add const_eval_select macro to reduce redundancy

Author: RalfJung

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
@@ -411,37 +412,35 @@ impl CStr {
     #[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]
-        fn rt_impl(bytes: &[u8]) -> &CStr {
-            // 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) }
-        }
-
-        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);
-            assert!(!bytes.is_empty() && bytes[i] == 0, "input was not nul-terminated");
-
-            // Ending nul byte exists, skip to the rest.
-            while i != 0 {
-                i -= 1;
-                let byte = bytes[i];
-                assert!(byte != 0, "input contained interior nul");
+        const_eval_select!(
+            (bytes: &[u8]) -> &CStr:
+            if const {
+                // 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);
+                assert!(!bytes.is_empty() && bytes[i] == 0, "input was not nul-terminated");
+
+                // Ending nul byte exists, skip to the rest.
+                while i != 0 {
+                    i -= 1;
+                    let byte = bytes[i];
+                    assert!(byte != 0, "input contained interior nul");
+                }
+
+                // SAFETY: See runtime cast comment below.
+                unsafe { &*(bytes as *const [u8] as *const CStr) }
+            } else #[inline] {
+                // 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) }
             }
-
-            // SAFETY: See `rt_impl` cast.
-            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.
@@ -735,29 +734,27 @@ impl AsRef<CStr> for 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 {
-        let mut len = 0;
-
-        // SAFETY: Outer caller has provided a pointer to a valid C string.
-        while unsafe { *s.add(len) } != 0 {
-            len += 1;
-        }
+    const_eval_select!(
+        (s: *const c_char = ptr) -> usize:
+        if const {
+            let mut len = 0;
+
+            // SAFETY: Outer caller has provided a pointer to a valid C string.
+            while unsafe { *s.add(len) } != 0 {
+                len += 1;
+            }
 
-        len
-    }
+            len
+        } else #[inline] {
+            extern "C" {
+                /// Provided by libc or compiler_builtins.
+                fn strlen(s: *const c_char) -> usize;
+            }
 
-    #[inline]
-    fn strlen_rt(s: *const c_char) -> usize {
-        extern "C" {
-            /// Provided by libc or compiler_builtins.
-            fn strlen(s: *const c_char) -> usize;
+            // SAFETY: Outer caller has provided a pointer to a valid C string.
+            unsafe { strlen(s) }
         }
-
-        // 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 +2788,65 @@ where
     unreachable!()
 }
 
+/// A macro to make it easier to invoke const_eval_select. Use as follows:
+/// ```rust,ignore (just a macro example)
+/// const_eval_select!(
+///     #[inline]
+///     (arg1: i32 = some_expr, arg2: T = other_expr) -> U:
+///     if const {
+///         // Compile-time code goes here.
+///     } else {
+///         // Run-time code goes here.
+///     }
+/// )
+/// ```
+pub(crate) macro const_eval_select {
+    (
+        $(#[$attr:meta])*
+        ($($arg:ident : $ty:ty = $val:expr),* $(,)?) -> $ret:ty:
+        if const
+            $(#[$compiletime_attr:meta])* $compiletime:block
+        else
+            $(#[$runtime_attr:meta])* $runtime:block
+    ) => {{
+        $(#[$attr])*
+        $(#[$runtime_attr])*
+        fn runtime($($arg: $ty),*) -> $ret {
+            $runtime
+        }
+
+        $(#[$attr])*
+        $(#[$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).
+    (
+        $(#[$attr:meta])*
+        ($($arg:ident : $ty:ty),* $(,)?) -> $ret:ty:
+        if const
+            $(#[$compiletime_attr:meta])* $compiletime:block
+        else
+            $(#[$runtime_attr:meta])* $runtime:block
+    ) => {
+        $crate::intrinsics::const_eval_select!(
+            $(#[$attr])*
+            ($($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.
 ///
@@ -3734,14 +3793,15 @@ 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) {
-        // SAFETY: this call is always safe.
-        unsafe {
-            miri_promise_symbolic_alignment(ptr, align);
+    const_eval_select!(
+        (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

@@ -24,18 +24,6 @@ macro_rules! panic {
 #[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.
@@ -44,7 +32,16 @@ pub macro const_panic {
         #[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)
+            $crate::intrinsics::const_eval_select!(
+                #[inline]
+                #[track_caller]
+                ($($arg: $ty),*) -> !:
+                if const {
+                    $crate::panic!($const_msg)
+                } else {
+                    $crate::panic!($runtime_msg)
+                }
+            )
         }
 
         do_panic($($val),*)