Auto merge of #132571 - RalfJung:const_eval_select_macro, r=<try>

add const_eval_select macro to reduce redundancy

I played around a bit with a macro to make const_eval_select invocations look a bit nicer and avoid repeating the argument lists. Here's what I got. What do you think?

I didn't apply this everywhere yet because I wanted to gather feedback first.

The second commit moves the macros from #132542 into a more sensible place. It didn't seem worth its own PR and would conflict with this PR if done separately.

Cc `@oli-obk` `@saethlin` `@tgross35`

try-job: dist-aarch64-msvc

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
@@ -411,37 +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!(
+            @capture { 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 {
+                // 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!(
+        @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.
+            while unsafe { *s.add(len) } != 0 {
+                len += 1;
+            }
 
-        len
-    }
+            len
+        } else {
+            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 +2788,68 @@ 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!(
+///     @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 @@ where
 /// # 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) {
-        // SAFETY: this call is always safe.
-        unsafe {
-            miri_promise_symbolic_alignment(ptr, align);
+    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 @@ pub macro assert_matches {
     },
 }
 
-/// 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};