add regions_that_outlive to FreeRegionMap

src/librustc/middle/free_region.rs

@@ -182,6 +182,19 @@ impl<'tcx> FreeRegionMap<'tcx> {
         debug!("lub_free_regions(r_a={:?}, r_b={:?}) = {:?}", r_a, r_b, result);
         result
     }
+
+    /// Returns all regions that are known to outlive `r_a`. For
+    /// example, in a function:
+    ///
+    /// ```
+    /// fn foo<'a, 'b: 'a, 'c: 'b>() { .. }
+    /// ```
+    ///
+    /// if `r_a` represents `'a`, this function would return `{'b, 'c}`.
+    pub fn regions_that_outlive<'a, 'gcx>(&self, r_a: Region<'tcx>) -> Vec<&Region<'tcx>> {
+        assert!(is_free(r_a));
+        self.relation.greater_than(&r_a)
+    }
 }
 
 fn is_free(r: Region) -> bool {