Re-do recursive const stability checks

Fundamentally, we have *three* disjoint categories of functions:
1. const-stable functions
2. private/unstable functions that are meant to be callable from const-stable functions
3. functions that can make use of unstable const features

This PR implements the following system:
- `#[rustc_const_stable]` puts functions in the first category. It may only be applied to `#[stable]` functions.
- `#[rustc_const_unstable]` by default puts functions in the third category. The new attribute `#[rustc_const_stable_indirect]` can be added to such a function to move it into the second category.
- `const fn` without a const stability marker are in the second category if they are still unstable. They automatically inherit the feature gate for regular calls, it can now also be used for const-calls.

Also, several holes in recursive const stability checking are being closed.
There's still one potential hole that is hard to avoid, which is when MIR
building automatically inserts calls to a particular function in stable
functions -- which happens in the panic machinery. Those need to *not* be
`rustc_const_unstable` (or manually get a `rustc_const_stable_indirect`) to be
sure they follow recursive const stability. But that's a fairly rare and special
case so IMO it's fine.

The net effect of this is that a `#[unstable]` or unmarked function can be
constified simply by marking it as `const fn`, and it will then be
const-callable from stable `const fn` and subject to recursive const stability
requirements. If it is publicly reachable (which implies it cannot be unmarked),
it will be const-unstable under the same feature gate. Only if the function ever
becomes `#[stable]` does it need a `#[rustc_const_unstable]` or
`#[rustc_const_stable]` marker to decide if this should also imply
const-stability.

Adding `#[rustc_const_unstable]` is only needed for (a) functions that need to
use unstable const lang features (including intrinsics), or (b) `#[stable]`
functions that are not yet intended to be const-stable. Adding
`#[rustc_const_stable]` is only needed for functions that are actually meant to
be directly callable from stable const code. `#[rustc_const_stable_indirect]` is
used to mark intrinsics as const-callable and for `#[rustc_const_unstable]`
functions that are actually called from other, exposed-on-stable `const fn`. No
other attributes are required.

alloc/src/raw_vec.rs

@@ -103,7 +103,7 @@ impl<T> RawVec<T, Global> {
     /// `RawVec` with capacity `usize::MAX`. Useful for implementing
     /// delayed allocation.
     #[must_use]
-    #[rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81")]
+    #[cfg_attr(bootstrap, rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81"))]
     pub const fn new() -> Self {
         Self::new_in(Global)
     }
@@ -179,7 +179,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     /// Like `new`, but parameterized over the choice of allocator for
     /// the returned `RawVec`.
     #[inline]
-    #[rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81")]
+    #[cfg_attr(bootstrap, rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81"))]
     pub const fn new_in(alloc: A) -> Self {
         Self { inner: RawVecInner::new_in(alloc, align_of::<T>()), _marker: PhantomData }
     }
@@ -409,7 +409,7 @@ unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
 
 impl<A: Allocator> RawVecInner<A> {
     #[inline]
-    #[rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81")]
+    #[cfg_attr(bootstrap, rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81"))]
     const fn new_in(alloc: A, align: usize) -> Self {
         let ptr = unsafe { core::mem::transmute(align) };
         // `cap: 0` means "unallocated". zero-sized types are ignored.

core/src/alloc/layout.rs

@@ -66,7 +66,6 @@ impl Layout {
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")]
     #[inline]
-    #[rustc_allow_const_fn_unstable(ptr_alignment_type)]
     pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError> {
         if Layout::is_size_align_valid(size, align) {
             // SAFETY: Layout::is_size_align_valid checks the preconditions for this call.
@@ -127,7 +126,6 @@ impl Layout {
     #[rustc_const_stable(feature = "const_alloc_layout_unchecked", since = "1.36.0")]
     #[must_use]
     #[inline]
-    #[rustc_allow_const_fn_unstable(ptr_alignment_type)]
     pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
         assert_unsafe_precondition!(
             check_library_ub,
@@ -159,7 +157,7 @@ impl Layout {
     #[must_use = "this returns the minimum alignment, \
                   without modifying the layout"]
     #[inline]
-    #[rustc_allow_const_fn_unstable(ptr_alignment_type)]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(ptr_alignment_type))]
     pub const fn align(&self) -> usize {
         self.align.as_usize()
     }

