add const_eval_select macro to reduce redundancy

also move internal const_panic helpers to a better location

library/core/src/char/methods.rs

@@ -1,7 +1,7 @@
 //! impl char {}
 
 use super::*;
-use crate::macros::const_panic;
+use crate::panic::const_panic;
 use crate::slice;
 use crate::str::from_utf8_unchecked_mut;
 use crate::unicode::printable::is_printable;

library/core/src/ffi/c_str.rs

@@ -3,11 +3,12 @@
 use crate::cmp::Ordering;
 use crate::error::Error;
 use crate::ffi::c_char;
+use crate::intrinsics::const_eval_select;
 use crate::iter::FusedIterator;
 use crate::marker::PhantomData;
 use crate::ptr::NonNull;
 use crate::slice::memchr;
-use crate::{fmt, intrinsics, ops, slice, str};
+use crate::{fmt, ops, slice, str};
 
 // FIXME: because this is doc(inline)d, we *have* to use intra-doc links because the actual link
 //   depends on where the item is being documented. however, since this is libcore, we can't
@@ -411,20 +412,9 @@ pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, FromBytesWithNul
     #[rustc_const_stable(feature = "const_cstr_unchecked", since = "1.59.0")]
     #[rustc_allow_const_fn_unstable(const_eval_select)]
     pub const unsafe fn from_bytes_with_nul_unchecked(bytes: &[u8]) -> &CStr {
-        #[inline]
+        const_eval_select!(
-        fn rt_impl(bytes: &[u8]) -> &CStr {
+            @capture { bytes: &[u8] } -> &CStr:
-            // Chance at catching some UB at runtime with debug builds.
+            if const {
-            debug_assert!(!bytes.is_empty() && bytes[bytes.len() - 1] == 0);
-
-            // SAFETY: Casting to CStr is safe because its internal representation
-            // is a [u8] too (safe only inside std).
-            // Dereferencing the obtained pointer is safe because it comes from a
-            // reference. Making a reference is then safe because its lifetime
-            // is bound by the lifetime of the given `bytes`.
-            unsafe { &*(bytes as *const [u8] as *const CStr) }
-        }
-
-        const fn const_impl(bytes: &[u8]) -> &CStr {
                 // Saturating so that an empty slice panics in the assert with a good
                 // message, not here due to underflow.
                 let mut i = bytes.len().saturating_sub(1);
@@ -437,11 +427,20 @@ const fn const_impl(bytes: &[u8]) -> &CStr {
                     assert!(byte != 0, "input contained interior nul");
                 }
 
-            // SAFETY: See `rt_impl` cast.
+                // SAFETY: See runtime cast comment below.
+                unsafe { &*(bytes as *const [u8] as *const CStr) }
+            } else {
+                // Chance at catching some UB at runtime with debug builds.
+                debug_assert!(!bytes.is_empty() && bytes[bytes.len() - 1] == 0);
+
+                // SAFETY: Casting to CStr is safe because its internal representation
+                // is a [u8] too (safe only inside std).
+                // Dereferencing the obtained pointer is safe because it comes from a
+                // reference. Making a reference is then safe because its lifetime
+                // is bound by the lifetime of the given `bytes`.
                 unsafe { &*(bytes as *const [u8] as *const CStr) }
             }
-
+        )
-        intrinsics::const_eval_select((bytes,), const_impl, rt_impl)
     }
 
     /// Returns the inner pointer to this C string.
@@ -735,7 +734,9 @@ fn as_ref(&self) -> &CStr {
 #[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 {
+    const_eval_select!(
+        @capture { s: *const c_char = ptr } -> usize:
+        if const {
             let mut len = 0;
 
             // SAFETY: Outer caller has provided a pointer to a valid C string.
@@ -744,10 +745,7 @@ const fn strlen_ct(s: *const c_char) -> usize {
             }
 
             len
-    }
+        } else {
-
-    #[inline]
-    fn strlen_rt(s: *const c_char) -> usize {
             extern "C" {
                 /// Provided by libc or compiler_builtins.
                 fn strlen(s: *const c_char) -> usize;
@@ -756,8 +754,7 @@ fn strlen_rt(s: *const c_char) -> usize {
             // SAFETY: Outer caller has provided a pointer to a valid C string.
             unsafe { strlen(s) }
         }
-
+    )
-    intrinsics::const_eval_select((ptr,), strlen_ct, strlen_rt)
 }
 
