Enable CARGO_CFG_DEBUG_ASSERTIONS in build scripts based on profile

crates/build-rs-test-lib/build.rs

@@ -11,6 +11,7 @@ fn smoke_test_inputs() {
     dbg!(cargo());
     dbg!(cargo_cfg_feature());
     dbg!(cargo_cfg("careful"));
+    dbg!(cargo_cfg_debug_assertions());
     #[cfg(feature = "unstable")]
     dbg!(cargo_cfg_fmt_debug());
     #[cfg(feature = "unstable")]

crates/build-rs/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "build-rs"
-version = "0.3.2"
+version = "0.3.3"
 rust-version.workspace = true
 edition.workspace = true
 license.workspace = true

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

@@ -407,9 +407,19 @@ fn build_work(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResul
         "feature",
         unit.features.iter().map(|s| s.as_str()).collect::<Vec<_>>(),
     );
+    // Manually inject debug_assertions based on the profile setting.
+    // The cfg query from rustc doesn't include profile settings and would always be true,
+    // so we override it with the actual profile setting.
+    if unit.profile.debug_assertions {
+        cfg_map.insert("debug_assertions", Vec::new());
+    }
     for cfg in bcx.target_data.cfg(unit.kind) {
         match *cfg {
             Cfg::Name(ref n) => {
+                // Skip debug_assertions from rustc query; we use the profile setting instead
+                if n.as_str() == "debug_assertions" {
+                    continue;
+                }
                 cfg_map.insert(n.as_str(), Vec::new());
             }
             Cfg::KeyPair(ref k, ref v) => {
@@ -419,11 +429,6 @@ fn build_work(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResul
         }
     }
     for (k, v) in cfg_map {
-        if k == "debug_assertions" {
-            // This cfg is always true and misleading, so avoid setting it.
-            // That is because Cargo queries rustc without any profile settings.
-            continue;
-        }
         // FIXME: We should handle raw-idents somehow instead of predenting they
         // don't exist here
         let k = format!("CARGO_CFG_{}", super::envify(k));

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
     }

tests/testsuite/build.rs

@@ -5887,7 +5887,7 @@ fn user_specific_cfgs_are_filtered_out() {
             r#"
             fn main() {
                 assert!(std::env::var_os("CARGO_CFG_PROC_MACRO").is_none());
-                assert!(std::env::var_os("CARGO_CFG_DEBUG_ASSERTIONS").is_none());
+                assert!(std::env::var_os("CARGO_CFG_DEBUG_ASSERTIONS").is_some());
             }
             "#,
         )

tests/testsuite/build_script_env.rs

@@ -452,3 +452,22 @@ fn rerun_if_env_newly_added_in_config() {
 "#]])
         .run();
 }
+
+#[cargo_test]
+fn build_script_debug_assertions_dev() {
+    // Test that CARGO_CFG_DEBUG_ASSERTIONS is set in dev profile (default)
+    let build_rs = r#"
+        fn main() {
+            let has_debug_assertions = std::env::var_os("CARGO_CFG_DEBUG_ASSERTIONS").is_some();
+            assert!(has_debug_assertions, "CARGO_CFG_DEBUG_ASSERTIONS should be set in dev profile");
+        }
+    "#;
+
+    let p = project()
+        .file("src/lib.rs", r#""#)
+        .file("build.rs", build_rs)
+        .build();
+
+    // Default dev profile has debug-assertions enabled
+    p.cargo("check").run();
+}