Auto merge of #30245 - Aatch:dynamic-align-dst, r=pnkfelix

Fixes #26403

This adjusts the pointer, if needed, to the correct alignment by using the alignment information in the vtable.

Handling zero might not be necessary, as it shouldn't actually occur. I've left it as it's own commit so it can be removed fairly easily if people don't think it's worth doing. The way it's handled though means that there shouldn't be much impact on performance.

src/test/run-pass/dst-field-align.rs

@@ -0,0 +1,87 @@
+// Copyright 2015 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.
+
+struct Foo<T: ?Sized> {
+    a: u16,
+    b: T
+}
+
+trait Bar {
+    fn get(&self) -> usize;
+}
+
+impl Bar for usize {
+    fn get(&self) -> usize { *self }
+}
+
+struct Baz<T: ?Sized> {
+    a: T
+}
+
+#[repr(packed)]
+struct Packed<T: ?Sized> {
+    a: u8,
+    b: T
+}
+
+struct HasDrop<T: ?Sized> {
+    ptr: Box<usize>,
+    data: T
+}
+
+fn main() {
+    // Test that zero-offset works properly
+    let b : Baz<usize> = Baz { a: 7 };
+    assert_eq!(b.a.get(), 7);
+    let b : &Baz<Bar> = &b;
+    assert_eq!(b.a.get(), 7);
+
+    // Test that the field is aligned properly
+    let f : Foo<usize> = Foo { a: 0, b: 11 };
+    assert_eq!(f.b.get(), 11);
+    let ptr1 : *const u8 = &f.b as *const _ as *const u8;
+
+    let f : &Foo<Bar> = &f;
+    let ptr2 : *const u8 = &f.b as *const _ as *const u8;
+    assert_eq!(f.b.get(), 11);
+
+    // The pointers should be the same
+    assert_eq!(ptr1, ptr2);
+
+    // Test that packed structs are handled correctly
+    let p : Packed<usize> = Packed { a: 0, b: 13 };
+    assert_eq!(p.b.get(), 13);
+    let p : &Packed<Bar> = &p;
+    assert_eq!(p.b.get(), 13);
+
+    // Test that nested DSTs work properly
+    let f : Foo<Foo<usize>> = Foo { a: 0, b: Foo { a: 1, b: 17 }};
+    assert_eq!(f.b.b.get(), 17);
+    let f : &Foo<Foo<Bar>> = &f;
+    assert_eq!(f.b.b.get(), 17);
+
+    // Test that get the pointer via destructuring works
+
+    let f : Foo<usize> = Foo { a: 0, b: 11 };
+    let f : &Foo<Bar> = &f;
+    let &Foo { a: _, b: ref bar } = f;
+    assert_eq!(bar.get(), 11);
+
+    // Make sure that drop flags don't screw things up
+
+    let d : HasDrop<Baz<[i32; 4]>> = HasDrop {
+        ptr: Box::new(0),
+        data: Baz { a: [1,2,3,4] }
+    };
+    assert_eq!([1,2,3,4], d.data.a);
+
+    let d : &HasDrop<Baz<[i32]>> = &d;
+    assert_eq!(&[1,2,3,4], &d.data.a);
+}