Make align_of behave like min_align_of.

This removes a footgun, since it is a reasonable assumption to make that
pointers to `T` will be aligned to `align_of::<T>()`. This also matches
the behaviour of C/C++. `min_align_of` is now deprecated.

Closes #21611.

src/liballoc/arc.rs

@@ -77,7 +77,7 @@ use core::atomic;
 use core::atomic::Ordering::{Relaxed, Release, Acquire, SeqCst};
 use core::fmt;
 use core::cmp::Ordering;
-use core::mem::{min_align_of_val, size_of_val};
+use core::mem::{align_of_val, size_of_val};
 use core::intrinsics::drop_in_place;
 use core::mem;
 use core::nonzero::NonZero;
@@ -241,7 +241,7 @@ impl<T: ?Sized> Arc<T> {
 
         if self.inner().weak.fetch_sub(1, Release) == 1 {
             atomic::fence(Acquire);
-            deallocate(ptr as *mut u8, size_of_val(&*ptr), min_align_of_val(&*ptr))
+            deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
         }
     }
 }
@@ -565,7 +565,7 @@ impl<T: ?Sized> Drop for Weak<T> {
             atomic::fence(Acquire);
             unsafe { deallocate(ptr as *mut u8,
                                 size_of_val(&*ptr),
-                                min_align_of_val(&*ptr)) }
+                                align_of_val(&*ptr)) }
         }
     }
 }

src/liballoc/rc.rs

@@ -162,7 +162,7 @@ use core::fmt;
 use core::hash::{Hasher, Hash};
 use core::intrinsics::{assume, drop_in_place};
 use core::marker::{self, Unsize};
-use core::mem::{self, min_align_of, size_of, min_align_of_val, size_of_val, forget};
+use core::mem::{self, align_of, size_of, align_of_val, size_of_val, forget};
 use core::nonzero::NonZero;
 use core::ops::{CoerceUnsized, Deref};
 use core::ptr;
@@ -246,7 +246,7 @@ impl<T> Rc<T> {
                 // destruct the box and skip our Drop
                 // we can ignore the refcounts because we know we're unique
                 deallocate(*rc._ptr as *mut u8, size_of::<RcBox<T>>(),
-                            min_align_of::<RcBox<T>>());
+                            align_of::<RcBox<T>>());
                 forget(rc);
                 Ok(val)
             }
@@ -496,7 +496,7 @@ impl<T: ?Sized> Drop for Rc<T> {
                     if self.weak() == 0 {
                         deallocate(ptr as *mut u8,
                                    size_of_val(&*ptr),
-                                   min_align_of_val(&*ptr))
+                                   align_of_val(&*ptr))
                     }
                 }
             }
@@ -805,7 +805,7 @@ impl<T: ?Sized> Drop for Weak<T> {
                 // the strong pointers have disappeared.
                 if self.weak() == 0 {
                     deallocate(ptr as *mut u8, size_of_val(&*ptr),
-                               min_align_of_val(&*ptr))
+                               align_of_val(&*ptr))
                 }
             }
         }

src/libarena/lib.rs

@@ -244,7 +244,7 @@ impl<'longer_than_self> Arena<'longer_than_self> {
     fn alloc_copy<T, F>(&self, op: F) -> &mut T where F: FnOnce() -> T {
         unsafe {
             let ptr = self.alloc_copy_inner(mem::size_of::<T>(),
-                                            mem::min_align_of::<T>());
+                                            mem::align_of::<T>());
             let ptr = ptr as *mut T;
             ptr::write(&mut (*ptr), op());
             return &mut *ptr;
@@ -300,7 +300,7 @@ impl<'longer_than_self> Arena<'longer_than_self> {
             let tydesc = get_tydesc::<T>();
             let (ty_ptr, ptr) =
                 self.alloc_noncopy_inner(mem::size_of::<T>(),
-                                         mem::min_align_of::<T>());
+                                         mem::align_of::<T>());
             let ty_ptr = ty_ptr as *mut usize;
             let ptr = ptr as *mut T;
             // Write in our tydesc along with a bit indicating that it
@@ -393,7 +393,7 @@ struct TypedArenaChunk<T> {
 
