      BiOr                      =>  1
-  }
-}
-
-/// Precedence of the `as` operator, which is a binary operator
-/// not appearing in the prior table.
-pub const AS_PREC: usize = 12;
-
 pub fn empty_generics() -> Generics {
     Generics {
         lifetimes: Vec::new(),

src/libsyntax/lib.rs

@@ -66,6 +66,7 @@ pub mod util {
     #[cfg(test)]
     pub mod parser_testing;
     pub mod small_vector;
+    pub mod parser;
 }
 
 pub mod diagnostics {

src/libsyntax/print/pprust.rs

@@ -14,6 +14,7 @@ use abi;
 use ast;
 use ast::{RegionTyParamBound, TraitTyParamBound, TraitBoundModifier};
 use ast_util;
+use util::parser::AssocOp;
 use attr;
 use owned_slice::OwnedSlice;
 use attr::{AttrMetaMethods, AttributeMethods};
@@ -445,7 +446,8 @@ fn needs_parentheses(expr: &ast::Expr) -> bool {
     match expr.node {
         ast::ExprAssign(..) | ast::ExprBinary(..) |
         ast::ExprClosure(..) |
-        ast::ExprAssignOp(..) | ast::ExprCast(..) => true,
+        ast::ExprAssignOp(..) | ast::ExprCast(..) |
+        ast::ExprInPlace(..) => true,
         _ => false,
     }
 }
@@ -1776,8 +1778,8 @@ impl<'a> State<'a> {
                                       binop: ast::BinOp) -> bool {
         match sub_expr.node {
             ast::ExprBinary(ref sub_op, _, _) => {
-                if ast_util::operator_prec(sub_op.node) <
-                    ast_util::operator_prec(binop.node) {
+                if AssocOp::from_ast_binop(sub_op.node).precedence() <
+                    AssocOp::from_ast_binop(binop.node).precedence() {
                     true
                 } else {
                     false
@@ -1802,10 +1804,10 @@ impl<'a> State<'a> {
     fn print_expr_in_place(&mut self,
                            place: &ast::Expr,
                            expr: &ast::Expr) -> io::Result<()> {
-        try!(self.word_space("in"));
-        try!(self.print_expr(place));
+        try!(self.print_expr_maybe_paren(place));
         try!(space(&mut self.s));
-        self.print_expr(expr)
+        try!(self.word_space("<-"));
+        self.print_expr_maybe_paren(expr)
     }
 
