Auto merge of #55366 - Amanieu:stable_layout, r=Amanieu

Add tracking issue for Layout methods (and some API changes)

These methods are already useful when used with the stable global allocator API (stabilized in #51241).

```rust
pub fn align_to(&self, align: usize) -> Result<Layout, LayoutErr>;
pub fn padding_needed_for(&self, align: usize) -> usize;
pub fn repeat(&self, n: usize) -> Result<(Layout, usize), LayoutErr>;
pub fn extend(&self, next: Layout) -> Result<(Layout, usize), LayoutErr>;
pub fn repeat_packed(&self, n: usize) -> Result<Layout, LayoutErr>;
pub fn extend_packed(&self, next: Layout) -> Result<Layout, LayoutErr>;
pub fn array<T>(n: usize) -> Result<Layout, LayoutErr>;
```

cc #32838

r? @SimonSapin

src/liballoc/lib.rs

@@ -119,6 +119,7 @@
 #![feature(const_vec_new)]
 #![feature(slice_partition_dedup)]
 #![feature(maybe_uninit)]
+#![feature(alloc_layout_extra)]
 
 // Allow testing this library
 

src/libcore/alloc.rs

@@ -164,15 +164,13 @@ impl Layout {
     /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
     /// will *still* have size 16.
     ///
-    /// # Panics
-    ///
-    /// Panics if the combination of `self.size()` and the given `align`
-    /// violates the conditions listed in
+    /// Returns an error if the combination of `self.size()` and the given
+    /// `align` violates the conditions listed in
     /// [`Layout::from_size_align`](#method.from_size_align).
-    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
-    pub fn align_to(&self, align: usize) -> Self {
-        Layout::from_size_align(self.size(), cmp::max(self.align(), align)).unwrap()
+    pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
+        Layout::from_size_align(self.size(), cmp::max(self.align(), align))
     }
 
     /// Returns the amount of padding we must insert after `self`
@@ -191,7 +189,7 @@ impl Layout {
     /// to be less than or equal to the alignment of the starting
     /// address for the whole allocated block of memory. One way to
     /// satisfy this constraint is to ensure `align <= self.align()`.
-    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn padding_needed_for(&self, align: usize) -> usize {
         let len = self.size();
@@ -228,7 +226,7 @@ impl Layout {
     /// of each element in the array.
     ///
     /// On arithmetic overflow, returns `LayoutErr`.
-    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
         let padded_size = self.size().checked_add(self.padding_needed_for(self.align()))
@@ -248,13 +246,16 @@ impl Layout {
     /// will be properly aligned. Note that the result layout will
     /// satisfy the alignment properties of both `self` and `next`.
     ///
+    /// The resulting layout will be the same as that of a C struct containing
+    /// two fields with the layouts of `self` and `next`, in that order.
+    ///
     /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
     /// record and `offset` is the relative location, in bytes, of the
     /// start of the `next` embedded within the concatenated record
     /// (assuming that the record itself starts at offset 0).
     ///
     /// On arithmetic overflow, returns `LayoutErr`.
-    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
         let new_align = cmp::max(self.align(), next.align());
@@ -281,7 +282,7 @@ impl Layout {
     /// aligned.
     ///
     /// On arithmetic overflow, returns `LayoutErr`.
-    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
         let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
@@ -293,29 +294,20 @@ impl Layout {
     /// padding is inserted, the alignment of `next` is irrelevant,
     /// and is not incorporated *at all* into the resulting layout.
     ///
-    /// Returns `(k, offset)`, where `k` is layout of the concatenated
-    /// record and `offset` is the relative location, in bytes, of the
-    /// start of the `next` embedded within the concatenated record
-    /// (assuming that the record itself starts at offset 0).
-    ///
-    /// (The `offset` is always the same as `self.size()`; we use this
-    ///  signature out of convenience in matching the signature of
-    ///  `extend`.)
-    ///
     /// On arithmetic overflow, returns `LayoutErr`.
-    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
-    pub fn extend_packed(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
+    pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
         let new_size = self.size().checked_add(next.size())
             .ok_or(LayoutErr { private: () })?;
         let layout = Layout::from_size_align(new_size, self.align())?;
-        Ok((layout, self.size()))
+        Ok(layout)
     }
 
     /// Creates a layout describing the record for a `[T; n]`.
     ///
     /// On arithmetic overflow, returns `LayoutErr`.
-    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
         Layout::new::<T>()

src/libstd/lib.rs

@@ -310,6 +310,7 @@
 #![feature(doc_keyword)]
 #![feature(panic_info_message)]
 #![feature(non_exhaustive)]
+#![feature(alloc_layout_extra)]
 
 #![default_lib_allocator]