 fn calculate_size<T>(capacity: usize) -> usize {
     let mut size = mem::size_of::<TypedArenaChunk<T>>();
-    size = round_up(size, mem::min_align_of::<T>());
+    size = round_up(size, mem::align_of::<T>());
     let elem_size = mem::size_of::<T>();
     let elems_size = elem_size.checked_mul(capacity).unwrap();
     size = size.checked_add(elems_size).unwrap();
@@ -405,7 +405,7 @@ impl<T> TypedArenaChunk<T> {
     unsafe fn new(next: *mut TypedArenaChunk<T>, capacity: usize)
            -> *mut TypedArenaChunk<T> {
         let size = calculate_size::<T>(capacity);
-        let chunk = allocate(size, mem::min_align_of::<TypedArenaChunk<T>>())
+        let chunk = allocate(size, mem::align_of::<TypedArenaChunk<T>>())
                     as *mut TypedArenaChunk<T>;
         if chunk.is_null() { alloc::oom() }
         (*chunk).next = next;
@@ -431,7 +431,7 @@ impl<T> TypedArenaChunk<T> {
         let size = calculate_size::<T>(self.capacity);
         let self_ptr: *mut TypedArenaChunk<T> = self;
         deallocate(self_ptr as *mut u8, size,
-                   mem::min_align_of::<TypedArenaChunk<T>>());
+                   mem::align_of::<TypedArenaChunk<T>>());
         if !next.is_null() {
             let capacity = (*next).capacity;
             (*next).destroy(capacity);
@@ -444,7 +444,7 @@ impl<T> TypedArenaChunk<T> {
         let this: *const TypedArenaChunk<T> = self;
         unsafe {
             mem::transmute(round_up(this.offset(1) as usize,
-                                    mem::min_align_of::<T>()))
+                                    mem::align_of::<T>()))
         }
     }
 

src/libcollections/btree/node.rs

@@ -163,12 +163,12 @@ fn test_offset_calculation() {
 }
 
 fn calculate_allocation_generic<K, V>(capacity: usize, is_leaf: bool) -> (usize, usize) {
-    let (keys_size, keys_align) = (capacity * mem::size_of::<K>(), mem::min_align_of::<K>());
-    let (vals_size, vals_align) = (capacity * mem::size_of::<V>(), mem::min_align_of::<V>());
+    let (keys_size, keys_align) = (capacity * mem::size_of::<K>(), mem::align_of::<K>());
+    let (vals_size, vals_align) = (capacity * mem::size_of::<V>(), mem::align_of::<V>());
     let (edges_size, edges_align) = if is_leaf {
         (0, 1)
     } else {
-        ((capacity + 1) * mem::size_of::<Node<K, V>>(), mem::min_align_of::<Node<K, V>>())
+        ((capacity + 1) * mem::size_of::<Node<K, V>>(), mem::align_of::<Node<K, V>>())
     };
 
     calculate_allocation(
@@ -181,11 +181,11 @@ fn calculate_allocation_generic<K, V>(capacity: usize, is_leaf: bool) -> (usize,
 fn calculate_offsets_generic<K, V>(capacity: usize, is_leaf: bool) -> (usize, usize) {
     let keys_size = capacity * mem::size_of::<K>();
     let vals_size = capacity * mem::size_of::<V>();
-    let vals_align = mem::min_align_of::<V>();
+    let vals_align = mem::align_of::<V>();
     let edges_align = if is_leaf {
         1
     } else {
-        mem::min_align_of::<Node<K, V>>()
+        mem::align_of::<Node<K, V>>()
     };
 
