Rollup merge of rust-lang#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 rust-lang#93957 (comment) 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.

Author: matthiaskrgr

compiler/rustc_passes/src/lib_features.rs

@@ -29,10 +29,11 @@ impl<'tcx> LibFeatureCollector<'tcx> {
     }
 
     fn extract(&self, attr: &Attribute) -> Option<(Symbol, Option<Symbol>, Span)> {
-        let stab_attrs = [sym::stable, sym::unstable, sym::rustc_const_unstable];
+        let stab_attrs =
+            [sym::stable, sym::unstable, sym::rustc_const_stable, sym::rustc_const_unstable];
 
-        // Find a stability attribute (i.e., `#[stable (..)]`, `#[unstable (..)]`,
-        // `#[rustc_const_unstable (..)]`).
+        // Find a stability attribute: one of #[stable(…)], #[unstable(…)],
+        // #[rustc_const_stable(…)], or #[rustc_const_unstable(…)].
         if let Some(stab_attr) = stab_attrs.iter().find(|stab_attr| attr.has_name(**stab_attr)) {
             let meta_kind = attr.meta_kind();
             if let Some(MetaItemKind::List(ref metas)) = meta_kind {
@@ -52,7 +53,9 @@ impl<'tcx> LibFeatureCollector<'tcx> {
                     // This additional check for stability is to make sure we
                     // don't emit additional, irrelevant errors for malformed
                     // attributes.
-                    if *stab_attr != sym::stable || since.is_some() {
+                    let is_unstable =
+                        matches!(*stab_attr, sym::unstable | sym::rustc_const_unstable);
+                    if since.is_some() || is_unstable {
                         return Some((feature, since, attr.span));
                     }
                 }

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]

library/core/src/char/methods.rs

@@ -1092,7 +1092,7 @@ impl char {
     /// ```
     #[must_use]
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
-    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.32.0")]
+    #[rustc_const_stable(feature = "const_char_is_ascii", since = "1.32.0")]
     #[inline]
     pub const fn is_ascii(&self) -> bool {
         *self as u32 <= 0x7F

library/core/src/intrinsics.rs

@@ -1638,7 +1638,7 @@ extern "rust-intrinsic" {
     /// let num_trailing = unsafe { cttz_nonzero(x) };
     /// assert_eq!(num_trailing, 3);
     /// ```
-    #[rustc_const_stable(feature = "const_cttz", since = "1.53.0")]
+    #[rustc_const_stable(feature = "const_cttz_nonzero", since = "1.53.0")]
     pub fn cttz_nonzero<T: Copy>(x: T) -> T;
 
     /// Reverses the bytes in an integer type `T`.
@@ -1718,15 +1718,15 @@ extern "rust-intrinsic" {
     /// Safe wrappers for this intrinsic are available on the integer
     /// primitives via the `checked_div` method. For example,
     /// [`u32::checked_div`]
-    #[rustc_const_stable(feature = "const_int_unchecked_arith", since = "1.52.0")]
+    #[rustc_const_stable(feature = "const_int_unchecked_div", since = "1.52.0")]
     pub fn unchecked_div<T: Copy>(x: T, y: T) -> T;
     /// Returns the remainder of an unchecked division, resulting in
     /// undefined behavior when `y == 0` or `x == T::MIN && y == -1`
     ///
     /// Safe wrappers for this intrinsic are available on the integer
     /// primitives via the `checked_rem` method. For example,
     /// [`u32::checked_rem`]
-    #[rustc_const_stable(feature = "const_int_unchecked_arith", since = "1.52.0")]
+    #[rustc_const_stable(feature = "const_int_unchecked_rem", since = "1.52.0")]
     pub fn unchecked_rem<T: Copy>(x: T, y: T) -> T;
 