 /// An iterator over the bytes of a [`CStr`], without the nul terminator.

library/core/src/intrinsics.rs

@@ -2788,6 +2788,68 @@ pub const fn const_eval_select<ARG: Tuple, F, G, RET>(
     unreachable!()
 }
 
+/// A macro to make it easier to invoke const_eval_select. Use as follows:
+/// ```rust,ignore (just a macro example)
+/// const_eval_select!(
+///     @capture { arg1: i32 = some_expr, arg2: T = other_expr } -> U:
+///     if const #[attributes_for_const_arm] {
+///         // Compile-time code goes here.
+///     } else #[attributes_for_runtime_arm] {
+///         // Run-time code goes here.
+///     }
+/// )
+/// ```
+/// The `@capture` block declares which surrounding variables / expressions can be
+/// used inside the `if const`.
+/// Note that the two arms of this `if` really each become their own function, which is why the
+/// macro supports setting attributes for those functions. The runtime function is always
+/// markes as `#[inline]`.
+///
+/// See [`const_eval_select()`] for the rules and requirements around that intrinsic.
+pub(crate) macro const_eval_select {
+    (
+        @capture { $($arg:ident : $ty:ty = $val:expr),* $(,)? } $( -> $ret:ty )? :
+        if const
+            $(#[$compiletime_attr:meta])* $compiletime:block
+        else
+            $(#[$runtime_attr:meta])* $runtime:block
+    ) => {{
+        #[inline] // avoid the overhead of an extra fn call
+        $(#[$runtime_attr])*
+        fn runtime($($arg: $ty),*) $( -> $ret )? {
+            $runtime
+        }
+
+        #[inline] // prevent codegen on this function
+        $(#[$compiletime_attr])*
+        const fn compiletime($($arg: $ty),*) $( -> $ret )? {
+            // Don't warn if one of the arguments is unused.
+            $(let _ = $arg;)*
+
+            $compiletime
+        }
+
+        const_eval_select(($($val,)*), compiletime, runtime)
+    }},
+    // We support leaving away the `val` expressions for *all* arguments
+    // (but not for *some* arguments, that's too tricky).
+    (
+        @capture { $($arg:ident : $ty:ty),* $(,)? } $( -> $ret:ty )? :
+        if const
+            $(#[$compiletime_attr:meta])* $compiletime:block
+        else
+            $(#[$runtime_attr:meta])* $runtime:block
+    ) => {
+        $crate::intrinsics::const_eval_select!(
+            @capture { $($arg : $ty = $arg),* } $(-> $ret)? :
+            if const
+                $(#[$compiletime_attr])* $compiletime
+            else
+                $(#[$runtime_attr])* $runtime
+        )
+    },
+}
+
 /// Returns whether the argument's value is statically known at
 /// compile-time.
 ///
@@ -2830,7 +2892,7 @@ pub const fn const_eval_select<ARG: Tuple, F, G, RET>(
 /// # Stability concerns
 ///
 /// While it is safe to call, this intrinsic may behave differently in
-/// a `const` context than otherwise. See the [`const_eval_select`]
+/// a `const` context than otherwise. See the [`const_eval_select()`]
 /// documentation for an explanation of the issues this can cause. Unlike
 /// `const_eval_select`, this intrinsic isn't guaranteed to behave
 /// deterministically even in a `const` context.
@@ -3734,14 +3796,15 @@ pub(crate) const fn miri_promise_symbolic_alignment(ptr: *const (), align: usize
         fn miri_promise_symbolic_alignment(ptr: *const (), align: usize);
     }
 
-    fn runtime(ptr: *const (), align: usize) {
+    const_eval_select!(
+        @capture { ptr: *const (), align: usize}:
+        if const {
+            // Do nothing.
+        } else {
             // SAFETY: this call is always safe.
             unsafe {
                 miri_promise_symbolic_alignment(ptr, align);
             }
         }
-
+    )
-    const fn compiletime(_ptr: *const (), _align: usize) {}
-
-    const_eval_select((ptr, align), compiletime, runtime);
 }