     calculate_offsets(

src/libcollections/vec.rs

@@ -219,7 +219,7 @@ impl<T> Vec<T> {
         } else {
             let size = capacity.checked_mul(mem::size_of::<T>())
                                .expect("capacity overflow");
-            let ptr = unsafe { allocate(size, mem::min_align_of::<T>()) };
+            let ptr = unsafe { allocate(size, mem::align_of::<T>()) };
             if ptr.is_null() { ::alloc::oom() }
             unsafe { Vec::from_raw_parts(ptr as *mut T, 0, capacity) }
         }
@@ -393,7 +393,7 @@ impl<T> Vec<T> {
                 let ptr = reallocate(*self.ptr as *mut u8,
                                      self.cap * mem::size_of::<T>(),
                                      self.len * mem::size_of::<T>(),
-                                     mem::min_align_of::<T>()) as *mut T;
+                                     mem::align_of::<T>()) as *mut T;
                 if ptr.is_null() { ::alloc::oom() }
                 self.ptr = Unique::new(ptr);
             }
@@ -866,9 +866,9 @@ impl<T> Vec<T> {
             // FIXME: Assert statically that the types `T` and `U` have the
             // same minimal alignment in case they are not zero-sized.
 
-            // These asserts are necessary because the `min_align_of` of the
+            // These asserts are necessary because the `align_of` of the
             // types are passed to the allocator by `Vec`.
-            assert!(mem::min_align_of::<T>() == mem::min_align_of::<U>());
+            assert!(mem::align_of::<T>() == mem::align_of::<U>());
 
             // This `as isize` cast is safe, because the size of the elements of the
             // vector is not 0, and:
@@ -1269,9 +1269,9 @@ impl<T> Vec<T> {
 #[inline(never)]
 unsafe fn alloc_or_realloc<T>(ptr: *mut T, old_size: usize, size: usize) -> *mut T {
     if old_size == 0 {
-        allocate(size, mem::min_align_of::<T>()) as *mut T
+        allocate(size, mem::align_of::<T>()) as *mut T
     } else {
-        reallocate(ptr as *mut u8, old_size, size, mem::min_align_of::<T>()) as *mut T
+        reallocate(ptr as *mut u8, old_size, size, mem::align_of::<T>()) as *mut T
     }
 }
 
@@ -1280,7 +1280,7 @@ unsafe fn dealloc<T>(ptr: *mut T, len: usize) {
     if mem::size_of::<T>() != 0 {
         deallocate(ptr as *mut u8,
                    len * mem::size_of::<T>(),
-                   mem::min_align_of::<T>())
+                   mem::align_of::<T>())
     }
 }
 

src/libcollections/vec_deque.rs

@@ -67,7 +67,7 @@ impl<T> Drop for VecDeque<T> {
             if mem::size_of::<T>() != 0 {
                 heap::deallocate(*self.ptr as *mut u8,
                                  self.cap * mem::size_of::<T>(),
-                                 mem::min_align_of::<T>())
+                                 mem::align_of::<T>())
             }
         }
     }
@@ -172,7 +172,7 @@ impl<T> VecDeque<T> {
 
         let ptr = unsafe {
             if mem::size_of::<T>() != 0 {
-                let ptr = heap::allocate(size, mem::min_align_of::<T>())  as *mut T;;
+                let ptr = heap::allocate(size, mem::align_of::<T>())  as *mut T;;
                 if ptr.is_null() { ::alloc::oom() }
                 Unique::new(ptr)
             } else {
@@ -340,7 +340,7 @@ impl<T> VecDeque<T> {
                     let ptr = heap::reallocate(*self.ptr as *mut u8,
                                                old,
                                                new,
-                                               mem::min_align_of::<T>()) as *mut T;
+                                               mem::align_of::<T>()) as *mut T;
                     if ptr.is_null() { ::alloc::oom() }
                     self.ptr = Unique::new(ptr);
                 }
@@ -460,7 +460,7 @@ impl<T> VecDeque<T> {
                     let ptr = heap::reallocate(*self.ptr as *mut u8,
                                                old,
                                                new_size,
-                                               mem::min_align_of::<T>()) as *mut T;
+                                               mem::align_of::<T>()) as *mut T;
                     if ptr.is_null() { ::alloc::oom() }
                     self.ptr = Unique::new(ptr);
                 }

src/libcore/mem.rs

@@ -155,6 +155,7 @@ pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[deprecated(reason = "use `align_of` instead", since = "1.1.0")]
 pub fn min_align_of<T>() -> usize {
     unsafe { intrinsics::min_align_of::<T>() }
 }
@@ -170,14 +171,14 @@ pub fn min_align_of<T>() -> usize {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[deprecated(reason = "use `align_of_val` instead", since = "1.1.0")]
 pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
     unsafe { intrinsics::min_align_of_val(val) }
 }
 
