improve error message for WF Tuples

src/librustc/traits/error_reporting.rs

@@ -768,6 +768,10 @@ fn note_obligation_cause_code<'a, 'tcx, T>(infcx: &InferCtxt<'a, 'tcx>,
             err.fileline_note(
                 cause_span,
                 "tuple elements must have `Sized` type");
+
+            err.fileline_warn(
+                cause_span,
+                "this is a new restriction added in rustc 1.10");
         }
         ObligationCauseCode::ProjectionWf(data) => {
             err.note(&format!("required so that the projection `{}` is well-formed",

src/test/compile-fail/unsized3.rs

@@ -60,7 +60,9 @@ fn f8<X: ?Sized>(x1: &S<X>, x2: &S<X>) {
 fn f9<X: ?Sized>(x1: Box<S<X>>, x2: Box<E<X>>) {
     f5(&(*x1, 34));
     //~^ ERROR `X: std::marker::Sized` is not satisfied
-    //~^^ ERROR `X: std::marker::Sized` is not satisfied
+    //~| WARNING this is a new restriction added in rustc 1.10
+    //~^^^ ERROR `X: std::marker::Sized` is not satisfied
+    //~| WARNING this is a new restriction added in rustc 1.10
 }
 
 fn f10<X: ?Sized>(x1: Box<S<X>>, x2: Box<E<X>>) {

src/test/compile-fail/unsized6.rs

@@ -15,12 +15,15 @@ trait T {}
 fn f1<X: ?Sized>(x: &X) {
     let _: X; // <-- this is OK, no bindings created, no initializer.
     let _: (isize, (X, isize)); //~ERROR `X: std::marker::Sized` is not satisfied
+    //~| WARNING this is a new restriction added in rustc 1.10
     let y: X; //~ERROR `X: std::marker::Sized` is not satisfied
     let y: (isize, (X, usize)); //~ERROR `X: std::marker::Sized` is not satisfied
+    //~| WARNING this is a new restriction added in rustc 1.10
 }
 fn f2<X: ?Sized + T>(x: &X) {
     let y: X; //~ERROR `X: std::marker::Sized` is not satisfied
     let y: (isize, (X, isize)); //~ERROR `X: std::marker::Sized` is not satisfied
+    //~| WARNING this is a new restriction added in rustc 1.10
 }
 
 fn f3<X: ?Sized>(x1: Box<X>, x2: Box<X>, x3: Box<X>) {