library/core/src/macros/mod.rs

@@ -12,54 +12,6 @@ macro_rules! panic {
     };
 }
 
-/// Helper macro for panicking in a `const fn`.
-/// Invoke as:
-/// ```rust,ignore (just an example)
-/// core::macros::const_panic!("boring message", "flavored message {a} {b:?}", a: u32 = foo.len(), b: Something = bar);
-/// ```
-/// where the first message will be printed in const-eval,
-/// and the second message will be printed at runtime.
-// All uses of this macro are FIXME(const-hack).
-#[unstable(feature = "panic_internals", issue = "none")]
-#[doc(hidden)]
-pub macro const_panic {
-    ($const_msg:literal, $runtime_msg:literal, $($arg:ident : $ty:ty = $val:expr),* $(,)?) => {{
-        #[inline]
-        #[track_caller]
-        fn runtime($($arg: $ty),*) -> ! {
-            $crate::panic!($runtime_msg);
-        }
-
-        #[inline]
-        #[track_caller]
-        const fn compiletime($(_: $ty),*) -> ! {
-            $crate::panic!($const_msg);
-        }
-
-        // Wrap call to `const_eval_select` in a function so that we can
-        // add the `rustc_allow_const_fn_unstable`. This is okay to do
-        // because both variants will panic, just with different messages.
-        #[rustc_allow_const_fn_unstable(const_eval_select)]
-        #[inline(always)]
-        #[track_caller]
-        #[cfg_attr(bootstrap, rustc_const_stable(feature = "const_panic", since = "CURRENT_RUSTC_VERSION"))]
-        const fn do_panic($($arg: $ty),*) -> ! {
-            $crate::intrinsics::const_eval_select(($($arg),* ,), compiletime, runtime)
-        }
-
-        do_panic($($val),*)
-    }},
-    // We support leaving away the `val` expressions for *all* arguments
-    // (but not for *some* arguments, that's too tricky).
-    ($const_msg:literal, $runtime_msg:literal, $($arg:ident : $ty:ty),* $(,)?) => {
-        $crate::macros::const_panic!(
-            $const_msg,
-            $runtime_msg,
-            $($arg: $ty = $arg),*
-        )
-    },
-}
-
 /// Asserts that two expressions are equal to each other (using [`PartialEq`]).
 ///
 /// Assertions are always checked in both debug and release builds, and cannot
@@ -244,19 +196,6 @@ macro_rules! assert_ne {
     },
 }
 
-/// A version of `assert` that prints a non-formatting message in const contexts.
-///
-/// See [`const_panic!`].
-#[unstable(feature = "panic_internals", issue = "none")]
-#[doc(hidden)]
-pub macro const_assert {
-    ($condition: expr, $const_msg:literal, $runtime_msg:literal, $($arg:tt)*) => {{
-        if !$crate::intrinsics::likely($condition) {
-            $crate::macros::const_panic!($const_msg, $runtime_msg, $($arg)*)
-        }
-    }}
-}
-
 /// A macro for defining `#[cfg]` match-like statements.
 ///
 /// It is similar to the `if/elif` C preprocessor macro by allowing definition of a cascade of

library/core/src/num/f128.rs

@@ -14,9 +14,9 @@
 use crate::convert::FloatToInt;
 #[cfg(not(test))]
 use crate::intrinsics;
-use crate::macros::const_assert;
 use crate::mem;
 use crate::num::FpCategory;
+use crate::panic::const_assert;
 
 /// Basic mathematical constants.
 #[unstable(feature = "f128", issue = "116909")]

library/core/src/num/f16.rs

@@ -14,9 +14,9 @@
 use crate::convert::FloatToInt;
 #[cfg(not(test))]
 use crate::intrinsics;
-use crate::macros::const_assert;
 use crate::mem;
 use crate::num::FpCategory;
+use crate::panic::const_assert;
 
 /// Basic mathematical constants.
 #[unstable(feature = "f16", issue = "116909")]

library/core/src/num/f32.rs

@@ -14,9 +14,9 @@
 use crate::convert::FloatToInt;
 #[cfg(not(test))]
 use crate::intrinsics;
-use crate::macros::const_assert;
 use crate::mem;
 use crate::num::FpCategory;
+use crate::panic::const_assert;
 
 /// The radix or base of the internal representation of `f32`.
 /// Use [`f32::RADIX`] instead.

library/core/src/num/f64.rs

@@ -14,9 +14,9 @@
 use crate::convert::FloatToInt;
 #[cfg(not(test))]
 use crate::intrinsics;
-use crate::macros::const_assert;
 use crate::mem;
 use crate::num::FpCategory;
+use crate::panic::const_assert;
 
 /// The radix or base of the internal representation of `f64`.
 /// Use [`f64::RADIX`] instead.