     fn print_expr_vec(&mut self, exprs: &[P<ast::Expr>]) -> io::Result<()> {

src/libsyntax/util/parser.rs

@@ -0,0 +1,191 @@
+use parse::token::{Token, BinOpToken, keywords};
+use ast;
+
+/// Associative operator with precedence.
+///
+/// This is the enum which specifies operator precedence and fixity to the parser.
+#[derive(Debug, PartialEq, Eq)]
+pub enum AssocOp {
+    /// `+`
+    Add,
+    /// `-`
+    Subtract,
+    /// `*`
+    Multiply,
+    /// `/`
+    Divide,
+    /// `%`
+    Modulus,
+    /// `&&`
+    LAnd,
+    /// `||`
+    LOr,
+    /// `^`
+    BitXor,
+    /// `&`
+    BitAnd,
+    /// `|`
+    BitOr,
+    /// `<<`
+    ShiftLeft,
+    /// `>>`
+    ShiftRight,
+    /// `==`
+    Equal,
+    /// `<`
+    Less,
+    /// `<=`
+    LessEqual,
+    /// `!=`
+    NotEqual,
+    /// `>`
+    Greater,
+    /// `>=`
+    GreaterEqual,
+    /// `=`
+    Assign,
+    /// `<-`
+    Inplace,
+    /// `?=` where ? is one of the BinOpToken
+    AssignOp(BinOpToken),
+    /// `as`
+    As,
+    /// `..` range
+    DotDot
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum Fixity {
+    /// The operator is left-associative
+    Left,
+    /// The operator is right-associative
+    Right,
+    /// The operator is not associative
+    None
+}
+
+impl AssocOp {
+    /// Create a new AssocOP from a token
+    pub fn from_token(t: &Token) -> Option<AssocOp> {
+        use self::AssocOp::*;
+        match *t {
+            Token::BinOpEq(k) => Some(AssignOp(k)),
+            Token::LArrow => Some(Inplace),
+            Token::Eq => Some(Assign),
+            Token::BinOp(BinOpToken::Star) => Some(Multiply),
+            Token::BinOp(BinOpToken::Slash) => Some(Divide),
+            Token::BinOp(BinOpToken::Percent) => Some(Modulus),
+            Token::BinOp(BinOpToken::Plus) => Some(Add),
+            Token::BinOp(BinOpToken::Minus) => Some(Subtract),
+            Token::BinOp(BinOpToken::Shl) => Some(ShiftLeft),
+            Token::BinOp(BinOpToken::Shr) => Some(ShiftRight),
+            Token::BinOp(BinOpToken::And) => Some(BitAnd),
+            Token::BinOp(BinOpToken::Caret) => Some(BitXor),
+            Token::BinOp(BinOpToken::Or) => Some(BitOr),
+            Token::Lt => Some(Less),
+            Token::Le => Some(LessEqual),
+            Token::Ge => Some(GreaterEqual),
+            Token::Gt => Some(Greater),
+            Token::EqEq => Some(Equal),
+            Token::Ne => Some(NotEqual),
+            Token::AndAnd => Some(LAnd),
+            Token::OrOr => Some(LOr),
+            Token::DotDot => Some(DotDot),
+            _ if t.is_keyword(keywords::As) => Some(As),
+            _ => None
+        }
+    }
+
+    /// Create a new AssocOp from ast::BinOp_.
+    pub fn from_ast_binop(op: ast::BinOp_) -> Self {
+        use self::AssocOp::*;
+        match op {
+            ast::BiLt => Less,
+            ast::BiGt => Greater,
+            ast::BiLe => LessEqual,
+            ast::BiGe => GreaterEqual,
+            ast::BiEq => Equal,
+            ast::BiNe => NotEqual,
+            ast::BiMul => Multiply,
+            ast::BiDiv => Divide,
+            ast::BiRem => Modulus,
+            ast::BiAdd => Add,
+            ast::BiSub => Subtract,
+            ast::BiShl => ShiftLeft,
+            ast::BiShr => ShiftRight,
+            ast::BiBitAnd => BitAnd,
+            ast::BiBitXor => BitXor,
+            ast::BiBitOr => BitOr,
+            ast::BiAnd => LAnd,
+            ast::BiOr => LOr
+        }
+    }
+
+    /// Gets the precedence of this operator
+    pub fn precedence(&self) -> usize {
+        use self::AssocOp::*;
+        match *self {
+            As => 14,
+            Multiply | Divide | Modulus => 13,
+            Add | Subtract => 12,
+            ShiftLeft | ShiftRight => 11,
+            BitAnd => 10,
+            BitXor => 9,
+            BitOr => 8,
+            Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual => 7,
+            LAnd => 6,
+            LOr => 5,
+            DotDot => 4,
+            Inplace => 3,
+            Assign | AssignOp(_) => 2,
+        }
+    }
+
+    /// Gets the fixity of this operator
+    pub fn fixity(&self) -> Fixity {
+        use self::AssocOp::*;
+        // NOTE: it is a bug to have an operators that has same precedence but different fixities!
+        match *self {
+            Inplace | Assign | AssignOp(_) => Fixity::Right,
+            As | Multiply | Divide | Modulus | Add | Subtract | ShiftLeft | ShiftRight | BitAnd |
+            BitXor | BitOr | Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual |
+            LAnd | LOr => Fixity::Left,
+            DotDot => Fixity::None
+        }
+    }
+
+    pub fn is_comparison(&self) -> bool {
+        use self::AssocOp::*;
+        match *self {
+            Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual => true,
+            Inplace | Assign | AssignOp(_) | As | Multiply | Divide | Modulus | Add | Subtract |
+            ShiftLeft | ShiftRight | BitAnd | BitXor | BitOr | LAnd | LOr | DotDot => false
+        }
+    }
+
+    pub fn to_ast_binop(&self) -> Option<ast::BinOp_> {
+        use self::AssocOp::*;
+        match *self {
+            Less => Some(ast::BiLt),
+            Greater => Some(ast::BiGt),
+            LessEqual => Some(ast::BiLe),
+            GreaterEqual => Some(ast::BiGe),
+            Equal => Some(ast::BiEq),
+            NotEqual => Some(ast::BiNe),
+            Multiply => Some(ast::BiMul),
+            Divide => Some(ast::BiDiv),
+            Modulus => Some(ast::BiRem),
+            Add => Some(ast::BiAdd),
+            Subtract => Some(ast::BiSub),
+            ShiftLeft => Some(ast::BiShl),
+            ShiftRight => Some(ast::BiShr),
+            BitAnd => Some(ast::BiBitAnd),
+            BitXor => Some(ast::BiBitXor),
+            BitOr => Some(ast::BiBitOr),
+            LAnd => Some(ast::BiAnd),
+            LOr => Some(ast::BiOr),
+            Inplace | Assign | AssignOp(_) | As | DotDot => None
+        }
+    }
+
+}

src/test/compile-fail/feature-gate-placement-expr.rs

@@ -19,6 +19,6 @@
 fn main() {
     use std::boxed::HEAP;
 
-    let x = in HEAP { 'c' }; //~ ERROR placement-in expression syntax is experimental
+    let x = HEAP <- 'c'; //~ ERROR placement-in expression syntax is experimental
     println!("x: {}", x);
 }

src/test/compile-fail/issue-14084.rs

@@ -12,6 +12,6 @@
 #![feature(placement_in_syntax)]
 
 fn main() {
-    in () { 0 };
+    () <- 0;
     //~^ ERROR: the trait `core::ops::Placer<_>` is not implemented
 }