Sort units once, not each call to dep_targets

The code lying around in `dep_targets` is pretty old at this point, but
given the current iteraiton there's no need to re-sort on each call to
`dep_targets`, only initially during construction!

Author: alexcrichton

src/cargo/core/compiler/context/mod.rs

@@ -380,9 +380,7 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
                 return Vec::new();
             }
         }
-        let mut deps = self.unit_dependencies[unit].clone();
-        deps.sort();
-        deps
+        self.unit_dependencies[unit].clone()
     }
 
     pub fn is_primary_package(&self, unit: &Unit<'a>) -> bool {

src/cargo/core/compiler/context/unit_dependencies.rs

@@ -87,6 +87,14 @@ pub fn build_unit_dependencies<'a, 'cfg>(
 
     connect_run_custom_build_deps(&mut state);
 
+    // Dependencies are used in tons of places throughout the backend, many of
+    // which affect the determinism of the build itself. As a result be sure
+    // that dependency lists are always sorted to ensure we've always got a
+    // deterministic output.
+    for list in state.deps.values_mut() {
+        list.sort();
+    }
+
     Ok(())
 }