library/core/src/num/mod.rs

@@ -2,7 +2,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use crate::macros::const_panic;
+use crate::panic::const_panic;
 use crate::str::FromStr;
 use crate::ub_checks::assert_unsafe_precondition;
 use crate::{ascii, intrinsics, mem};

library/core/src/panic.rs

@@ -189,3 +189,59 @@ fn as_str(&mut self) -> Option<&str> {
         None
     }
 }
+
+/// Helper macro for panicking in a `const fn`.
+/// Invoke as:
+/// ```rust,ignore (just an example)
+/// core::macros::const_panic!("boring message", "flavored message {a} {b:?}", a: u32 = foo.len(), b: Something = bar);
+/// ```
+/// where the first message will be printed in const-eval,
+/// and the second message will be printed at runtime.
+// All uses of this macro are FIXME(const-hack).
+#[unstable(feature = "panic_internals", issue = "none")]
+#[doc(hidden)]
+pub macro const_panic {
+    ($const_msg:literal, $runtime_msg:literal, $($arg:ident : $ty:ty = $val:expr),* $(,)?) => {{
+        // Wrap call to `const_eval_select` in a function so that we can
+        // add the `rustc_allow_const_fn_unstable`. This is okay to do
+        // because both variants will panic, just with different messages.
+        #[rustc_allow_const_fn_unstable(const_eval_select)]
+        #[inline(always)]
+        #[track_caller]
+        #[cfg_attr(bootstrap, rustc_const_stable(feature = "const_panic", since = "CURRENT_RUSTC_VERSION"))]
+        const fn do_panic($($arg: $ty),*) -> ! {
+            $crate::intrinsics::const_eval_select!(
+                @capture { $($arg: $ty),* } -> !:
+                if const #[track_caller] {
+                    $crate::panic!($const_msg)
+                } else #[track_caller] {
+                    $crate::panic!($runtime_msg)
+                }
+            )
+        }
+
+        do_panic($($val),*)
+    }},
+    // We support leaving away the `val` expressions for *all* arguments
+    // (but not for *some* arguments, that's too tricky).
+    ($const_msg:literal, $runtime_msg:literal, $($arg:ident : $ty:ty),* $(,)?) => {
+        $crate::panic::const_panic!(
+            $const_msg,
+            $runtime_msg,
+            $($arg: $ty = $arg),*
+        )
+    },
+}
+
+/// A version of `assert` that prints a non-formatting message in const contexts.
+///
+/// See [`const_panic!`].
+#[unstable(feature = "panic_internals", issue = "none")]
+#[doc(hidden)]
+pub macro const_assert {
+    ($condition: expr, $const_msg:literal, $runtime_msg:literal, $($arg:tt)*) => {{
+        if !$crate::intrinsics::likely($condition) {
+            $crate::panic::const_panic!($const_msg, $runtime_msg, $($arg)*)
+        }
+    }}
+}

library/core/src/panicking.rs

@@ -29,6 +29,7 @@
 )]
 
 use crate::fmt;
+use crate::intrinsics::const_eval_select;
 use crate::panic::{Location, PanicInfo};
 
 #[cfg(feature = "panic_immediate_abort")]
@@ -89,9 +90,12 @@ pub const fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
 #[cfg_attr(not(bootstrap), rustc_const_stable_indirect)] // must follow stable const rules since it is exposed to stable
 #[rustc_allow_const_fn_unstable(const_eval_select)]
 pub const fn panic_nounwind_fmt(fmt: fmt::Arguments<'_>, force_no_backtrace: bool) -> ! {
-    #[inline] // this should always be inlined into `panic_nounwind_fmt`
+    const_eval_select!(
-    #[track_caller]
+        @capture { fmt: fmt::Arguments<'_>, force_no_backtrace: bool } -> !:
-    fn runtime(fmt: fmt::Arguments<'_>, force_no_backtrace: bool) -> ! {
+        if const #[track_caller] {
+            // We don't unwind anyway at compile-time so we can call the regular `panic_fmt`.
+            panic_fmt(fmt)
+        } else #[track_caller] {
             if cfg!(feature = "panic_immediate_abort") {
                 super::intrinsics::abort()
             }
@@ -114,15 +118,7 @@ fn runtime(fmt: fmt::Arguments<'_>, force_no_backtrace: bool) -> ! {
             // SAFETY: `panic_impl` is defined in safe Rust code and thus is safe to call.
             unsafe { panic_impl(&pi) }
         }
-
+    )
-    #[inline]
-    #[track_caller]
-    const fn comptime(fmt: fmt::Arguments<'_>, _force_no_backtrace: bool) -> ! {
-        // We don't unwind anyway at compile-time so we can call the regular `panic_fmt`.
-        panic_fmt(fmt);
-    }
-
-    super::intrinsics::const_eval_select((fmt, force_no_backtrace), comptime, runtime);
 }
 
