Pass CARGO_CFG_DEBUG_ASSERTIONS to build scripts based on profile setting

Author: moabo3li

crates/build-rs/src/input.rs

@@ -172,11 +172,6 @@ mod cfg {
     }
 
     /// If we are compiling with debug assertions enabled.
-    ///
-    /// Build scripts are not passed this cfg because
-    /// this cfg is always true and misleading.
-    /// That is because Cargo queries rustc without any profile settings.
-    #[cfg(any())]
     #[track_caller]
     pub fn cargo_cfg_debug_assertions() -> bool {
         ENV.is_present("CARGO_CFG_DEBUG_ASSERTIONS")

src/cargo/core/compiler/custom_build.rs

@@ -430,6 +430,14 @@ fn build_work(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResul
         cmd.env(&k, v.join(","));
     }
 
+    // Manually set CARGO_CFG_DEBUG_ASSERTIONS based on the profile's debug_assertions setting.
+    // This is set separately because the cfg query from rustc doesn't include profile settings
+    // and would always be true (see filtering above). Build scripts need the actual profile
+    // setting to properly configure C/C++ code with matching assertion behavior.
+    if unit.profile.debug_assertions {
+        cmd.env("CARGO_CFG_DEBUG_ASSERTIONS", "");
+    }
+
     // Also inform the build script of the rustc compiler context.
     if let Some(wrapper) = bcx.rustc().wrapper.as_ref() {
         cmd.env("RUSTC_WRAPPER", wrapper);

src/cargo/core/profiles.rs

@@ -330,6 +330,7 @@ impl Profiles {
         result.root = for_unit_profile.root;
         result.debuginfo = for_unit_profile.debuginfo;
         result.opt_level = for_unit_profile.opt_level;
+        result.debug_assertions = for_unit_profile.debug_assertions;
         result.trim_paths = for_unit_profile.trim_paths.clone();
         result
     }