Rollup merge of rust-lang#130101 - RalfJung:const-cleanup, r=fee1-dead

some const cleanup: remove unnecessary attributes, add const-hack indications

I learned that we use `FIXME(const-hack)` on top of the "const-hack" label. That seems much better since it marks the right place in the code and moves around with the code. So I went through the PRs with that label and added appropriate FIXMEs in the code. IMO this means we can then remove the label -- Cc ``@rust-lang/wg-const-eval.``

I also noticed some const stability attributes that don't do anything useful, and removed them.

r? ``@fee1-dead``

Author: matthiaskrgr

Diff for: alloc/src/collections/vec_deque/mod.rs

@@ -554,8 +554,8 @@ impl<T> VecDeque<T> {
     #[rustc_const_stable(feature = "const_vec_deque_new", since = "1.68.0")]
     #[must_use]
     pub const fn new() -> VecDeque<T> {
-        // FIXME: This should just be `VecDeque::new_in(Global)` once that hits stable.
-        VecDeque { head: 0, len: 0, buf: RawVec::NEW }
+        // FIXME(const-hack): This should just be `VecDeque::new_in(Global)` once that hits stable.
+        VecDeque { head: 0, len: 0, buf: RawVec::new() }
     }
 
     /// Creates an empty deque with space for at least `capacity` elements.

Diff for: alloc/src/raw_vec.rs

@@ -96,13 +96,6 @@ struct RawVecInner<A: Allocator = Global> {
 }
 
 impl<T> RawVec<T, Global> {
-    /// HACK(Centril): This exists because stable `const fn` can only call stable `const fn`, so
-    /// they cannot call `Self::new()`.
-    ///
-    /// If you change `RawVec<T>::new` or dependencies, please take care to not introduce anything
-    /// that would truly const-call something unstable.
-    pub const NEW: Self = Self::new();
-
     /// Creates the biggest possible `RawVec` (on the system heap)
     /// without allocating. If `T` has positive size, then this makes a
     /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
@@ -111,7 +104,7 @@ impl<T> RawVec<T, Global> {
     #[must_use]
     #[rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81")]
     pub const fn new() -> Self {
-        Self { inner: RawVecInner::new::<T>(), _marker: PhantomData }
+        Self::new_in(Global)
     }
 
     /// Creates a `RawVec` (on the system heap) with exactly the
@@ -149,12 +142,6 @@ impl<T> RawVec<T, Global> {
 }
 
 impl RawVecInner<Global> {
-    #[must_use]
-    #[rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81")]
-    const fn new<T>() -> Self {
-        Self::new_in(Global, core::mem::align_of::<T>())
-    }
-
     #[cfg(not(any(no_global_oom_handling, test)))]
     #[must_use]
     #[inline]

Diff for: alloc/src/vec/in_place_collect.rs

@@ -191,7 +191,7 @@ const fn in_place_collectible<DEST, SRC>(
 
 const fn needs_realloc<SRC, DEST>(src_cap: usize, dst_cap: usize) -> bool {
     if const { mem::align_of::<SRC>() != mem::align_of::<DEST>() } {
-        // FIXME: use unreachable! once that works in const
+        // FIXME(const-hack): use unreachable! once that works in const
         panic!("in_place_collectible() prevents this");
     }
 

Diff for: alloc/src/vec/into_iter.rs

@@ -142,7 +142,7 @@ impl<T, A: Allocator> IntoIter<T, A> {
         // struct and then overwriting &mut self.
         // this creates less assembly
         self.cap = 0;
-        self.buf = RawVec::NEW.non_null();
+        self.buf = RawVec::new().non_null();
         self.ptr = self.buf;
         self.end = self.buf.as_ptr();
 

Diff for: alloc/src/vec/mod.rs

@@ -419,7 +419,7 @@ impl<T> Vec<T> {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
     pub const fn new() -> Self {
-        Vec { buf: RawVec::NEW, len: 0 }
+        Vec { buf: RawVec::new(), len: 0 }
     }
 
     /// Constructs a new, empty `Vec<T>` with at least the specified capacity.