 // Next we define a bunch of higher-level wrappers that all bottom out in the two core functions

library/core/src/ptr/const_ptr.rs

@@ -33,14 +33,12 @@ impl<T: ?Sized> *const T {
     #[rustc_diagnostic_item = "ptr_const_is_null"]
     #[inline]
     pub const fn is_null(self) -> bool {
-        #[inline]
+        // Compare via a cast to a thin pointer, so fat pointers are only
-        fn runtime_impl(ptr: *const u8) -> bool {
+        // considering their "data" part for null-ness.
-            ptr.addr() == 0
+        let ptr = self as *const u8;
-        }
+        const_eval_select!(
-
+            @capture { ptr: *const u8 } -> bool:
-        #[inline]
+            if const #[rustc_const_unstable(feature = "const_ptr_is_null", issue = "74939")] {
-        #[rustc_const_unstable(feature = "const_ptr_is_null", issue = "74939")]
-        const fn const_impl(ptr: *const u8) -> bool {
                 match (ptr).guaranteed_eq(null_mut()) {
                     Some(res) => res,
                     // To remain maximally convervative, we stop execution when we don't
@@ -48,11 +46,10 @@ const fn const_impl(ptr: *const u8) -> bool {
                     // We can *not* return `false` here, that would be unsound in `NonNull::new`!
                     None => panic!("null-ness of this pointer cannot be determined in const context"),
                 }
+            } else {
+                ptr.addr() == 0
             }
-
+        )
-        // Compare via a cast to a thin pointer, so fat pointers are only
-        // considering their "data" part for null-ness.
-        const_eval_select((self as *const u8,), const_impl, runtime_impl)
     }
 
     /// Casts to a pointer of another type.
@@ -410,22 +407,21 @@ pub const fn to_raw_parts(self) -> (*const (), <T as super::Pointee>::Metadata)
         #[inline]
         #[rustc_allow_const_fn_unstable(const_eval_select)]
         const fn runtime_offset_nowrap(this: *const (), count: isize, size: usize) -> bool {
-            #[inline]
+            // We can use const_eval_select here because this is only for UB checks.
-            fn runtime(this: *const (), count: isize, size: usize) -> bool {
+            const_eval_select!(
-                // We know `size <= isize::MAX` so the `as` cast here is not lossy.
+                @capture { this: *const (), count: isize, size: usize } -> bool:
+                if const {
+                    true
+                } else {
+                    // `size` is the size of a Rust type, so we know that
+                    // `size <= isize::MAX` and thus `as` cast here is not lossy.
                     let Some(byte_offset) = count.checked_mul(size as isize) else {
                         return false;
                     };
                     let (_, overflow) = this.addr().overflowing_add_signed(byte_offset);
                     !overflow
                 }
-
+            )
-            const fn comptime(_: *const (), _: isize, _: usize) -> bool {
-                true
-            }
-
-            // We can use const_eval_select here because this is only for UB checks.
-            intrinsics::const_eval_select((this, count, size), comptime, runtime)
         }
 
         ub_checks::assert_unsafe_precondition!(
@@ -763,14 +759,14 @@ pub fn mask(self, mask: usize) -> *const T {
     {
         #[rustc_allow_const_fn_unstable(const_eval_select)]
         const fn runtime_ptr_ge(this: *const (), origin: *const ()) -> bool {
-            fn runtime(this: *const (), origin: *const ()) -> bool {
+            const_eval_select!(
+                @capture { this: *const (), origin: *const () } -> bool:
+                if const {
+                    true
+                } else {
                     this >= origin
                 }
-            const fn comptime(_: *const (), _: *const ()) -> bool {
+            )
-                true
-            }
-
-            intrinsics::const_eval_select((this, origin), comptime, runtime)
         }
 
