Auto merge of #30184 - petrochenkov:ascr, r=nikomatsakis

This PR is a rebase of the original PR by @eddyb https://github.com/rust-lang/rust/pull/21836 with some unrebasable parts manually reapplied, feature gate added + type equality restriction added as described below.

This implementation is partial because the type equality restriction is applied to all type ascription expressions and not only those in lvalue contexts. Thus, all difficulties with detection of these contexts and translation of coercions having effect in runtime are avoided.
So, you can't write things with coercions like `let slice = &[1, 2, 3]: &[u8];`. It obviously makes type ascription less useful than it should be, but it's still much more useful than not having type ascription at all.
In particular, things like `let v = something.iter().collect(): Vec<_>;` and `let u = t.into(): U;` work as expected and I'm pretty happy with these improvements alone.

Part of https://github.com/rust-lang/rust/issues/23416

7 files changed, 36 insertions, 12 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 4b0ec8578c1..abc44407eff 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -942,6 +942,7 @@ pub enum Expr_ {
     ExprLit(P<Lit>),
     /// A cast (`foo as f64`)
     ExprCast(P<Expr>, P<Ty>),
+    ExprType(P<Expr>, P<Ty>),
     /// An `if` block, with an optional else block
     ///
     /// `if expr { block } else { expr }`
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 0d38411ac93..b2989c42a9e 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -233,6 +233,9 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Option<u32>, Status
 
     // Allows `#[deprecated]` attribute
     ("deprecated", "1.6.0", Some(29935), Active),
+
+    // allow using type ascription in expressions
+    ("type_ascription", "1.6.0", Some(23416), Active),
 ];
 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 
@@ -958,6 +961,10 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                                   "box expression syntax is experimental; \
                                    you can call `Box::new` instead.");
             }
+            ast::ExprType(..) => {
+                self.gate_feature("type_ascription", e.span,
+                                  "type ascription is experimental");
+            }
             _ => {}
         }
         visit::walk_expr(self, e);
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index cd2210c71b8..cb467f9016b 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -1203,6 +1203,9 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span, attrs}: Expr, folder: &mu
             ExprCast(expr, ty) => {
                 ExprCast(folder.fold_expr(expr), folder.fold_ty(ty))
             }
+            ExprType(expr, ty) => {
+                ExprType(folder.fold_expr(expr), folder.fold_ty(ty))
+            }
             ExprAddrOf(m, ohs) => ExprAddrOf(m, folder.fold_expr(ohs)),
             ExprIf(cond, tr, fl) => {
                 ExprIf(folder.fold_expr(cond),
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index ebfcf8c5180..b625277f2a7 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -26,7 +26,7 @@ use ast::{ExprBreak, ExprCall, ExprCast, ExprInPlace};
 use ast::{ExprField, ExprTupField, ExprClosure, ExprIf, ExprIfLet, ExprIndex};
 use ast::{ExprLit, ExprLoop, ExprMac, ExprRange};
 use ast::{ExprMethodCall, ExprParen, ExprPath};
-use ast::{ExprRepeat, ExprRet, ExprStruct, ExprTup, ExprUnary};
+use ast::{ExprRepeat, ExprRet, ExprStruct, ExprTup, ExprType, ExprUnary};
 use ast::{ExprVec, ExprWhile, ExprWhileLet, ExprForLoop, Field, FnDecl};
 use ast::{ForeignItem, ForeignItemStatic, ForeignItemFn, FunctionRetTy};
 use ast::{Ident, Inherited, ImplItem, Item, Item_, ItemStatic};
@@ -2787,6 +2787,11 @@ impl<'a> Parser<'a> {
                 lhs = self.mk_expr(lhs.span.lo, rhs.span.hi,
                                    ExprCast(lhs, rhs), None);
                 continue
+            } else if op == AssocOp::Colon {
+                let rhs = try!(self.parse_ty());
+                lhs = self.mk_expr(lhs.span.lo, rhs.span.hi,
+                                   ExprType(lhs, rhs), None);
+                continue
             } else if op == AssocOp::DotDot {
                     // If we didn’t have to handle `x..`, it would be pretty easy to generalise
                     // it to the Fixity::None code.
@@ -2809,7 +2814,6 @@ impl<'a> Parser<'a> {
                     break
             }
 
