Move if let behind a feature gate

Author: lilyball

src/doc/rust.md

@@ -2457,6 +2457,8 @@ The currently implemented features of the reference compiler are:
             whether this will continue as a feature or not. For these reasons,
             the glob import statement has been hidden behind this feature flag.
 
+* `if_let` - Allows use of the `if let` desugaring syntax.
+
 * `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics
                  are inherently unstable and no promise about them is made.
 

src/librustc/front/feature_gate.rs

@@ -70,6 +70,8 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     ("unboxed_closures", Active),
     ("import_shadowing", Active),
 
+    ("if_let", Active),
+
     // if you change this list without updating src/doc/rust.md, cmr will be sad
 
     // A temporary feature gate used to enable parser extensions needed
@@ -339,6 +341,14 @@ impl<'a> Visitor<()> for Context<'a> {
                                   "unboxed closures are a work-in-progress \
                                    feature with known bugs");
             }
+            ast::ExprIfLet(..) |
+            ast::ExprMatch(_, _, ast::MatchIfLetDesugar) => {
+                // note: at the point where we check feature-gates, the AST desugaring
+                // should not have happened, but the ExprMatch check is included just
+                // in case.
+                self.gate_feature("if_let", e.span,
+                                  "`if let` desugaring is experimental");
+            }
             _ => {}
         }
         visit::walk_expr(self, e, ());

src/test/compile-fail/if-let.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(macro_rules)]
+#![feature(macro_rules,if_let)]
 
 fn macros() {
     macro_rules! foo{

src/test/compile-fail/lint-unnecessary-parens.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![deny(unnecessary_parens)]
+#![feature(if_let)]
 
 #[deriving(Eq, PartialEq)]
 struct X { y: bool }

src/test/run-pass/if-let.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(if_let)]
+
 pub fn main() {
     let x = Some(3i);
     if let Some(y) = x {