Diff for: core/src/char/convert.rs

@@ -11,7 +11,7 @@ use crate::ub_checks::assert_unsafe_precondition;
 #[must_use]
 #[inline]
 pub(super) const fn from_u32(i: u32) -> Option<char> {
-    // FIXME: once Result::ok is const fn, use it here
+    // FIXME(const-hack): once Result::ok is const fn, use it here
     match char_try_from_u32(i) {
         Ok(c) => Some(c),
         Err(_) => None,

Diff for: core/src/char/methods.rs

@@ -383,7 +383,7 @@ impl char {
             // Force the 6th bit to be set to ensure ascii is lower case.
             digit = (self as u32 | 0b10_0000).wrapping_sub('a' as u32).saturating_add(10);
         }
-        // FIXME: once then_some is const fn, use it here
+        // FIXME(const-hack): once then_some is const fn, use it here
         if digit < radix { Some(digit) } else { None }
     }
 

Diff for: core/src/lib.rs

@@ -149,7 +149,6 @@
 #![feature(const_size_of_val_raw)]
 #![feature(const_slice_from_raw_parts_mut)]
 #![feature(const_slice_from_ref)]
-#![feature(const_slice_index)]
 #![feature(const_slice_split_at_mut)]
 #![feature(const_str_from_utf8_unchecked_mut)]
 #![feature(const_strict_overflow_ops)]

Diff for: core/src/num/mod.rs

@@ -6,7 +6,7 @@ use crate::str::FromStr;
 use crate::ub_checks::assert_unsafe_precondition;
 use crate::{ascii, intrinsics, mem};
 
-// Used because the `?` operator is not allowed in a const context.
+// FIXME(const-hack): Used because the `?` operator is not allowed in a const context.
 macro_rules! try_opt {
     ($e:expr) => {
         match $e {

Diff for: core/src/option.rs

@@ -739,6 +739,7 @@ impl<T> Option<T> {
     #[stable(feature = "pin", since = "1.33.0")]
     #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
     pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
+        // FIXME(const-hack): use `map` once that is possible
         match Pin::get_ref(self).as_ref() {
             // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
             // which is pinned.
@@ -758,6 +759,7 @@ impl<T> Option<T> {
         // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
         // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
         unsafe {
+            // FIXME(const-hack): use `map` once that is possible
             match Pin::get_unchecked_mut(self).as_mut() {
                 Some(x) => Some(Pin::new_unchecked(x)),
                 None => None,
@@ -1290,10 +1292,7 @@ impl<T> Option<T> {
     where
         T: Deref,
     {
-        match self.as_ref() {
-            Some(t) => Some(t.deref()),
-            None => None,
-        }
+        self.as_ref().map(|t| t.deref())
     }
 
     /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
@@ -1316,10 +1315,7 @@ impl<T> Option<T> {
     where
         T: DerefMut,
     {
-        match self.as_mut() {
-            Some(t) => Some(t.deref_mut()),
-            None => None,
-        }
+        self.as_mut().map(|t| t.deref_mut())
     }
 
     /////////////////////////////////////////////////////////////////////////
@@ -1632,13 +1628,7 @@ impl<T> Option<T> {
     #[inline]
     #[stable(feature = "option_entry", since = "1.20.0")]
     pub fn get_or_insert(&mut self, value: T) -> &mut T {
-        if let None = *self {
-            *self = Some(value);
-        }
-
-        // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
-        // variant in the code above.
-        unsafe { self.as_mut().unwrap_unchecked() }
+        self.get_or_insert_with(|| value)
     }
 
     /// Inserts the default value into the option if it is [`None`], then
@@ -1724,7 +1714,7 @@ impl<T> Option<T> {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_option", issue = "67441")]
     pub const fn take(&mut self) -> Option<T> {
-        // FIXME replace `mem::replace` by `mem::take` when the latter is const ready
+        // FIXME(const-hack) replace `mem::replace` by `mem::take` when the latter is const ready
         mem::replace(self, None)
     }
 

Diff for: core/src/ptr/alignment.rs

@@ -199,7 +199,6 @@ impl From<Alignment> for usize {
     }
 }
 
-#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
 #[unstable(feature = "ptr_alignment_type", issue = "102070")]
 impl cmp::Ord for Alignment {
     #[inline]
@@ -208,7 +207,6 @@ impl cmp::Ord for Alignment {
     }
 }
 
-#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
 #[unstable(feature = "ptr_alignment_type", issue = "102070")]
 impl cmp::PartialOrd for Alignment {
     #[inline]

Diff for: core/src/slice/index.rs

@@ -31,12 +31,12 @@ where
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
+    // FIXME(const-hack): once integer formatting in panics is possible, we
+    // should use the same implementation at compiletime and runtime.
     const_eval_select((index, len), slice_start_index_len_fail_ct, slice_start_index_len_fail_rt)
 }
 
