more tiny clippy cleanups

compiler/rustc_mir/src/borrow_check/type_check/constraint_conversion.rs

@@ -62,8 +62,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
         // `self.constraints`, but we also want to be mutating
         // `self.member_constraints`. For now, just swap out the value
         // we want and replace at the end.
-        let mut tmp =
-            std::mem::replace(&mut self.constraints.member_constraints, Default::default());
+        let mut tmp = std::mem::take(&mut self.constraints.member_constraints);
         for member_constraint in member_constraints {
             tmp.push_constraint(member_constraint, |r| self.to_region_vid(r));
         }

compiler/rustc_mir/src/transform/early_otherwise_branch.rs

@@ -91,7 +91,7 @@ impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
             let not_equal_rvalue = Rvalue::BinaryOp(
                 not_equal,
                 Operand::Copy(Place::from(second_discriminant_temp)),
-                Operand::Copy(Place::from(first_descriminant_place)),
+                Operand::Copy(first_descriminant_place),
             );
             patch.add_statement(
                 end_of_block_location,

compiler/rustc_typeck/src/check/writeback.rs

@@ -70,10 +70,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         debug!("used_trait_imports({:?}) = {:?}", item_def_id, used_trait_imports);
         wbcx.typeck_results.used_trait_imports = used_trait_imports;
 
-        wbcx.typeck_results.closure_captures = mem::replace(
-            &mut self.typeck_results.borrow_mut().closure_captures,
-            Default::default(),
-        );
+        wbcx.typeck_results.closure_captures =
+            mem::take(&mut self.typeck_results.borrow_mut().closure_captures);
 
         if self.is_tainted_by_errors() {
             // FIXME(eddyb) keep track of `ErrorReported` from where the error was emitted.