core/src/cell.rs

@@ -2287,6 +2287,7 @@ impl<T> SyncUnsafeCell<T> {
 
     /// Unwraps the value, consuming the cell.
     #[inline]
+    #[rustc_const_unstable(feature = "sync_unsafe_cell", issue = "95439")]
     pub const fn into_inner(self) -> T {
         self.value.into_inner()
     }

core/src/cell/lazy.rs

@@ -79,6 +79,7 @@ impl<T, F: FnOnce() -> T> LazyCell<T, F> {
     /// assert_eq!(LazyCell::into_inner(lazy).ok(), Some("HELLO, WORLD!".to_string()));
     /// ```
     #[unstable(feature = "lazy_cell_into_inner", issue = "125623")]
+    #[rustc_const_unstable(feature = "lazy_cell_into_inner", issue = "125623")]
     pub const fn into_inner(this: Self) -> Result<T, F> {
         match this.state.into_inner() {
             State::Init(data) => Ok(data),

core/src/char/methods.rs

@@ -1770,7 +1770,7 @@ const fn len_utf16(code: u32) -> usize {
 /// Panics if the buffer is not large enough.
 /// A buffer of length four is large enough to encode any `char`.
 #[unstable(feature = "char_internals", reason = "exposed only for libstd", issue = "none")]
-#[rustc_const_stable(feature = "const_char_encode_utf8", since = "1.83.0")]
+#[cfg_attr(bootstrap, rustc_const_stable(feature = "const_char_encode_utf8", since = "1.83.0"))]
 #[doc(hidden)]
 #[inline]
 #[rustc_allow_const_fn_unstable(const_eval_select)]

core/src/ffi/c_str.rs

@@ -137,11 +137,11 @@ enum FromBytesWithNulErrorKind {
 
 // FIXME: const stability attributes should not be required here, I think
 impl FromBytesWithNulError {
-    #[rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0")]
+    #[cfg_attr(bootstrap, rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0"))]
     const fn interior_nul(pos: usize) -> FromBytesWithNulError {
         FromBytesWithNulError { kind: FromBytesWithNulErrorKind::InteriorNul(pos) }
     }
-    #[rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0")]
+    #[cfg_attr(bootstrap, rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0"))]
     const fn not_nul_terminated() -> FromBytesWithNulError {
         FromBytesWithNulError { kind: FromBytesWithNulErrorKind::NotNulTerminated }
     }
@@ -730,7 +730,7 @@ impl AsRef<CStr> for CStr {
 /// located within `isize::MAX` from `ptr`.
 #[inline]
 #[unstable(feature = "cstr_internals", issue = "none")]
-#[rustc_const_stable(feature = "const_cstr_from_ptr", since = "1.81.0")]
+#[cfg_attr(bootstrap, rustc_const_stable(feature = "const_cstr_from_ptr", since = "1.81.0"))]
 #[rustc_allow_const_fn_unstable(const_eval_select)]
 const unsafe fn strlen(ptr: *const c_char) -> usize {
     const fn strlen_ct(s: *const c_char) -> usize {

core/src/fmt/mod.rs

@@ -333,7 +333,10 @@ pub struct Arguments<'a> {
 #[unstable(feature = "fmt_internals", issue = "none")]
 impl<'a> Arguments<'a> {
     #[inline]
-    #[rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")]
+    #[cfg_attr(
+        bootstrap,
+        rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")
+    )]
     pub const fn new_const<const N: usize>(pieces: &'a [&'static str; N]) -> Self {
         const { assert!(N <= 1) };
         Arguments { pieces, fmt: None, args: &[] }
@@ -438,6 +441,7 @@ impl<'a> Arguments<'a> {
     #[rustc_const_unstable(feature = "const_arguments_as_str", issue = "103900")]
     #[must_use]
     #[inline]
+    #[cfg_attr(not(bootstrap), rustc_const_stable_indirect)]
     pub const fn as_str(&self) -> Option<&'static str> {
         match (self.pieces, self.args) {
             ([], []) => Some(""),

core/src/hint.rs

@@ -506,7 +506,7 @@ pub const fn black_box<T>(dummy: T) -> T {
 ///   # }
 ///   ```
 #[unstable(feature = "hint_must_use", issue = "94745")]
-#[rustc_const_unstable(feature = "hint_must_use", issue = "94745")]
+#[cfg_attr(bootstrap, rustc_const_unstable(feature = "hint_must_use", issue = "94745"))]
 #[must_use] // <-- :)
 #[inline(always)]
 pub const fn must_use<T>(value: T) -> T {