restore emplacement syntax (obsolete)

src/librustc/hir/lowering.rs

@@ -2935,7 +2935,10 @@ impl<'a> LoweringContext<'a> {
     fn lower_expr(&mut self, e: &Expr) -> hir::Expr {
         let kind = match e.node {
             ExprKind::Box(ref inner) => hir::ExprBox(P(self.lower_expr(inner))),
-
+            ExprKind::ObsoleteInPlace(..) => {
+                self.sess.abort_if_errors();
+                span_bug!(e.span, "encountered ObsoleteInPlace expr during lowering");
+            }
             ExprKind::Array(ref exprs) => {
                 hir::ExprArray(exprs.iter().map(|x| self.lower_expr(x)).collect())
             }

src/librustc_passes/ast_validation.rs

@@ -172,6 +172,12 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
                 span_err!(self.session, expr.span, E0472, "asm! is unsupported on this target");
             }
+            ExprKind::ObsoleteInPlace(..) => {
+                self.err_handler()
+                    .struct_span_err(expr.span, "emplacement syntax is obsolete (for now, anyway)")
+                    .note("for more information, see <https://github.com/rust-lang/rust/issues/27779#issuecomment-378416911>")
+                    .emit();
+            }
             _ => {}
         }
 

src/libsyntax/ast.rs

@@ -1008,6 +1008,7 @@ impl Expr {
     pub fn precedence(&self) -> ExprPrecedence {
         match self.node {
             ExprKind::Box(_) => ExprPrecedence::Box,
+            ExprKind::ObsoleteInPlace(..) => ExprPrecedence::ObsoleteInPlace,
             ExprKind::Array(_) => ExprPrecedence::Array,
             ExprKind::Call(..) => ExprPrecedence::Call,
             ExprKind::MethodCall(..) => ExprPrecedence::MethodCall,
@@ -1066,6 +1067,8 @@ pub enum RangeLimits {
 pub enum ExprKind {
     /// A `box x` expression.
     Box(P<Expr>),
+    /// First expr is the place; second expr is the value.
+    ObsoleteInPlace(P<Expr>, P<Expr>),
     /// An array (`[a, b, c, d]`)
     Array(Vec<P<Expr>>),
     /// A function call

src/libsyntax/feature_gate.rs

@@ -1684,6 +1684,9 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 gate_feature_post!(&self, type_ascription, e.span,
                                   "type ascription is experimental");
             }
+            ast::ExprKind::ObsoleteInPlace(..) => {
+                // these get a hard error in ast-validation
+            }
             ast::ExprKind::Yield(..) => {
                 gate_feature_post!(&self, generators,
                                   e.span,

src/libsyntax/fold.rs

@@ -1194,6 +1194,9 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span, attrs}: Expr, folder: &mu
             ExprKind::Box(e) => {
                 ExprKind::Box(folder.fold_expr(e))
             }
+            ExprKind::ObsoleteInPlace(a, b) => {
+                ExprKind::ObsoleteInPlace(folder.fold_expr(a), folder.fold_expr(b))
+            }
             ExprKind::Array(exprs) => {
                 ExprKind::Array(folder.fold_exprs(exprs))
             }

src/libsyntax/parse/parser.rs

@@ -2839,6 +2839,17 @@ impl<'a> Parser<'a> {
                 let (span, e) = self.interpolated_or_expr_span(e)?;
                 (lo.to(span), ExprKind::AddrOf(m, e))
             }
+            token::Ident(..) if self.token.is_keyword(keywords::In) => {
+                self.bump();
+                let place = self.parse_expr_res(
+                    Restrictions::NO_STRUCT_LITERAL,
+                    None,
+                )?;
+                let blk = self.parse_block()?;
+                let span = blk.span;
+                let blk_expr = self.mk_expr(span, ExprKind::Block(blk, None), ThinVec::new());
+                (lo.to(span), ExprKind::ObsoleteInPlace(place, blk_expr))
+            }
             token::Ident(..) if self.token.is_keyword(keywords::Box) => {
                 self.bump();
                 let e = self.parse_prefix_expr(None);
@@ -3042,6 +3053,8 @@ impl<'a> Parser<'a> {
                 }
                 AssocOp::Assign =>
                     self.mk_expr(span, ExprKind::Assign(lhs, rhs), ThinVec::new()),
+                AssocOp::ObsoleteInPlace =>
+                    self.mk_expr(span, ExprKind::ObsoleteInPlace(lhs, rhs), ThinVec::new()),
                 AssocOp::AssignOp(k) => {
                     let aop = match k {
                         token::Plus =>    BinOpKind::Add,

src/libsyntax/print/pprust.rs

@@ -2057,6 +2057,13 @@ impl<'a> State<'a> {
                 self.word_space("box")?;
                 self.print_expr_maybe_paren(expr, parser::PREC_PREFIX)?;
             }
+            ast::ExprKind::ObsoleteInPlace(ref place, ref expr) => {
+                let prec = AssocOp::ObsoleteInPlace.precedence() as i8;
+                self.print_expr_maybe_paren(place, prec + 1)?;
+                self.s.space()?;
+                self.word_space("<-")?;
+                self.print_expr_maybe_paren(expr, prec)?;
+            }
             ast::ExprKind::Array(ref exprs) => {
                 self.print_expr_vec(&exprs[..], attrs)?;
             }

