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

This PR is a rebase of the original PR by @eddyb #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 #23416

Author: bors

src/doc/reference.md

@@ -2388,6 +2388,8 @@ The currently implemented features of the reference compiler are:
 
 * - `deprecated` - Allows using the `#[deprecated]` attribute.
 
+* - `type_ascription` - Allows type ascription expressions `expr: Type`.
+
 If a feature is promoted to a language feature, then all existing programs will
 start to receive compilation warnings about `#![feature]` directives which enabled
 the new feature (because the directive is no longer necessary). However, if a

src/librustc/middle/cfg/construct.rs

@@ -352,6 +352,7 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
             hir::ExprBox(ref e) |
             hir::ExprAddrOf(_, ref e) |
             hir::ExprCast(ref e, _) |
+            hir::ExprType(ref e, _) |
             hir::ExprUnary(_, ref e) |
             hir::ExprField(ref e, _) |
             hir::ExprTupField(ref e, _) => {

src/librustc/middle/check_const.rs

@@ -784,6 +784,7 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>,
         hir::ExprField(..) |
         hir::ExprTupField(..) |
         hir::ExprVec(_) |
+        hir::ExprType(..) |
         hir::ExprTup(..) => {}
 
         // Conditional control flow (possible to implement).

src/librustc/middle/const_eval.rs

@@ -1126,6 +1126,7 @@ pub fn eval_const_expr_partial<'tcx>(tcx: &ty::ctxt<'tcx>,
             None => unreachable!(),
         }
       }
+      hir::ExprType(ref e, _) => try!(eval_const_expr_partial(tcx, &**e, ty_hint, fn_args)),
       hir::ExprTup(_) => Tuple(e.id),
       hir::ExprStruct(..) => Struct(e.id),
       hir::ExprIndex(ref arr, ref idx) => {

src/librustc/middle/expr_use_visitor.rs

@@ -373,6 +373,10 @@ impl<'d,'t,'a,'tcx> ExprUseVisitor<'d,'t,'a,'tcx> {
         match expr.node {
             hir::ExprPath(..) => { }
 
+            hir::ExprType(ref subexpr, _) => {
+                self.walk_expr(&**subexpr)
+            }
+
             hir::ExprUnary(hir::UnDeref, ref base) => {      // *base
                 if !self.walk_overloaded_operator(expr, &**base, Vec::new(), PassArgs::ByRef) {
                     self.select_from_expr(&**base);

src/librustc/middle/liveness.rs

@@ -496,7 +496,7 @@ fn visit_expr(ir: &mut IrMaps, expr: &Expr) {
       hir::ExprBlock(..) | hir::ExprAssign(..) | hir::ExprAssignOp(..) |
       hir::ExprStruct(..) | hir::ExprRepeat(..) |
       hir::ExprInlineAsm(..) | hir::ExprBox(..) |
-      hir::ExprRange(..) => {
+      hir::ExprRange(..) | hir::ExprType(..) => {
           intravisit::walk_expr(ir, expr);
       }
     }
@@ -1160,6 +1160,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
           hir::ExprBox(ref e) |
           hir::ExprAddrOf(_, ref e) |
           hir::ExprCast(ref e, _) |
+          hir::ExprType(ref e, _) |
           hir::ExprUnary(_, ref e) => {
             self.propagate_through_expr(&**e, succ)
           }
@@ -1443,7 +1444,7 @@ fn check_expr(this: &mut Liveness, expr: &Expr) {
       hir::ExprBlock(..) | hir::ExprAddrOf(..) |
       hir::ExprStruct(..) | hir::ExprRepeat(..) |
       hir::ExprClosure(..) | hir::ExprPath(..) | hir::ExprBox(..) |
-      hir::ExprRange(..) => {
+      hir::ExprRange(..) | hir::ExprType(..) => {
         intravisit::walk_expr(this, expr);
       }
     }

src/librustc/middle/mem_categorization.rs

@@ -518,6 +518,10 @@ impl<'t, 'a,'tcx> MemCategorizationContext<'t, 'a, 'tcx> {
             self.cat_def(expr.id, expr.span, expr_ty, def)
           }
 
+          hir::ExprType(ref e, _) => {
+            self.cat_expr(&**e)
+          }
+
           hir::ExprAddrOf(..) | hir::ExprCall(..) |
           hir::ExprAssign(..) | hir::ExprAssignOp(..) |
           hir::ExprClosure(..) | hir::ExprRet(..) |

src/librustc/middle/ty/mod.rs

@@ -2114,6 +2114,10 @@ impl<'tcx> ctxt<'tcx> {
                 }
             }
 
+            hir::ExprType(ref e, _) => {
+                self.expr_is_lval(e)
+            }
+
             hir::ExprUnary(hir::UnDeref, _) |
             hir::ExprField(..) |
             hir::ExprTupField(..) |

src/librustc_back/svh.rs

@@ -234,6 +234,7 @@ mod svh_visitor {
         SawExprUnary(hir::UnOp),
         SawExprLit(ast::Lit_),
         SawExprCast,
+        SawExprType,
         SawExprIf,
         SawExprWhile,
         SawExprMatch,
@@ -262,6 +263,7 @@ mod svh_visitor {
             ExprUnary(op, _)         => SawExprUnary(op),
             ExprLit(ref lit)         => SawExprLit(lit.node.clone()),
             ExprCast(..)             => SawExprCast,
+            ExprType(..)             => SawExprType,
             ExprIf(..)               => SawExprIf,
             ExprWhile(..)            => SawExprWhile,
             ExprLoop(_, id)          => SawExprLoop(id.map(|id| id.name.as_str())),

src/librustc_front/fold.rs

@@ -1043,6 +1043,9 @@ pub fn noop_fold_expr<T: Folder>(Expr { id, node, span, attrs }: Expr, folder: &
             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),