Rollup merge of rust-lang#121894 - RalfJung:const_eval_select, r=oli-obk

const_eval_select: make it safe but be careful with what we expose on stable for now

As this is all still nightly-only I think `````@rust-lang/wg-const-eval````` can do that without involving t-lang.

r? `````@oli-obk`````
Cc `````@Nilstrieb````` -- the updated version of your RFC would basically say that we can remove these comments about not making behavior differences visible in stable `const fn`

Author: matthiaskrgr

Diff for: alloc/src/alloc.rs

@@ -385,6 +385,7 @@ pub const fn handle_alloc_error(layout: Layout) -> ! {
     }
 
     #[cfg(not(feature = "panic_immediate_abort"))]
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     unsafe {
         core::intrinsics::const_eval_select((layout,), ct_error, rt_error)
     }

Diff for: core/src/ffi/c_str.rs

@@ -428,10 +428,13 @@ impl CStr {
             unsafe { &*(bytes as *const [u8] as *const CStr) }
         }
 
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The const and runtime versions have identical behavior
         // unless the safety contract of `from_bytes_with_nul_unchecked` is
         // violated, which is UB.
-        unsafe { intrinsics::const_eval_select((bytes,), const_impl, rt_impl) }
+        unsafe {
+            intrinsics::const_eval_select((bytes,), const_impl, rt_impl)
+        }
     }
 
     /// Returns the inner pointer to this C string.
@@ -719,6 +722,9 @@ const unsafe fn const_strlen(ptr: *const c_char) -> usize {
         unsafe { strlen(s) }
     }
 
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: the two functions always provide equivalent functionality
-    unsafe { intrinsics::const_eval_select((ptr,), strlen_ct, strlen_rt) }
+    unsafe {
+        intrinsics::const_eval_select((ptr,), strlen_ct, strlen_rt)
+    }
 }

Diff for: core/src/intrinsics.rs

@@ -2514,6 +2514,8 @@ extern "rust-intrinsic" {
     /// intrinsic will be replaced with a call to `called_in_const`. It gets
     /// replaced with a call to `called_at_rt` otherwise.
     ///
+    /// This function is safe to call, but note the stability concerns below.
+    ///
     /// # Type Requirements
     ///
     /// The two functions must be both function items. They cannot be function
@@ -2523,45 +2525,47 @@ extern "rust-intrinsic" {
     /// the two functions, therefore, both functions must accept the same type of
     /// arguments. Both functions must return RET.
     ///
-    /// # Safety
+    /// # Stability concerns
     ///
-    /// The two functions must behave observably equivalent. Safe code in other
-    /// crates may assume that calling a `const fn` at compile-time and at run-time
-    /// produces the same result. A function that produces a different result when
-    /// evaluated at run-time, or has any other observable side-effects, is
-    /// *unsound*.
+    /// Rust has not yet decided that `const fn` are allowed to tell whether
+    /// they run at compile-time or at runtime. Therefore, when using this
+    /// intrinsic anywhere that can be reached from stable, it is crucial that
+    /// the end-to-end behavior of the stable `const fn` is the same for both
+    /// modes of execution. (Here, Undefined Behavior is considered "the same"
+    /// as any other behavior, so if the function exhibits UB at runtime then
+    /// it may do whatever it wants at compile-time.)
     ///
     /// Here is an example of how this could cause a problem:
     /// ```no_run
     /// #![feature(const_eval_select)]
     /// #![feature(core_intrinsics)]
     /// # #![allow(internal_features)]
-    /// use std::hint::unreachable_unchecked;
+    /// # #![cfg_attr(bootstrap, allow(unused))]
     /// use std::intrinsics::const_eval_select;
     ///
-    /// // Crate A
+    /// // Standard library
+    /// # #[cfg(not(bootstrap))]
     /// pub const fn inconsistent() -> i32 {
     ///     fn runtime() -> i32 { 1 }
     ///     const fn compiletime() -> i32 { 2 }
     ///
-    ///     unsafe {
-    //          // ⚠ This code violates the required equivalence of `compiletime`
-    ///         // and `runtime`.
-    ///         const_eval_select((), compiletime, runtime)
-    ///     }
+    //      // ⚠ This code violates the required equivalence of `compiletime`
+    ///     // and `runtime`.
+    ///     const_eval_select((), compiletime, runtime)
     /// }
+    /// # #[cfg(bootstrap)]
+    /// # pub const fn inconsistent() -> i32 { 0 }
     ///
-    /// // Crate B
+    /// // User Crate
     /// const X: i32 = inconsistent();
     /// let x = inconsistent();
-    /// if x != X { unsafe { unreachable_unchecked(); }}
+    /// assert_eq!(x, X);
     /// ```
     ///
-    /// This code causes Undefined Behavior when being run, since the
-    /// `unreachable_unchecked` is actually being reached. The bug is in *crate A*,
-    /// which violates the principle that a `const fn` must behave the same at
-    /// compile-time and at run-time. The unsafe code in crate B is fine.
+    /// Currently such an assertion would always succeed; until Rust decides
+    /// otherwise, that principle should not be violated.
     #[rustc_const_unstable(feature = "const_eval_select", issue = "none")]
+    #[cfg_attr(not(bootstrap), rustc_safe_intrinsic)]
     pub fn const_eval_select<ARG: Tuple, F, G, RET>(
         arg: ARG,
         called_in_const: F,

Diff for: core/src/num/f32.rs

@@ -1153,8 +1153,11 @@ impl f32 {
             // Stability concerns.
             unsafe { mem::transmute(x) }
         }
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: We use internal implementations that either always work or fail at compile time.
-        unsafe { intrinsics::const_eval_select((self,), ct_f32_to_u32, rt_f32_to_u32) }
+        unsafe {
+            intrinsics::const_eval_select((self,), ct_f32_to_u32, rt_f32_to_u32)
+        }
     }
 
