Rollup merge of #95354 - dtolnay:rustc_const_stable, r=lcnr

Handle rustc_const_stable attribute in library feature collector

The library feature collector in [compiler/rustc_passes/src/lib_features.rs](https://github.com/rust-lang/rust/blob/551b4fa395fa588d91cbecfb0cdfe1baa02670cf/compiler/rustc_passes/src/lib_features.rs) has only been looking at `#[stable(…)]`, `#[unstable(…)]`, and `#[rustc_const_unstable(…)]` attributes, while ignoring `#[rustc_const_stable(…)]`. The consequences of this were:

- When any const feature got stabilized (changing one or more `rustc_const_unstable` to `rustc_const_stable`), users who had previously enabled that unstable feature using `#![feature(…)]` would get told "unknown feature", rather than rustc's nicer "the feature … has been stable since … and no longer requires an attribute to enable".

    This can be seen in the way that https://github.com/rust-lang/rust/pull/93957#issuecomment-1079794660 failed after rebase:

    ```console
    error[E0635]: unknown feature `const_ptr_offset`
      --> $DIR/offset_from_ub.rs:1:35
       |
    LL | #![feature(const_ptr_offset_from, const_ptr_offset)]
       |                                   ^^^^^^^^^^^^^^^^
    ```

- We weren't enforcing that a particular feature is either stable everywhere or unstable everywhere, and that a feature that has been stabilized has the same stabilization version everywhere, both of which we enforce for the other stability attributes.

This PR updates the library feature collector to handle `rustc_const_stable`, and fixes places in the standard library and test suite where `rustc_const_stable` was being used in a way that does not meet the rules for a stability attribute.

1 files changed, 4 insertions, 4 deletions

diff --git a/library/core/src/alloc/layout.rs b/library/core/src/alloc/layout.rs
index 0639d6eed62..db6898c1308 100644
--- a/library/core/src/alloc/layout.rs
+++ b/library/core/src/alloc/layout.rs
@@ -56,7 +56,7 @@ impl Layout {
     ///    must not overflow (i.e., the rounded value must be less than
     ///    or equal to `usize::MAX`).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
+    #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
     #[inline]
     pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError> {
         if !align.is_power_of_two() {
@@ -93,7 +93,7 @@ impl Layout {
     /// This function is unsafe as it does not verify the preconditions from
     /// [`Layout::from_size_align`].
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_stable(feature = "alloc_layout", since = "1.36.0")]
+    #[rustc_const_stable(feature = "const_alloc_layout_unchecked", since = "1.36.0")]
     #[must_use]
     #[inline]
     pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
@@ -103,7 +103,7 @@ impl Layout {
 
     /// The minimum size in bytes for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
+    #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
     #[must_use]
     #[inline]
     pub const fn size(&self) -> usize {
@@ -112,7 +112,7 @@ impl Layout {
 
     /// The minimum byte alignment for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
+    #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
     #[must_use = "this returns the minimum alignment, \
                   without modifying the layout"]
     #[inline]