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

1 files changed, 9 insertions, 3 deletions

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 }