Free some memory instead of just dropping elements

Author: oli-obk

src/librustc/traits/select.rs

@@ -3789,14 +3789,16 @@ impl<'tcx> TraitObligation<'tcx> {
 }
 
 impl<'tcx> SelectionCache<'tcx> {
+    /// Actually frees the underlying memory in contrast to what stdlib containers do on `clear`
     pub fn clear(&self) {
-        self.hashmap.borrow_mut().clear();
+        *self.hashmap.borrow_mut() = Default::default();
     }
 }
 
 impl<'tcx> EvaluationCache<'tcx> {
+    /// Actually frees the underlying memory in contrast to what stdlib containers do on `clear`
     pub fn clear(&self) {
-        self.hashmap.borrow_mut().clear();
+        *self.hashmap.borrow_mut() = Default::default();
     }
 }
 

src/librustc_errors/lib.rs

@@ -398,7 +398,8 @@ impl Handler {
     /// tools that want to reuse a `Parser` cleaning the previously emitted diagnostics as well as
     /// the overall count of emitted error diagnostics.
     pub fn reset_err_count(&self) {
-        self.emitted_diagnostics.borrow_mut().clear();
+        // actually frees the underlying memory (which `clear` would not do)
+        *self.emitted_diagnostics.borrow_mut() = Default::default();
         self.err_count.store(0, SeqCst);
     }