 /// Returns the alignment in memory for a type.
 ///
-/// This function will return the alignment, in bytes, of a type in memory. If the alignment
-/// returned is adhered to, then the type is guaranteed to function properly.
+/// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
 ///
 /// # Examples
 ///
@@ -189,17 +190,10 @@ pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn align_of<T>() -> usize {
-    // We use the preferred alignment as the default alignment for a type. This
-    // appears to be what clang migrated towards as well:
-    //
-    // http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20110725/044411.html
-    unsafe { intrinsics::pref_align_of::<T>() }
+    unsafe { intrinsics::min_align_of::<T>() }
 }
 
-/// Returns the alignment of the type of the value that `_val` points to.
-///
-/// This is similar to `align_of`, but function will properly handle types such as trait objects
-/// (in the future), returning the alignment for an arbitrary value at runtime.
+/// Returns the ABI-required minimum alignment of the type of the value that `val` points to
 ///
 /// # Examples
 ///
@@ -210,8 +204,8 @@ pub fn align_of<T>() -> usize {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub fn align_of_val<T>(_val: &T) -> usize {
-    align_of::<T>()
+pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
+    unsafe { intrinsics::min_align_of_val(val) }
 }
 
 /// Creates a value initialized to zero.

src/libstd/collections/hash/table.rs

@@ -15,7 +15,7 @@ use cmp;
 use hash::{Hash, Hasher};
 use iter::{Iterator, ExactSizeIterator};
 use marker::{Copy, Send, Sync, Sized, self};
-use mem::{min_align_of, size_of};
+use mem::{align_of, size_of};
 use mem;
 use num::wrapping::OverflowingOps;
 use ops::{Deref, DerefMut, Drop};
@@ -553,9 +553,9 @@ fn calculate_allocation(hash_size: usize, hash_align: usize,
                                                               vals_align);
     let (end_of_vals, oflo2) = vals_offset.overflowing_add(vals_size);
 
-    let min_align = cmp::max(hash_align, cmp::max(keys_align, vals_align));
+    let align = cmp::max(hash_align, cmp::max(keys_align, vals_align));
 
-    (min_align, hash_offset, end_of_vals, oflo || oflo2)
+    (align, hash_offset, end_of_vals, oflo || oflo2)
 }
 
 #[test]
@@ -597,9 +597,9 @@ impl<K, V> RawTable<K, V> {
         // factored out into a different function.
         let (malloc_alignment, hash_offset, size, oflo) =
             calculate_allocation(
-                hashes_size, min_align_of::<u64>(),
-                keys_size,   min_align_of::< K >(),
-                vals_size,   min_align_of::< V >());
+                hashes_size, align_of::<u64>(),
+                keys_size,   align_of::< K >(),
+                vals_size,   align_of::< V >());
 
         assert!(!oflo, "capacity overflow");
 
@@ -630,8 +630,8 @@ impl<K, V> RawTable<K, V> {
         let buffer = *self.hashes as *mut u8;
         let (keys_offset, vals_offset, oflo) =
             calculate_offsets(hashes_size,
-                              keys_size, min_align_of::<K>(),
-                              min_align_of::<V>());
+                              keys_size, align_of::<K>(),
+                              align_of::<V>());
         debug_assert!(!oflo, "capacity overflow");
         unsafe {
             RawBucket {
@@ -1005,9 +1005,9 @@ impl<K, V> Drop for RawTable<K, V> {
         let keys_size = self.capacity * size_of::<K>();
         let vals_size = self.capacity * size_of::<V>();
         let (align, _, size, oflo) =
-            calculate_allocation(hashes_size, min_align_of::<u64>(),
-                                 keys_size, min_align_of::<K>(),
-                                 vals_size, min_align_of::<V>());
+            calculate_allocation(hashes_size, align_of::<u64>(),
+                                 keys_size, align_of::<K>(),
+                                 vals_size, align_of::<V>());
 
         debug_assert!(!oflo, "should be impossible");