Add unmarked_api feature (fixes #21884)

Author: Manishearth

src/librustc/middle/stability.rs

@@ -253,10 +253,19 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
             None => {
                 // This is an 'unmarked' API, which should not exist
                 // in the standard library.
-                self.tcx.sess.span_err(span, "use of unmarked library feature");
-                self.tcx.sess.span_note(span, "this is either a bug in the library you are \
-                                               using or a bug in the compiler - there is \
-                                               no way to use this feature");
+                if self.tcx.sess.features.borrow().unmarked_api {
+                    self.tcx.sess.span_warn(span, "use of unmarked library feature");
+                    self.tcx.sess.span_note(span, "this is either a bug in the library you are \
+                                                   using and a bug in the compiler - please \
+                                                   report it in both places");
+                } else {
+                    self.tcx.sess.span_err(span, "use of unmarked library feature");
+                    self.tcx.sess.span_note(span, "this is either a bug in the library you are \
+                                                   using and a bug in the compiler - please \
+                                                   report it in both places");
+                    self.tcx.sess.span_note(span, "use #![feature(unmarked_api)] in the \
+                                                   crate attributes to override this");
+                }
             }
         }
     }

src/libsyntax/feature_gate.rs

@@ -119,6 +119,9 @@ static KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
 
     // Allows use of #[staged_api]
     ("staged_api", "1.0.0", Active),
+
+    // Allows using items which are missing stability attributes
+    ("unmarked_api", "1.0.0", Active)
 ];
 
 enum Status {
@@ -145,6 +148,7 @@ pub struct Features {
     pub quote: bool,
     pub old_orphan_check: bool,
     pub simd_ffi: bool,
+    pub unmarked_api: bool,
     pub lib_features: Vec<(InternedString, Span)>
 }
 
@@ -157,6 +161,7 @@ impl Features {
             quote: false,
             old_orphan_check: false,
             simd_ffi: false,
+            unmarked_api: false,
             lib_features: Vec::new()
         }
     }
@@ -566,6 +571,7 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler, krate: &ast::C
         quote: cx.has_feature("quote"),
         old_orphan_check: cx.has_feature("old_orphan_check"),
         simd_ffi: cx.has_feature("simd_ffi"),
+        unmarked_api: cx.has_feature("unmarked_api"),
         lib_features: unknown_features
     }
 }