src/libsyntax/util/parser.rs

@@ -56,6 +56,8 @@ pub enum AssocOp {
     GreaterEqual,
     /// `=`
     Assign,
+    /// `<-`
+    ObsoleteInPlace,
     /// `?=` where ? is one of the BinOpToken
     AssignOp(BinOpToken),
     /// `as`
@@ -84,6 +86,7 @@ impl AssocOp {
         use self::AssocOp::*;
         match *t {
             Token::BinOpEq(k) => Some(AssignOp(k)),
+            Token::LArrow => Some(ObsoleteInPlace),
             Token::Eq => Some(Assign),
             Token::BinOp(BinOpToken::Star) => Some(Multiply),
             Token::BinOp(BinOpToken::Slash) => Some(Divide),
@@ -153,6 +156,7 @@ impl AssocOp {
             LAnd => 6,
             LOr => 5,
             DotDot | DotDotEq => 4,
+            ObsoleteInPlace => 3,
             Assign | AssignOp(_) => 2,
         }
     }
@@ -162,7 +166,7 @@ impl AssocOp {
         use self::AssocOp::*;
         // NOTE: it is a bug to have an operators that has same precedence but different fixities!
         match *self {
-            Assign | AssignOp(_) => Fixity::Right,
+            ObsoleteInPlace | Assign | AssignOp(_) => Fixity::Right,
             As | Multiply | Divide | Modulus | Add | Subtract | ShiftLeft | ShiftRight | BitAnd |
             BitXor | BitOr | Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual |
             LAnd | LOr | Colon => Fixity::Left,
@@ -174,16 +178,16 @@ impl AssocOp {
         use self::AssocOp::*;
         match *self {
             Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual => true,
-            Assign | AssignOp(_) | As | Multiply | Divide | Modulus | Add | Subtract |
-            ShiftLeft | ShiftRight | BitAnd | BitXor | BitOr | LAnd | LOr |
+            ObsoleteInPlace | Assign | AssignOp(_) | As | Multiply | Divide | Modulus | Add |
+            Subtract | ShiftLeft | ShiftRight | BitAnd | BitXor | BitOr | LAnd | LOr |
             DotDot | DotDotEq | Colon => false
         }
     }
 
     pub fn is_assign_like(&self) -> bool {
         use self::AssocOp::*;
         match *self {
-            Assign | AssignOp(_) => true,
+            Assign | AssignOp(_) | ObsoleteInPlace => true,
             Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual | As | Multiply | Divide |
             Modulus | Add | Subtract | ShiftLeft | ShiftRight | BitAnd | BitXor | BitOr | LAnd |
             LOr | DotDot | DotDotEq | Colon => false
@@ -211,7 +215,7 @@ impl AssocOp {
             BitOr => Some(BinOpKind::BitOr),
             LAnd => Some(BinOpKind::And),
             LOr => Some(BinOpKind::Or),
-            Assign | AssignOp(_) | As | DotDot | DotDotEq | Colon => None
+            ObsoleteInPlace | Assign | AssignOp(_) | As | DotDot | DotDotEq | Colon => None
         }
     }
 }
@@ -238,6 +242,7 @@ pub enum ExprPrecedence {
 
     Binary(BinOpKind),
 
+    ObsoleteInPlace,
     Cast,
     Type,
 
@@ -304,6 +309,7 @@ impl ExprPrecedence {
 
             // Binop-like expr kinds, handled by `AssocOp`.
             ExprPrecedence::Binary(op) => AssocOp::from_ast_binop(op).precedence() as i8,
+            ExprPrecedence::ObsoleteInPlace => AssocOp::ObsoleteInPlace.precedence() as i8,
             ExprPrecedence::Cast => AssocOp::As.precedence() as i8,
             ExprPrecedence::Type => AssocOp::Colon.precedence() as i8,
 

src/libsyntax/visit.rs

@@ -660,6 +660,10 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::Box(ref subexpression) => {
             visitor.visit_expr(subexpression)
         }
+        ExprKind::ObsoleteInPlace(ref place, ref subexpression) => {
+            visitor.visit_expr(place);
+            visitor.visit_expr(subexpression)
+        }
         ExprKind::Array(ref subexpressions) => {
             walk_list!(visitor, visit_expr, subexpressions);
         }

src/test/ui/obsolete-in-place/bad.bad.stderr

@@ -0,0 +1,18 @@
+error: emplacement syntax is obsolete (for now, anyway)
+  --> $DIR/bad.rs:19:5
+   |
+LL |     x <- y; //[bad]~ ERROR emplacement syntax is obsolete
+   |     ^^^^^^
+   |
+   = note: for more information, see <https://github.com/rust-lang/rust/issues/27779#issuecomment-378416911>
+
+error: emplacement syntax is obsolete (for now, anyway)
+  --> $DIR/bad.rs:20:5
+   |
+LL |     in(foo) { bar }; //[bad]~ ERROR emplacement syntax is obsolete
+   |     ^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see <https://github.com/rust-lang/rust/issues/27779#issuecomment-378416911>
+
+error: aborting due to 2 previous errors
+

src/test/ui/obsolete-in-place/bad.rs

@@ -0,0 +1,25 @@
+// 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.
+
+// Check that `<-` and `in` syntax gets a hard error.
+
+// revisions: good bad
+//[good] run-pass
+
+#[cfg(bad)]
+fn main() {
+    let (x, y, foo, bar);
+    x <- y; //[bad]~ ERROR emplacement syntax is obsolete
+    in(foo) { bar }; //[bad]~ ERROR emplacement syntax is obsolete
+}
+
+#[cfg(good)]
+fn main() {
+}