     /// Raw transmutation from `u32`.
@@ -1245,8 +1248,11 @@ impl f32 {
             // Stability concerns.
             unsafe { mem::transmute(x) }
         }
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: We use internal implementations that either always work or fail at compile time.
-        unsafe { intrinsics::const_eval_select((v,), ct_u32_to_f32, rt_u32_to_f32) }
+        unsafe {
+            intrinsics::const_eval_select((v,), ct_u32_to_f32, rt_u32_to_f32)
+        }
     }
 
     /// Return the memory representation of this floating point number as a byte array in

Diff for: core/src/num/f64.rs

@@ -1146,8 +1146,11 @@ impl f64 {
             // Stability concerns.
             unsafe { mem::transmute::<f64, u64>(rt) }
         }
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: We use internal implementations that either always work or fail at compile time.
-        unsafe { intrinsics::const_eval_select((self,), ct_f64_to_u64, rt_f64_to_u64) }
+        unsafe {
+            intrinsics::const_eval_select((self,), ct_f64_to_u64, rt_f64_to_u64)
+        }
     }
 
     /// Raw transmutation from `u64`.
@@ -1243,8 +1246,11 @@ impl f64 {
             // Stability concerns.
             unsafe { mem::transmute::<u64, f64>(rt) }
         }
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: We use internal implementations that either always work or fail at compile time.
-        unsafe { intrinsics::const_eval_select((v,), ct_u64_to_f64, rt_u64_to_f64) }
+        unsafe {
+            intrinsics::const_eval_select((v,), ct_u64_to_f64, rt_u64_to_f64)
+        }
     }
 
     /// Return the memory representation of this floating point number as a byte array in

Diff for: core/src/panicking.rs

@@ -117,6 +117,7 @@ pub const fn panic_nounwind_fmt(fmt: fmt::Arguments<'_>, force_no_backtrace: boo
         panic_fmt(fmt);
     }
 
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: const panic does not care about unwinding
     unsafe {
         super::intrinsics::const_eval_select((fmt, force_no_backtrace), comptime, runtime);

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

@@ -48,8 +48,11 @@ impl<T: ?Sized> *const T {
             }
         }
 
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The two versions are equivalent at runtime.
-        unsafe { const_eval_select((self as *const u8,), const_impl, runtime_impl) }
+        unsafe {
+            const_eval_select((self as *const u8,), const_impl, runtime_impl)
+        }
     }
 
     /// Casts to a pointer of another type.
@@ -806,6 +809,7 @@ impl<T: ?Sized> *const T {
     where
         T: Sized,
     {
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The comparison has no side-effects, and the intrinsic
         // does this check internally in the CTFE implementation.
         unsafe {
@@ -1623,8 +1627,11 @@ impl<T: ?Sized> *const T {
             ptr.align_offset(align) == 0
         }
 
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The two versions are equivalent at runtime.
-        unsafe { const_eval_select((self.cast::<()>(), align), const_impl, runtime_impl) }
+        unsafe {
+            const_eval_select((self.cast::<()>(), align), const_impl, runtime_impl)
+        }
     }
 }
 

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

@@ -995,6 +995,7 @@ pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
         };
     }
 
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: the caller must guarantee that `x` and `y` are
     // valid for writes and properly aligned.
     unsafe {

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

@@ -48,8 +48,11 @@ impl<T: ?Sized> *mut T {
             }
         }
 
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The two versions are equivalent at runtime.
-        unsafe { const_eval_select((self as *mut u8,), const_impl, runtime_impl) }
+        unsafe {
+            const_eval_select((self as *mut u8,), const_impl, runtime_impl)
+        }
     }
 
     /// Casts to a pointer of another type.
@@ -1896,8 +1899,11 @@ impl<T: ?Sized> *mut T {
             ptr.align_offset(align) == 0
         }
 
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The two versions are equivalent at runtime.
-        unsafe { const_eval_select((self.cast::<()>(), align), const_impl, runtime_impl) }
+        unsafe {
+            const_eval_select((self.cast::<()>(), align), const_impl, runtime_impl)
+        }
     }
 }
 

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

@@ -35,6 +35,7 @@ where
 #[track_caller]
 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: we are just panicking here
     unsafe {
         const_eval_select(
@@ -63,6 +64,7 @@ const fn slice_start_index_len_fail_ct(_: usize, _: usize) -> ! {
 #[track_caller]
 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: we are just panicking here
     unsafe {
         const_eval_select((index, len), slice_end_index_len_fail_ct, slice_end_index_len_fail_rt)
@@ -87,8 +89,11 @@ const fn slice_end_index_len_fail_ct(_: usize, _: usize) -> ! {
 #[track_caller]
 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_index_order_fail(index: usize, end: usize) -> ! {
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: we are just panicking here
-    unsafe { const_eval_select((index, end), slice_index_order_fail_ct, slice_index_order_fail_rt) }
+    unsafe {
+        const_eval_select((index, end), slice_index_order_fail_ct, slice_index_order_fail_rt)
+    }
 }
 
 // FIXME const-hack

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

@@ -86,6 +86,7 @@ use iter::{MatchesInternal, SplitNInternal};
 #[rustc_allow_const_fn_unstable(const_eval_select)]
 #[cfg(not(feature = "panic_immediate_abort"))]
 const fn slice_error_fail(s: &str, begin: usize, end: usize) -> ! {
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: panics for both branches
     unsafe {
         crate::intrinsics::const_eval_select(