     /// Performs an unchecked left shift, resulting in undefined behavior when

library/core/src/lib.rs

@@ -97,7 +97,6 @@
 // Library features:
 #![feature(const_align_offset)]
 #![feature(const_align_of_val)]
-#![feature(const_alloc_layout)]
 #![feature(const_arguments_as_str)]
 #![feature(const_array_into_iter_constructors)]
 #![feature(const_bigint_helper_methods)]

library/core/src/mem/maybe_uninit.rs

@@ -622,7 +622,7 @@ impl<T> MaybeUninit<T> {
     /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️
     /// ```
     #[stable(feature = "maybe_uninit", since = "1.36.0")]
-    #[rustc_const_stable(feature = "const_maybe_uninit_assume_init", since = "1.59.0")]
+    #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_by_value", since = "1.59.0")]
     #[inline(always)]
     #[rustc_diagnostic_item = "assume_init"]
     #[track_caller]
@@ -788,7 +788,7 @@ impl<T> MaybeUninit<T> {
     /// }
     /// ```
     #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
-    #[rustc_const_stable(feature = "const_maybe_uninit_assume_init", since = "1.59.0")]
+    #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_ref", since = "1.59.0")]
     #[inline(always)]
     pub const unsafe fn assume_init_ref(&self) -> &T {
         // SAFETY: the caller must guarantee that `self` is initialized.

library/core/src/mem/mod.rs

@@ -299,7 +299,7 @@ pub fn forget_unsized<T: ?Sized>(t: T) {
 #[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_promotable]
-#[rustc_const_stable(feature = "const_size_of", since = "1.24.0")]
+#[rustc_const_stable(feature = "const_mem_size_of", since = "1.24.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_size_of")]
 pub const fn size_of<T>() -> usize {
     intrinsics::size_of::<T>()
@@ -581,7 +581,7 @@ pub const unsafe fn align_of_val_raw<T: ?Sized>(val: *const T) -> usize {
 #[inline]
 #[must_use]
 #[stable(feature = "needs_drop", since = "1.21.0")]
-#[rustc_const_stable(feature = "const_needs_drop", since = "1.36.0")]
+#[rustc_const_stable(feature = "const_mem_needs_drop", since = "1.36.0")]
 #[rustc_diagnostic_item = "needs_drop"]
 pub const fn needs_drop<T>() -> bool {
     intrinsics::needs_drop::<T>()

library/core/src/num/int_macros.rs

@@ -270,7 +270,7 @@ macro_rules! int_impl {
         #[doc = concat!("assert_eq!(0, 0", stringify!($SelfT), ".reverse_bits());")]
         /// ```
         #[stable(feature = "reverse_bits", since = "1.37.0")]
-        #[rustc_const_stable(feature = "const_int_methods", since = "1.37.0")]
+        #[rustc_const_stable(feature = "reverse_bits", since = "1.37.0")]
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline(always)]
@@ -603,7 +603,7 @@ macro_rules! int_impl {
         #[doc = concat!("assert_eq!((1", stringify!($SelfT), ").checked_div(0), None);")]
         /// ```
         #[stable(feature = "rust1", since = "1.0.0")]
-        #[rustc_const_stable(feature = "const_checked_int_methods", since = "1.52.0")]
+        #[rustc_const_stable(feature = "const_checked_int_div", since = "1.52.0")]
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline]
@@ -656,7 +656,7 @@ macro_rules! int_impl {
         #[doc = concat!("assert_eq!(", stringify!($SelfT), "::MIN.checked_rem(-1), None);")]
         /// ```
         #[stable(feature = "wrapping", since = "1.7.0")]
-        #[rustc_const_stable(feature = "const_checked_int_methods", since = "1.52.0")]
+        #[rustc_const_stable(feature = "const_checked_int_div", since = "1.52.0")]
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline]

library/core/src/num/mod.rs

@@ -281,7 +281,7 @@ impl u8 {
     /// ```
     #[must_use]
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
-    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.43.0")]
+    #[rustc_const_stable(feature = "const_u8_is_ascii", since = "1.43.0")]
     #[inline]
     pub const fn is_ascii(&self) -> bool {
         *self & 128 == 0

library/core/src/num/nonzero.rs

@@ -74,7 +74,7 @@ macro_rules! nonzero_integers {
                 /// Returns the value as a primitive type.
                 #[$stability]
                 #[inline]
-                #[rustc_const_stable(feature = "nonzero", since = "1.34.0")]
+                #[rustc_const_stable(feature = "const_nonzero_get", since = "1.34.0")]
                 pub const fn get(self) -> $Int {
                     self.0
                 }