-
             let rhs = try!(match op.fixity() {
                 Fixity::Right => self.with_res(restrictions, |this|{
                     this.parse_assoc_expr_with(op.precedence(), LhsExpr::NotYetParsed)
@@ -2856,7 +2860,9 @@ impl<'a> Parser<'a> {
                     let aopexpr = self.mk_assign_op(codemap::respan(cur_op_span, aop), lhs, rhs);
                     self.mk_expr(lhs_span.lo, rhs_span.hi, aopexpr, None)
                 }
-                AssocOp::As | AssocOp::DotDot => self.bug("As or DotDot branch reached")
+                AssocOp::As | AssocOp::Colon | AssocOp::DotDot => {
+                    self.bug("As, Colon or DotDot branch reached")
+                }
             };
 
             if op.fixity() == Fixity::None { break }
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 1f296dc5d59..ff01f0ac67e 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -444,7 +444,7 @@ fn needs_parentheses(expr: &ast::Expr) -> bool {
         ast::ExprAssign(..) | ast::ExprBinary(..) |
         ast::ExprClosure(..) |
         ast::ExprAssignOp(..) | ast::ExprCast(..) |
-        ast::ExprInPlace(..) => true,
+        ast::ExprInPlace(..) | ast::ExprType(..) => true,
         _ => false,
     }
 }
@@ -2035,6 +2035,11 @@ impl<'a> State<'a> {
                 try!(self.word_space("as"));
                 try!(self.print_type(&**ty));
             }
+            ast::ExprType(ref expr, ref ty) => {
+                try!(self.print_expr(&**expr));
+                try!(self.word_space(":"));
+                try!(self.print_type(&**ty));
+            }
             ast::ExprIf(ref test, ref blk, ref elseopt) => {
                 try!(self.print_if(&**test, &**blk, elseopt.as_ref().map(|e| &**e)));
             }
diff --git a/src/libsyntax/util/parser.rs b/src/libsyntax/util/parser.rs
index bf3a8def390..87ef96d87ff 100644
--- a/src/libsyntax/util/parser.rs
+++ b/src/libsyntax/util/parser.rs
@@ -60,7 +60,9 @@ pub enum AssocOp {
     /// `as`
     As,
     /// `..` range
-    DotDot
+    DotDot,
+    /// `:`
+    Colon,
 }
 
 #[derive(Debug, PartialEq, Eq)]
@@ -100,6 +102,7 @@ impl AssocOp {
             Token::AndAnd => Some(LAnd),
             Token::OrOr => Some(LOr),
             Token::DotDot => Some(DotDot),
+            Token::Colon => Some(Colon),
             _ if t.is_keyword(keywords::As) => Some(As),
             _ => None
         }
@@ -134,7 +137,7 @@ impl AssocOp {
     pub fn precedence(&self) -> usize {
         use self::AssocOp::*;
         match *self {
-            As => 14,
+            As | Colon => 14,
             Multiply | Divide | Modulus => 13,
             Add | Subtract => 12,
             ShiftLeft | ShiftRight => 11,
@@ -158,7 +161,7 @@ impl AssocOp {
             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,
+            LAnd | LOr | Colon => Fixity::Left,
             DotDot => Fixity::None
         }
     }
@@ -168,7 +171,7 @@ impl 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
+            ShiftLeft | ShiftRight | BitAnd | BitXor | BitOr | LAnd | LOr | DotDot | Colon => false
         }
     }
 
@@ -178,7 +181,7 @@ impl AssocOp {
             Assign | AssignOp(_) | Inplace => true,
             Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual | As | Multiply | Divide |
             Modulus | Add | Subtract | ShiftLeft | ShiftRight | BitAnd | BitXor | BitOr | LAnd |
-            LOr | DotDot => false
+            LOr | DotDot | Colon => false
         }
     }
 
@@ -203,8 +206,7 @@ impl AssocOp {
             BitOr => Some(ast::BiBitOr),
             LAnd => Some(ast::BiAnd),
             LOr => Some(ast::BiOr),
-            Inplace | Assign | AssignOp(_) | As | DotDot => None
+            Inplace | Assign | AssignOp(_) | As | DotDot | Colon => None
         }
     }
-
 }
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 22bf135f4f9..b8dd54790ce 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -693,7 +693,7 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr) {
             visitor.visit_expr(subexpression)
         }
         ExprLit(_) => {}
-        ExprCast(ref subexpression, ref typ) => {
+        ExprCast(ref subexpression, ref typ) | ExprType(ref subexpression, ref typ) => {
             visitor.visit_expr(subexpression);
             visitor.visit_ty(typ)
         }