2 files changed, 36 insertions, 3 deletions
diff --git a/src/librustc/middle/typeck/collect.rs b/src/librustc/middle/typeck/collect.rs
index 62ec991f779..7fc0b65d10c 100644
--- a/src/librustc/middle/typeck/collect.rs
+++ b/src/librustc/middle/typeck/collect.rs
@@ -254,16 +254,26 @@ fn ensure_trait_methods(ccx: @crate_ctxt, id: ast::node_id, trait_ty: ty::t) {
 
 fn ensure_supertraits(ccx: @crate_ctxt,
                       id: ast::node_id,
+                      sp: codemap::span,
                       rp: Option<ty::region_variance>,
                       trait_refs: &[@ast::trait_ref]) {
-    if ccx.tcx.supertraits.contains_key(local_def(id)) { return; }
+    let tcx = ccx.tcx;
+    if tcx.supertraits.contains_key(local_def(id)) { return; }
 
     let instantiated = dvec::DVec();
     for trait_refs.each |trait_ref| {
         let (did, tpt) = instantiate_trait_ref(ccx, *trait_ref, rp);
+        if instantiated.any(|other_trait: &InstantiatedTraitRef|
+                            { (*other_trait).def_id == did }) {
+            // This means a trait inherited from the same supertrait more
+            // than once.
+            tcx.sess.span_err(sp, ~"Duplicate supertrait in trait \
+                                     declaration");
+            return;
+        }
         instantiated.push(InstantiatedTraitRef { def_id: did, tpt: tpt });
     }
-    ccx.tcx.supertraits.insert(local_def(id),
+    tcx.supertraits.insert(local_def(id),
                                @dvec::unwrap(move instantiated));
 }
 
@@ -551,7 +561,7 @@ fn convert(ccx: @crate_ctxt, it: @ast::item) {
                it.id, ty_to_str(tcx, tpt.ty));
         write_ty_to_tcx(tcx, it.id, tpt.ty);
         ensure_trait_methods(ccx, it.id, tpt.ty);
-        ensure_supertraits(ccx, it.id, rp, supertraits);
+        ensure_supertraits(ccx, it.id, it.span, rp, supertraits);
 
         let (_, provided_methods) = split_trait_methods(trait_methods);
         let {bounds, _} = mk_substs(ccx, tps, rp);
diff --git a/src/test/compile-fail/issue-3953.rs b/src/test/compile-fail/issue-3953.rs
new file mode 100644
index 00000000000..45fa00bd672
--- /dev/null
+++ b/src/test/compile-fail/issue-3953.rs
@@ -0,0 +1,23 @@
+use cmp::Eq;
+
+trait Hahaha: Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, //~ ERROR Duplicate supertrait in trait declaration
+              Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq,
+              Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq,
+              Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq,
+              Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq,
+              Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq, Eq {}
+
+enum Lol = int;
+
+pub impl Lol: Hahaha {
+    pure fn eq(other: &Lol) -> bool { *self != **other }
+    pure fn ne(other: &Lol) -> bool { *self == **other }
+}
+
+fn main() {
+    if Lol(2) == Lol(4) {
+        io::println("2 == 4");
+    } else {
+        io::println("2 != 4");
+    }
+}
\ No newline at end of file