Revert "pre-expansion gate type_ascription"

This reverts commit 15a6c09.

Author: eddyb

src/libsyntax/feature_gate/check.rs

@@ -502,6 +502,21 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         }
     }
 
+    fn visit_expr(&mut self, e: &'a ast::Expr) {
+        match e.kind {
+            ast::ExprKind::Type(..) => {
+                // To avoid noise about type ascription in common syntax errors, only emit if it
+                // is the *only* error.
+                if self.parse_sess.span_diagnostic.err_count() == 0 {
+                    gate_feature_post!(&self, type_ascription, e.span,
+                                       "type ascription is experimental");
+                }
+            }
+            _ => {}
+        }
+        visit::walk_expr(self, e)
+    }
+
     fn visit_pat(&mut self, pattern: &'a ast::Pat) {
         match &pattern.kind {
             PatKind::Slice(pats) => {

src/test/ui/feature-gates/feature-gate-type_ascription.rs

@@ -1,8 +1,5 @@
 // Type ascription is unstable
 
-#[cfg(FALSE)]
-fn foo() {
+fn main() {
     let a = 10: u8; //~ ERROR type ascription is experimental
 }
-
-fn main() {}

src/test/ui/feature-gates/feature-gate-type_ascription.stderr

@@ -1,5 +1,5 @@
 error[E0658]: type ascription is experimental
-  --> $DIR/feature-gate-type_ascription.rs:5:13
+  --> $DIR/feature-gate-type_ascription.rs:4:13
    |
 LL |     let a = 10: u8;
    |             ^^^^^^