Rollup merge of #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

compiler/rustc_mir_transform/src/pass_manager.rs

@@ -42,7 +42,7 @@ fn to_profiler_name(type_name: &'static str) -> &'static str {
 
 // const wrapper for `if let Some((_, tail)) = name.rsplit_once(':') { tail } else { name }`
 const fn c_name(name: &'static str) -> &'static str {
-    // FIXME Simplify the implementation once more `str` methods get const-stable.
+    // FIXME(const-hack) Simplify the implementation once more `str` methods get const-stable.
     // and inline into call site
     let bytes = name.as_bytes();
     let mut i = bytes.len();
@@ -61,7 +61,7 @@ const fn c_name(name: &'static str) -> &'static str {
 /// loop that goes over each available MIR and applies `run_pass`.
 pub(super) trait MirPass<'tcx> {
     fn name(&self) -> &'static str {
-        // FIXME Simplify the implementation once more `str` methods get const-stable.
+        // FIXME(const-hack) Simplify the implementation once more `str` methods get const-stable.
         // See copypaste in `MirLint`
         const {
             let name = std::any::type_name::<Self>();
@@ -89,7 +89,7 @@ pub(super) trait MirPass<'tcx> {
 /// disabled (via the `Lint` adapter).
 pub(super) trait MirLint<'tcx> {
     fn name(&self) -> &'static str {
-        // FIXME Simplify the implementation once more `str` methods get const-stable.
+        // FIXME(const-hack) Simplify the implementation once more `str` methods get const-stable.
         // See copypaste in `MirPass`
         const {
             let name = std::any::type_name::<Self>();

library/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.

library/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]

library/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");
     }
 

library/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();
 

library/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.

library/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,

library/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 }
     }
 

library/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)]

library/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 {