         ub_checks::assert_unsafe_precondition!(
@@ -924,20 +920,18 @@ pub const fn guaranteed_ne(self, other: *const T) -> Option<bool>
         #[inline]
         #[rustc_allow_const_fn_unstable(const_eval_select)]
         const fn runtime_add_nowrap(this: *const (), count: usize, size: usize) -> bool {
-            #[inline]
+            const_eval_select!(
-            fn runtime(this: *const (), count: usize, size: usize) -> bool {
+                @capture { this: *const (), count: usize, size: usize } -> bool:
+                if const {
+                    true
+                } else {
                     let Some(byte_offset) = count.checked_mul(size) else {
                         return false;
                     };
                     let (_, overflow) = this.addr().overflowing_add(byte_offset);
                     byte_offset <= (isize::MAX as usize) && !overflow
                 }
-
+            )
-            const fn comptime(_: *const (), _: usize, _: usize) -> bool {
-                true
-            }
-
-            intrinsics::const_eval_select((this, count, size), comptime, runtime)
         }
 
         #[cfg(debug_assertions)] // Expensive, and doesn't catch much in the wild.
@@ -1033,19 +1027,17 @@ const fn comptime(_: *const (), _: usize, _: usize) -> bool {
         #[inline]
         #[rustc_allow_const_fn_unstable(const_eval_select)]
         const fn runtime_sub_nowrap(this: *const (), count: usize, size: usize) -> bool {
-            #[inline]
+            const_eval_select!(
-            fn runtime(this: *const (), count: usize, size: usize) -> bool {
+                @capture { this: *const (), count: usize, size: usize } -> bool:
+                if const {
+                    true
+                } else {
                     let Some(byte_offset) = count.checked_mul(size) else {
                         return false;
                     };
                     byte_offset <= (isize::MAX as usize) && this.addr() >= byte_offset
                 }
-
+            )
-            const fn comptime(_: *const (), _: usize, _: usize) -> bool {
-                true
-            }
-
-            intrinsics::const_eval_select((this, count, size), comptime, runtime)
         }
 
         #[cfg(debug_assertions)] // Expensive, and doesn't catch much in the wild.

library/core/src/ptr/mut_ptr.rs

@@ -1,5 +1,6 @@
 use super::*;
 use crate::cmp::Ordering::{Equal, Greater, Less};
+use crate::intrinsics::const_eval_select;
 use crate::mem::SizedTypeProperties;
 use crate::slice::{self, SliceIndex};
 
@@ -404,8 +405,12 @@ pub const fn to_raw_parts(self) -> (*mut (), <T as super::Pointee>::Metadata) {
         #[inline]
         #[rustc_allow_const_fn_unstable(const_eval_select)]
         const fn runtime_offset_nowrap(this: *const (), count: isize, size: usize) -> bool {
-            #[inline]
+            // We can use const_eval_select here because this is only for UB checks.
-            fn runtime(this: *const (), count: isize, size: usize) -> bool {
+            const_eval_select!(
+                @capture { this: *const (), count: isize, size: usize } -> bool:
+                if const {
+                    true
+                } else {
                     // `size` is the size of a Rust type, so we know that
                     // `size <= isize::MAX` and thus `as` cast here is not lossy.
                     let Some(byte_offset) = count.checked_mul(size as isize) else {
@@ -414,13 +419,7 @@ fn runtime(this: *const (), count: isize, size: usize) -> bool {
                     let (_, overflow) = this.addr().overflowing_add_signed(byte_offset);
                     !overflow
                 }
-
+            )
-            const fn comptime(_: *const (), _: isize, _: usize) -> bool {
-                true
-            }
-
-            // We can use const_eval_select here because this is only for UB checks.
-            intrinsics::const_eval_select((this, count, size), comptime, runtime)
         }
 
         ub_checks::assert_unsafe_precondition!(
@@ -1002,20 +1001,18 @@ pub const fn guaranteed_ne(self, other: *mut T) -> Option<bool>
         #[inline]
         #[rustc_allow_const_fn_unstable(const_eval_select)]
         const fn runtime_add_nowrap(this: *const (), count: usize, size: usize) -> bool {
-            #[inline]
+            const_eval_select!(
-            fn runtime(this: *const (), count: usize, size: usize) -> bool {
+                @capture { this: *const (), count: usize, size: usize } -> bool:
+                if const {
+                    true
+                } else {
                     let Some(byte_offset) = count.checked_mul(size) else {
                         return false;
                     };
                     let (_, overflow) = this.addr().overflowing_add(byte_offset);
                     byte_offset <= (isize::MAX as usize) && !overflow
                 }
-
+            )
-            const fn comptime(_: *const (), _: usize, _: usize) -> bool {
-                true
-            }
-
-            intrinsics::const_eval_select((this, count, size), comptime, runtime)
         }
 