-// FIXME const-hack
 #[inline]
 #[track_caller]
 fn slice_start_index_len_fail_rt(index: usize, len: usize) -> ! {
@@ -52,12 +52,12 @@ const fn slice_start_index_len_fail_ct(_: usize, _: usize) -> ! {
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
+    // FIXME(const-hack): once integer formatting in panics is possible, we
+    // should use the same implementation at compiletime and runtime.
     const_eval_select((index, len), slice_end_index_len_fail_ct, slice_end_index_len_fail_rt)
 }
 
-// FIXME const-hack
 #[inline]
 #[track_caller]
 fn slice_end_index_len_fail_rt(index: usize, len: usize) -> ! {
@@ -73,12 +73,12 @@ const fn slice_end_index_len_fail_ct(_: usize, _: usize) -> ! {
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_index_order_fail(index: usize, end: usize) -> ! {
+    // FIXME(const-hack): once integer formatting in panics is possible, we
+    // should use the same implementation at compiletime and runtime.
     const_eval_select((index, end), slice_index_order_fail_ct, slice_index_order_fail_rt)
 }
 
-// FIXME const-hack
 #[inline]
 #[track_caller]
 fn slice_index_order_fail_rt(index: usize, end: usize) -> ! {
@@ -246,7 +246,6 @@ pub unsafe trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
 
 /// The methods `index` and `index_mut` panic if the index is out of bounds.
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> SliceIndex<[T]> for usize {
     type Output = T;
 
@@ -386,7 +385,6 @@ unsafe impl<T> SliceIndex<[T]> for ops::IndexRange {
 /// - the start of the range is greater than the end of the range or
 /// - the end of the range is out of bounds.
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> SliceIndex<[T]> for ops::Range<usize> {
     type Output = [T];
 
@@ -522,7 +520,6 @@ unsafe impl<T> SliceIndex<[T]> for range::Range<usize> {
 
 /// The methods `index` and `index_mut` panic if the end of the range is out of bounds.
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> SliceIndex<[T]> for ops::RangeTo<usize> {
     type Output = [T];
 
@@ -561,7 +558,6 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeTo<usize> {
 
 /// The methods `index` and `index_mut` panic if the start of the range is out of bounds.
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> SliceIndex<[T]> for ops::RangeFrom<usize> {
     type Output = [T];
 
@@ -644,7 +640,6 @@ unsafe impl<T> SliceIndex<[T]> for range::RangeFrom<usize> {
 }
 
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> SliceIndex<[T]> for ops::RangeFull {
     type Output = [T];
 
@@ -684,7 +679,6 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeFull {
 /// - the start of the range is greater than the end of the range or
 /// - the end of the range is out of bounds.
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> SliceIndex<[T]> for ops::RangeInclusive<usize> {
     type Output = [T];
 
@@ -766,7 +760,6 @@ unsafe impl<T> SliceIndex<[T]> for range::RangeInclusive<usize> {
 
 /// The methods `index` and `index_mut` panic if the end of the range is out of bounds.
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> SliceIndex<[T]> for ops::RangeToInclusive<usize> {
     type Output = [T];
 

Diff for: core/src/slice/memchr.rs

@@ -51,7 +51,6 @@ const fn memchr_naive(x: u8, text: &[u8]) -> Option<usize> {
 }
 
 #[rustc_allow_const_fn_unstable(const_cmp)]
-#[rustc_allow_const_fn_unstable(const_slice_index)]
 #[rustc_allow_const_fn_unstable(const_align_offset)]
 #[rustc_const_stable(feature = "const_memchr", since = "1.65.0")]
 const fn memchr_aligned(x: u8, text: &[u8]) -> Option<usize> {

Diff for: core/src/slice/mod.rs

@@ -529,7 +529,7 @@ impl<T> [T] {
             None
         } else {
             // SAFETY: We manually verified the bounds of the slice.
-            // FIXME: Without const traits, we need this instead of `get_unchecked`.
+            // FIXME(const-hack): Without const traits, we need this instead of `get_unchecked`.
             let last = unsafe { self.split_at_unchecked(self.len() - N).1 };
 
             // SAFETY: We explicitly check for the correct number of elements,
@@ -563,7 +563,7 @@ impl<T> [T] {
             None
         } else {
             // SAFETY: We manually verified the bounds of the slice.
-            // FIXME: Without const traits, we need this instead of `get_unchecked`.
+            // FIXME(const-hack): Without const traits, we need this instead of `get_unchecked`.
             let last = unsafe { self.split_at_mut_unchecked(self.len() - N).1 };
 
