Fix the order in which struct fields drop

When a struct implements Drop, its fields should still drop in
declaration order (just as they do when the struct does not implement
Drop).

Fixes #16492.

src/librustc/middle/trans/glue.rs

@@ -250,8 +250,9 @@ fn trans_struct_drop<'a>(bcx: &'a Block<'a>,
         let args = vec!(self_arg);
 
         // Add all the fields as a value which needs to be cleaned at the end of
-        // this scope.
-        for (i, ty) in st.fields.iter().enumerate() {
+        // this scope. Iterate in reverse order so a Drop impl doesn't reverse
+        // the order in which fields get dropped.
+        for (i, ty) in st.fields.iter().enumerate().rev() {
             let llfld_a = adt::struct_field_ptr(variant_cx, &*st, value, i, false);
             variant_cx.fcx.schedule_drop_mem(cleanup::CustomScope(field_scope),
                                              llfld_a, *ty);

src/test/run-pass/issue-16492.rs

@@ -0,0 +1,80 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-pretty
+
+#![feature(unsafe_destructor)]
+
+use std::rc::Rc;
+use std::cell::Cell;
+
+struct Field {
+    number: uint,
+    state: Rc<Cell<uint>>
+}
+
+impl Field {
+    fn new(number: uint, state: Rc<Cell<uint>>) -> Field {
+        Field {
+            number: number,
+            state: state
+        }
+    }
+}
+
+#[unsafe_destructor] // because Field isn't Send
+impl Drop for Field {
+    fn drop(&mut self) {
+        println!("Dropping field {}", self.number);
+        assert_eq!(self.state.get(), self.number);
+        self.state.set(self.state.get()+1);
+    }
+}
+
+struct NoDropImpl {
+    _one: Field,
+    _two: Field,
+    _three: Field
+}
+
+struct HasDropImpl {
+    _one: Field,
+    _two: Field,
+    _three: Field
+}
+
+#[unsafe_destructor] // because HasDropImpl isn't Send
+impl Drop for HasDropImpl {
+    fn drop(&mut self) {
+        println!("HasDropImpl.drop()");
+        assert_eq!(self._one.state.get(), 0);
+        self._one.state.set(1);
+    }
+}
+
+pub fn main() {
+    let state = Rc::new(Cell::new(1));
+    let noImpl = NoDropImpl {
+        _one: Field::new(1, state.clone()),
+        _two: Field::new(2, state.clone()),
+        _three: Field::new(3, state.clone())
+    };
+    drop(noImpl);
+    assert_eq!(state.get(), 4);
+
+    state.set(0);
+    let hasImpl = HasDropImpl {
+        _one: Field::new(1, state.clone()),
+        _two: Field::new(2, state.clone()),
+        _three: Field::new(3, state.clone())
+    };
+    drop(hasImpl);
+    assert_eq!(state.get(), 4);
+}