diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index 8a58dad292420..3da653eb7ade9 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -14,7 +14,7 @@ use dep_graph::DepGraph;
 use dep_graph::{DepNode, DepConstructor};
 use errors::DiagnosticBuilder;
 use session::Session;
-use session::config::{BorrowckMode, OutputFilenames};
+use session::config::{BorrowckMode, OutputFilenames, OptLevel};
 use middle;
 use hir::{TraitCandidate, HirId, ItemLocalId};
 use hir::def::{Def, Export};
@@ -1503,8 +1503,18 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     #[inline]
     pub fn share_generics(self) -> bool {
         match self.sess.opts.debugging_opts.share_generics {
-            Some(true) => true,
-            Some(false) | None => false,
+            Some(setting) => setting,
+            None => {
+                self.sess.opts.incremental.is_some() ||
+                match self.sess.opts.optimize {
+                    OptLevel::No   |
+                    OptLevel::Less |
+                    OptLevel::Size |
+                    OptLevel::SizeMin => true,
+                    OptLevel::Default    |
+                    OptLevel::Aggressive => false,
+                }
+            }
         }
     }
 }