make const_alloc_layout feature gate only about functions that are already stable

the rest has their constness guarded by their usual feature gate

library/core/src/alloc/layout.rs

@@ -216,7 +216,7 @@ pub const fn for_value<T: ?Sized>(t: &T) -> Self {
     /// [trait object]: ../../book/ch17-02-trait-objects.html
     /// [extern type]: ../../unstable-book/language-features/extern-types.html
     #[unstable(feature = "layout_for_ptr", issue = "69835")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[rustc_const_unstable(feature = "layout_for_ptr", issue = "69835")]
     #[must_use]
     pub const unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self {
         // SAFETY: we pass along the prerequisites of these functions to the caller
@@ -232,7 +232,6 @@ pub const fn for_value<T: ?Sized>(t: &T) -> Self {
     /// sentinel value. Types that lazily allocate must track initialization by
     /// some other means.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
-    #[rustc_const_unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[must_use]
     #[inline]
     pub const fn dangling(&self) -> NonNull<u8> {
@@ -256,6 +255,7 @@ pub const fn dangling(&self) -> NonNull<u8> {
     /// `align` violates the conditions listed in [`Layout::from_size_align`].
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
     #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[cfg_attr(not(bootstrap), rustc_const_stable_indirect)]
     #[inline]
     pub const fn align_to(&self, align: usize) -> Result<Self, LayoutError> {
         if let Some(align) = Alignment::new(align) {
@@ -282,7 +282,6 @@ pub const fn align_to(&self, align: usize) -> Result<Self, LayoutError> {
     /// address for the whole allocated block of memory. One way to
     /// satisfy this constraint is to ensure `align <= self.align()`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
     #[must_use = "this returns the padding needed, \
                   without modifying the `Layout`"]
     #[inline]
@@ -332,6 +331,7 @@ const fn size_rounded_up_to_custom_align(&self, align: Alignment) -> usize {
     /// to the layout's current size.
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
     #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[cfg_attr(not(bootstrap), rustc_const_stable_indirect)]
     #[must_use = "this returns a new `Layout`, \
                   without modifying the original"]
     #[inline]
@@ -374,7 +374,6 @@ pub const fn pad_to_align(&self) -> Layout {
     /// assert_eq!(repeated, (Layout::from_size_align(24, 4).unwrap(), 8));
     /// ```
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
     #[inline]
     pub const fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError> {
         let padded = self.pad_to_align();
@@ -432,6 +431,7 @@ pub const fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError> {
     /// ```
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
     #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[cfg_attr(not(bootstrap), rustc_const_stable_indirect)]
     #[inline]
     pub const fn extend(&self, next: Self) -> Result<(Self, usize), LayoutError> {
         let new_align = Alignment::max(self.align, next.align);
@@ -463,7 +463,6 @@ pub const fn extend(&self, next: Self) -> Result<(Self, usize), LayoutError> {
     ///
     /// On arithmetic overflow, returns `LayoutError`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
     #[inline]
     pub const fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError> {
         if let Some(size) = self.size.checked_mul(n) {
@@ -481,7 +480,6 @@ pub const fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError> {
     ///
     /// On arithmetic overflow, returns `LayoutError`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
     #[inline]
     pub const fn extend_packed(&self, next: Self) -> Result<Self, LayoutError> {
         // SAFETY: each `size` is at most `isize::MAX == usize::MAX/2`, so the
@@ -497,6 +495,7 @@ pub const fn extend_packed(&self, next: Self) -> Result<Self, LayoutError> {
     /// `isize::MAX`, returns `LayoutError`.
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
     #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[cfg_attr(not(bootstrap), rustc_const_stable_indirect)]
     #[inline]
     pub const fn array<T>(n: usize) -> Result<Self, LayoutError> {
         // Reduce the amount of code we need to monomorphize per `T`.