Drop function parameters in expected order

Given the function

fn foo((_x, _): (LogDrop, LogDrop), (_, _y): (LogDrop, LogDrop)) {}

Prior to 1.12 we dropped both `_x` and `_y` before the rest of their
respective parameters, since then we dropped `_x` and `_y` after. The
original order appears to be the correct order, as the value created
later is dropped first, so we revert to that order and add a test for
it.

src/librustc_mir/build/mod.rs

@@ -915,6 +915,13 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             let place = Place::Local(local);
             let &ArgInfo(ty, opt_ty_info, pattern, ref self_binding) = arg_info;
 
+            // Make sure we drop (parts of) the argument even when not matched on.
+            self.schedule_drop(
+                pattern.as_ref().map_or(ast_body.span, |pat| pat.span),
+                argument_scope, &place, ty,
+                DropKind::Value { cached_block: CachedBlock::default() },
+            );
+
             if let Some(pattern) = pattern {
                 let pattern = self.hir.pattern_from_hir(pattern);
                 let span = pattern.span;
@@ -946,13 +953,6 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                     }
                 }
             }
-
-            // Make sure we drop (parts of) the argument even when not matched on.
-            self.schedule_drop(
-                pattern.as_ref().map_or(ast_body.span, |pat| pat.span),
-                argument_scope, &place, ty,
-                DropKind::Value { cached_block: CachedBlock::default() },
-            );
         }
 
         // Enter the argument pattern bindings source scope, if it exists.

src/test/run-pass/binding/fn-arg-incomplete-pattern-drop-order.rs

@@ -0,0 +1,54 @@
+// Check that partially moved from function parameters are dropped after the
+// named bindings that move from them.
+
+use std::{panic, cell::RefCell};
+
+struct LogDrop<'a>(i32, Context<'a>);
+
+#[derive(Copy, Clone)]
+struct Context<'a> {
+    panic_on: i32,
+    drops: &'a RefCell<Vec<i32>>,
+}
+
+impl<'a> Context<'a> {
+    fn record_drop(self, index: i32) {
+        self.drops.borrow_mut().push(index);
+        if index == self.panic_on {
+            panic!();
+        }
+    }
+}
+
+impl<'a> Drop for LogDrop<'a> {
+    fn drop(&mut self) {
+        self.1.record_drop(self.0);
+    }
+}
+
+fn foo((_x, _): (LogDrop, LogDrop), (_, _y): (LogDrop, LogDrop)) {}
+
+fn test_drop_order(panic_on: i32) {
+    let context = Context {
+        panic_on,
+        drops: &RefCell::new(Vec::new()),
+    };
+    let one = LogDrop(1, context);
+    let two = LogDrop(2, context);
+    let three = LogDrop(3, context);
+    let four = LogDrop(4, context);
+
+    let res = panic::catch_unwind(panic::AssertUnwindSafe(|| {
+        foo((three, four), (two, one));
+    }));
+    if panic_on == 0 {
+        assert!(res.is_ok(), "should not have panicked");
+    } else {
+        assert!(res.is_err(), "should have panicked");
+    }
+    assert_eq!(*context.drops.borrow(), [1, 2, 3, 4], "incorrect drop order");
+}
+
+fn main() {
+    (0..=4).for_each(test_drop_order);
+}