Move while let behind while_let feature gate

Author: hydhknn

src/doc/reference.md

@@ -2503,6 +2503,8 @@ The currently implemented features of the reference compiler are:
 
 * `if_let` - Allows use of the `if let` syntax.
 
+* `while_let` - Allows use of the `while let` syntax.
+
 * `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics
                  are inherently unstable and no promise about them is made.
 

src/libsyntax/feature_gate.rs

@@ -73,6 +73,7 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     ("slicing_syntax", Active),
 
     ("if_let", Active),
+    ("while_let", Active),
 
     // if you change this list without updating src/doc/reference.md, cmr will be sad
 
@@ -356,6 +357,10 @@ impl<'a, 'v> Visitor<'v> for Context<'a> {
                                   e.span,
                                   "slicing syntax is experimental");
             }
+            ast::ExprWhileLet(..) => {
+                self.gate_feature("while_let", e.span,
+                                  "`while let` syntax is experimental");
+            }
             _ => {}
         }
         visit::walk_expr(self, e);