Experiment: force debug_assertions in consteval context

library/core/src/char/convert.rs

@@ -5,6 +5,7 @@ use crate::convert::TryFrom;
 use crate::fmt;
 use crate::mem::transmute;
 use crate::str::FromStr;
+use crate::use_debug_assertions::use_debug_assertions;
 
 /// Converts a `u32` to a `char`. See [`char::from_u32`].
 #[must_use]
@@ -23,7 +24,7 @@ pub(super) const fn from_u32(i: u32) -> Option<char> {
 #[must_use]
 pub(super) const unsafe fn from_u32_unchecked(i: u32) -> char {
     // SAFETY: the caller must guarantee that `i` is a valid char value.
-    if cfg!(debug_assertions) { char::from_u32(i).unwrap() } else { unsafe { transmute(i) } }
+    if use_debug_assertions!() { char::from_u32(i).unwrap() } else { unsafe { transmute(i) } }
 }
 
 #[stable(feature = "char_convert", since = "1.13.0")]

library/core/src/lib.rs

@@ -144,6 +144,7 @@
 #![feature(const_type_name)]
 #![feature(const_default_impls)]
 #![feature(const_unsafecell_get_mut)]
+#![feature(consteval_debug_assertions)]
 #![feature(core_panic)]
 #![feature(duration_consts_float)]
 #![feature(maybe_uninit_uninit_array)]
@@ -243,6 +244,15 @@ pub mod assert_matches {
     pub use crate::macros::{assert_matches, debug_assert_matches};
 }
 
+// We don't export this through #[macro_export] for now, to avoid breakage.
+// See https://github.com/rust-lang/rust/issues/82913
+#[unstable(feature = "consteval_debug_assertions", issue = "none")]
+/// Unstable module containing the unstable `use_debug_assertions` macro.
+pub mod use_debug_assertions {
+    #[unstable(feature = "consteval_debug_assertions", issue = "none")]
+    pub use crate::macros::use_debug_assertions;
+}
+
 #[macro_use]
 mod internal_macros;
 

library/core/src/macros/mod.rs

@@ -168,6 +168,29 @@ pub macro assert_matches {
     },
 }
 
+/// Part of an experiment to enable debug assertions in consteval regardless of actual setting.
+///
+/// This returns `true` in consteval context and `cfg!(debug_assertions)` at runtime.
+#[macro_export]
+#[unstable(feature = "consteval_debug_assertions", issue = "none")]
+#[allow_internal_unstable(const_eval_select)]
+#[allow(unused_unsafe)]
+#[rustc_macro_transparency = "semitransparent"]
+pub macro use_debug_assertions() {{
+    // FIXME: Currently no unsafe hygiene inside macros; see https://github.com/rust-lang/rust/issues/74838
+    const fn use_debug_assertions() -> bool {
+        const fn always_true() -> bool {
+            true
+        }
+
+        // SAFETY: Code isn't going to rely on these values being the same, so, we're okay here.
+        unsafe {
+            $crate::intrinsics::const_eval_select((), always_true, || $crate::cfg!(debug_assertions))
+        }
+    }
+    use_debug_assertions()
+}}
+
 /// Asserts that a boolean expression is `true` at runtime.
 ///
 /// This will invoke the [`panic!`] macro if the provided expression cannot be
@@ -212,10 +235,10 @@ pub macro assert_matches {
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_diagnostic_item = "debug_assert_macro"]
-#[allow_internal_unstable(edition_panic)]
+#[allow_internal_unstable(edition_panic, consteval_debug_assertions)]
 macro_rules! debug_assert {
     ($($arg:tt)*) => {
-        if $crate::cfg!(debug_assertions) {
+        if $crate::use_debug_assertions::use_debug_assertions!() {
             $crate::assert!($($arg)*);
         }
     };
@@ -243,9 +266,10 @@ macro_rules! debug_assert {
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "debug_assert_eq_macro")]
+#[allow_internal_unstable(consteval_debug_assertions)]
 macro_rules! debug_assert_eq {
     ($($arg:tt)*) => {
-        if $crate::cfg!(debug_assertions) {
+        if $crate::use_debug_assertions::use_debug_assertions!() {
             $crate::assert_eq!($($arg)*);
         }
     };
@@ -273,9 +297,10 @@ macro_rules! debug_assert_eq {
 #[macro_export]
 #[stable(feature = "assert_ne", since = "1.13.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "debug_assert_ne_macro")]
+#[allow_internal_unstable(consteval_debug_assertions)]
 macro_rules! debug_assert_ne {
     ($($arg:tt)*) => {
-        if $crate::cfg!(debug_assertions) {
+        if $crate::use_debug_assertions::use_debug_assertions!() {
             $crate::assert_ne!($($arg)*);
         }
     };
@@ -314,10 +339,10 @@ macro_rules! debug_assert_ne {
 /// ```
 #[macro_export]
 #[unstable(feature = "assert_matches", issue = "82775")]
-#[allow_internal_unstable(assert_matches)]
+#[allow_internal_unstable(assert_matches, consteval_debug_assertions)]
 #[rustc_macro_transparency = "semitransparent"]
 pub macro debug_assert_matches($($arg:tt)*) {
-    if $crate::cfg!(debug_assertions) {
+    if $crate::use_debug_assertions::use_debug_assertions!() {
         $crate::assert_matches::assert_matches!($($arg)*);
     }
 }