             // SAFETY: We explicitly check for the correct number of elements,
@@ -1952,7 +1952,7 @@ impl<T> [T] {
     #[inline]
     #[must_use]
     pub const unsafe fn split_at_unchecked(&self, mid: usize) -> (&[T], &[T]) {
-        // HACK: the const function `from_raw_parts` is used to make this
+        // FIXME(const-hack): the const function `from_raw_parts` is used to make this
         // function const; previously the implementation used
         // `(self.get_unchecked(..mid), self.get_unchecked(mid..))`
 

Diff for: core/src/str/converts.rs

@@ -85,7 +85,7 @@ use crate::{mem, ptr};
 #[rustc_allow_const_fn_unstable(str_internals)]
 #[rustc_diagnostic_item = "str_from_utf8"]
 pub const fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error> {
-    // FIXME: This should use `?` again, once it's `const`
+    // FIXME(const-hack): This should use `?` again, once it's `const`
     match run_utf8_validation(v) {
         Ok(_) => {
             // SAFETY: validation succeeded.
@@ -129,7 +129,7 @@ pub const fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error> {
 #[rustc_const_unstable(feature = "const_str_from_utf8", issue = "91006")]
 #[rustc_diagnostic_item = "str_from_utf8_mut"]
 pub const fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error> {
-    // This should use `?` again, once it's `const`
+    // FIXME(const-hack): This should use `?` again, once it's `const`
     match run_utf8_validation(v) {
         Ok(_) => {
             // SAFETY: validation succeeded.

Diff for: core/src/str/error.rs

@@ -100,7 +100,7 @@ impl Utf8Error {
     #[must_use]
     #[inline]
     pub const fn error_len(&self) -> Option<usize> {
-        // FIXME: This should become `map` again, once it's `const`
+        // FIXME(const-hack): This should become `map` again, once it's `const`
         match self.error_len {
             Some(len) => Some(len as usize),
             None => None,

Diff for: core/src/str/traits.rs

@@ -92,7 +92,6 @@ const fn str_index_overflow_fail() -> ! {
 ///
 /// Equivalent to `&self[0 .. len]` or `&mut self[0 .. len]`.
 #[stable(feature = "str_checked_slicing", since = "1.20.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl SliceIndex<str> for ops::RangeFull {
     type Output = str;
     #[inline]
@@ -157,7 +156,6 @@ unsafe impl SliceIndex<str> for ops::RangeFull {
 /// // &s[3 .. 100];
 /// ```
 #[stable(feature = "str_checked_slicing", since = "1.20.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl SliceIndex<str> for ops::Range<usize> {
     type Output = str;
     #[inline]
@@ -429,7 +427,6 @@ unsafe impl SliceIndex<str> for (ops::Bound<usize>, ops::Bound<usize>) {
 /// Panics if `end` does not point to the starting byte offset of a
 /// character (as defined by `is_char_boundary`), or if `end > len`.
 #[stable(feature = "str_checked_slicing", since = "1.20.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl SliceIndex<str> for ops::RangeTo<usize> {
     type Output = str;
     #[inline]
@@ -498,7 +495,6 @@ unsafe impl SliceIndex<str> for ops::RangeTo<usize> {
 /// Panics if `begin` does not point to the starting byte offset of
 /// a character (as defined by `is_char_boundary`), or if `begin > len`.
 #[stable(feature = "str_checked_slicing", since = "1.20.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl SliceIndex<str> for ops::RangeFrom<usize> {
     type Output = str;
     #[inline]
@@ -625,7 +621,6 @@ unsafe impl SliceIndex<str> for range::RangeFrom<usize> {
 /// to the ending byte offset of a character (`end + 1` is either a starting
 /// byte offset or equal to `len`), if `begin > end`, or if `end >= len`.
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl SliceIndex<str> for ops::RangeInclusive<usize> {
     type Output = str;
     #[inline]
@@ -714,7 +709,6 @@ unsafe impl SliceIndex<str> for range::RangeInclusive<usize> {
 /// (`end + 1` is either a starting byte offset as defined by
 /// `is_char_boundary`, or equal to `len`), or if `end >= len`.
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl SliceIndex<str> for ops::RangeToInclusive<usize> {
     type Output = str;
     #[inline]