         #[cfg(debug_assertions)] // Expensive, and doesn't catch much in the wild.
@@ -1111,19 +1108,17 @@ const fn comptime(_: *const (), _: usize, _: usize) -> bool {
         #[inline]
         #[rustc_allow_const_fn_unstable(const_eval_select)]
         const fn runtime_sub_nowrap(this: *const (), count: usize, size: usize) -> bool {
-            #[inline]
+            const_eval_select!(
-            fn runtime(this: *const (), count: usize, size: usize) -> bool {
+                @capture { this: *const (), count: usize, size: usize } -> bool:
+                if const {
+                    true
+                } else {
                     let Some(byte_offset) = count.checked_mul(size) else {
                         return false;
                     };
                     byte_offset <= (isize::MAX as usize) && this.addr() >= byte_offset
                 }
-
+            )
-            const fn comptime(_: *const (), _: usize, _: usize) -> bool {
-                true
-            }
-
-            intrinsics::const_eval_select((this, count, size), comptime, runtime)
         }
 
         #[cfg(debug_assertions)] // Expensive, and doesn't catch much in the wild.

library/core/src/slice/ascii.rs

@@ -351,14 +351,11 @@ pub const fn is_ascii_simple(mut bytes: &[u8]) -> bool {
 const fn is_ascii(s: &[u8]) -> bool {
     // The runtime version behaves the same as the compiletime version, it's
     // just more optimized.
-    return const_eval_select((s,), compiletime, runtime);
+    const_eval_select!(
-
+        @capture { s: &[u8] } -> bool:
-    const fn compiletime(s: &[u8]) -> bool {
+        if const {
             is_ascii_simple(s)
-    }
+        } else {
-
-    #[inline]
-    fn runtime(s: &[u8]) -> bool {
             const USIZE_SIZE: usize = mem::size_of::<usize>();
 
             let len = s.len();
@@ -436,4 +433,5 @@ fn runtime(s: &[u8]) -> bool {
 
             !contains_nonascii(last_word)
         }
+    )
 }

library/core/src/slice/index.rs

@@ -1,6 +1,6 @@
 //! Indexing implementations for `[T]`.
 
-use crate::macros::const_panic;
+use crate::panic::const_panic;
 use crate::ub_checks::assert_unsafe_precondition;
 use crate::{ops, range};
 

library/core/src/slice/memchr.rs

@@ -56,14 +56,11 @@ const fn memchr_naive(x: u8, text: &[u8]) -> Option<usize> {
 const fn memchr_aligned(x: u8, text: &[u8]) -> Option<usize> {
     // The runtime version behaves the same as the compiletime version, it's
     // just more optimized.
-    return const_eval_select((x, text), compiletime, runtime);
+    const_eval_select!(
-
+        @capture { x: u8, text: &[u8] } -> Option<usize>:
-    const fn compiletime(x: u8, text: &[u8]) -> Option<usize> {
+        if const {
             memchr_naive(x, text)
-    }
+        } else {
-
-    #[inline]
-    fn runtime(x: u8, text: &[u8]) -> Option<usize> {
             // Scan for a single byte value by reading two `usize` words at a time.
             //
             // Split `text` in three parts
@@ -111,6 +108,7 @@ fn runtime(x: u8, text: &[u8]) -> Option<usize> {
                 unsafe { super::from_raw_parts(text.as_ptr().add(offset), text.len() - offset) };
             if let Some(i) = memchr_naive(x, slice) { Some(offset + i) } else { None }
         }
+    )
 }
 
 /// Returns the last index matching the byte `x` in `text`.

library/core/src/str/validations.rs

@@ -132,19 +132,16 @@ pub(super) const fn run_utf8_validation(v: &[u8]) -> Result<(), Utf8Error> {
 
     let ascii_block_size = 2 * USIZE_BYTES;
     let blocks_end = if len >= ascii_block_size { len - ascii_block_size + 1 } else { 0 };
-    let align = {
-        const fn compiletime(_v: &[u8]) -> usize {
-            usize::MAX
-        }
-
-        fn runtime(v: &[u8]) -> usize {
-            v.as_ptr().align_offset(USIZE_BYTES)
-        }
-
     // Below, we safely fall back to a slower codepath if the offset is `usize::MAX`,
     // so the end-to-end behavior is the same at compiletime and runtime.
-        const_eval_select((v,), compiletime, runtime)
+    let align = const_eval_select!(
-    };
+        @capture { v: &[u8] } -> usize:
+        if const {
+            usize::MAX
+        } else {
+            v.as_ptr().align_offset(USIZE_BYTES)
+        }
+    );
 
