added unary_negate feature gate.

Author: pnkfelix

src/librustc_typeck/check/mod.rs

@@ -120,6 +120,7 @@ use syntax::attr::AttrMetaMethods;
 use syntax::ast::{self, DefId, Visibility};
 use syntax::ast_util::{self, local_def};
 use syntax::codemap::{self, Span};
+use syntax::feature_gate;
 use syntax::owned_slice::OwnedSlice;
 use syntax::parse::token;
 use syntax::print::pprust;
@@ -3258,6 +3259,15 @@ fn check_expr_with_unifier<'a, 'tcx, F>(fcx: &FnCtxt<'a, 'tcx>,
                                                       tcx.lang_items.neg_trait(),
                                                       expr, &**oprnd, oprnd_t, unop);
                     }
+                    if let ty::ty_uint(_) = oprnd_t.sty {
+                        if !tcx.sess.features.borrow().negate_unsigned {
+                            feature_gate::emit_feature_err(
+                                &tcx.sess.parse_sess.span_diagnostic,
+                                "negate_unsigned",
+                                expr.span,
+                                "unary negation of unsigned integers may be removed in the future");
+                        }
+                    }
                 }
             }
         }

src/libsyntax/feature_gate.rs

@@ -150,6 +150,9 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
 
     // #23121. Array patterns have some hazards yet.
     ("slice_patterns", "1.0.0", Active),
+
+    // Allows use of unary negate on unsigned integers, e.g. -e for e: u8
+    ("negate_unsigned", "1.0.0", Active),
 ];
 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 
@@ -319,6 +322,7 @@ pub struct Features {
     pub allow_custom_derive: bool,
     pub simd_ffi: bool,
     pub unmarked_api: bool,
+    pub negate_unsigned: bool,
     /// spans of #![feature] attrs for stable language features. for error reporting
     pub declared_stable_lang_features: Vec<Span>,
     /// #![feature] attrs for non-language (library) features
@@ -340,6 +344,7 @@ impl Features {
             allow_custom_derive: false,
             simd_ffi: false,
             unmarked_api: false,
+            negate_unsigned: false,
             declared_stable_lang_features: Vec::new(),
             declared_lib_features: Vec::new()
         }
@@ -712,6 +717,7 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler,
         allow_custom_derive: cx.has_feature("custom_derive"),
         simd_ffi: cx.has_feature("simd_ffi"),
         unmarked_api: cx.has_feature("unmarked_api"),
+        negate_unsigned: cx.has_feature("negate_unsigned"),
         declared_stable_lang_features: accepted_features,
         declared_lib_features: unknown_features
     }