     while index < len {
         let old_offset = index;

library/core/src/ub_checks.rs

@@ -95,20 +95,18 @@ const fn precondition_check($($name:$ty),*) {
 #[inline]
 #[rustc_allow_const_fn_unstable(const_eval_select)]
 pub(crate) const fn check_language_ub() -> bool {
-    #[inline]
+    // Only used for UB checks so we may const_eval_select.
-    fn runtime() -> bool {
+    intrinsics::ub_checks()
+        && const_eval_select!(
+            @capture { } -> bool:
+            if const {
+                // Always disable UB checks.
+                false
+            } else {
                 // Disable UB checks in Miri.
                 !cfg!(miri)
             }
-
+        )
-    #[inline]
-    const fn comptime() -> bool {
-        // Always disable UB checks.
-        false
-    }
-
-    // Only used for UB checks so we may const_eval_select.
-    intrinsics::ub_checks() && const_eval_select((), comptime, runtime)
 }
 
 /// Checks whether `ptr` is properly aligned with respect to the given alignment, and
@@ -120,19 +118,15 @@ const fn comptime() -> bool {
 #[inline]
 #[rustc_const_unstable(feature = "const_ub_checks", issue = "none")]
 pub(crate) const fn is_aligned_and_not_null(ptr: *const (), align: usize, is_zst: bool) -> bool {
-    #[inline]
+    // This is just for safety checks so we can const_eval_select.
-    fn runtime(ptr: *const (), align: usize, is_zst: bool) -> bool {
+    const_eval_select!(
+        @capture { ptr: *const (), align: usize, is_zst: bool } -> bool:
+        if const #[rustc_const_unstable(feature = "const_ub_checks", issue = "none")] {
+            is_zst || !ptr.is_null()
+        } else {
             ptr.is_aligned_to(align) && (is_zst || !ptr.is_null())
         }
-
+    )
-    #[inline]
-    #[rustc_const_unstable(feature = "const_ub_checks", issue = "none")]
-    const fn comptime(ptr: *const (), _align: usize, is_zst: bool) -> bool {
-        is_zst || !ptr.is_null()
-    }
-
-    // This is just for safety checks so we can const_eval_select.
-    const_eval_select((ptr, align, is_zst), comptime, runtime)
 }
 
 #[inline]
@@ -154,8 +148,12 @@ pub(crate) const fn is_nonoverlapping(
     size: usize,
     count: usize,
 ) -> bool {
-    #[inline]
+    // This is just for safety checks so we can const_eval_select.
-    fn runtime(src: *const (), dst: *const (), size: usize, count: usize) -> bool {
+    const_eval_select!(
+        @capture { src: *const (), dst: *const (), size: usize, count: usize } -> bool:
+        if const {
+            true
+        } else {
             let src_usize = src.addr();
             let dst_usize = dst.addr();
             let Some(size) = size.checked_mul(count) else {
@@ -168,12 +166,5 @@ fn runtime(src: *const (), dst: *const (), size: usize, count: usize) -> bool {
             // they do not overlap.
             diff >= size
         }
-
+    )
-    #[inline]
-    const fn comptime(_: *const (), _: *const (), _: usize, _: usize) -> bool {
-        true
-    }
-
-    // This is just for safety checks so we can const_eval_select.
-    const_eval_select((src, dst, size, count), comptime, runtime)
 }

tests/ui/consts/const-ptr-is-null.stderr

@@ -3,7 +3,7 @@ error[E0080]: evaluation of constant value failed
    |
    = note: the evaluated program panicked at 'null-ness of this pointer cannot be determined in const context', $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
    |
-note: inside `std::ptr::const_ptr::<impl *const T>::is_null::const_impl`
+note: inside `std::ptr::const_ptr::<impl *const T>::is_null::compiletime`
   --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
 note: inside `std::ptr::const_ptr::<impl *const i32>::is_null`
   --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
@@ -12,7 +12,7 @@ note: inside `MAYBE_NULL`
    |
 LL |     assert!(!ptr.wrapping_sub(512).is_null());
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+   = note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `const_eval_select` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 1 previous error