Enable CARGO_CFG_DEBUG_ASSERTIONS in build scripts based on profile (#16160)

Build scripts need to know whether debug assertions are enabled to
properly configure dependencies with matching assertion behavior.
Previously, `CARGO_CFG_DEBUG_ASSERTIONS` was filtered out because the
cfg query to rustc doesn't include profile settings.

This PR manually sets `CARGO_CFG_DEBUG_ASSERTIONS` based on the
profile's `debug-assertions` setting, allowing build scripts to
configure dependencies appropriately.

FCP:
#16160 (comment)

Closes #15760

Author: epage

Cargo.lock

@@ -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-test-lib/build.rs

@@ -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/Cargo.toml

@@ -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")

crates/build-rs/src/input.rs

@@ -404,9 +404,19 @@ 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) => {
@@ -416,11 +426,6 @@ 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/compiler/custom_build.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
     }

src/cargo/core/profiles.rs

@@ -363,7 +363,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
   > Note that different [target triples][Target Triple] have different sets of `cfg` values,
   > hence variables present in one target triple might not be available in the other.
   >
-  > Some cfg values like `debug_assertions` and `test` are not available.
+  > Some cfg values like `test` are not available.
 * `OUT_DIR` --- the folder in which all output and intermediate artifacts should
   be placed. This folder is inside the build directory for the package being built,
   and it is unique for the package in question.

src/doc/src/reference/environment-variables.md

@@ -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.rs

@@ -452,3 +452,142 @@ 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();
+}
+
+#[cargo_test]
+fn build_script_debug_assertions_release() {
+    // Test that CARGO_CFG_DEBUG_ASSERTIONS is NOT set in release 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 NOT be set in release profile");
+        }
+    "#;
+
+    let p = project()
+        .file("src/lib.rs", r#""#)
+        .file("build.rs", build_rs)
+        .build();
+
+    // Release profile has debug-assertions disabled by default
+    p.cargo("check --release").run();
+}
+
+#[cargo_test]
+fn build_script_debug_assertions_override_dev() {
+    // Test that CARGO_CFG_DEBUG_ASSERTIONS respects profile overrides
+    // Dev profile with debug-assertions explicitly DISABLED
+    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 NOT be set when dev profile disables it");
+        }
+    "#;
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2024"
+
+                [profile.dev]
+                debug-assertions = false
+            "#,
+        )
+        .file("src/lib.rs", r#""#)
+        .file("build.rs", build_rs)
+        .build();
+
+    // Dev profile with debug-assertions explicitly disabled
+    p.cargo("check").run();
+}
+
+#[cargo_test]
+fn build_script_debug_assertions_override_release() {
+    // Test that CARGO_CFG_DEBUG_ASSERTIONS respects profile overrides
+    // Release profile with debug-assertions explicitly ENABLED
+    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 when release profile enables it");
+        }
+    "#;
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2024"
+
+                [profile.release]
+                debug-assertions = true
+            "#,
+        )
+        .file("src/lib.rs", r#""#)
+        .file("build.rs", build_rs)
+        .build();
+
+    // Release profile with debug-assertions explicitly enabled
+    p.cargo("check --release").run();
+}
+
+#[cargo_test]
+fn build_script_debug_assertions_build_override() {
+    let build_rs = r#"
+        fn main() {
+            let profile = std::env::var("PROFILE").unwrap();
+            if profile == "debug" {
+                assert!(!cfg!(debug_assertions));
+            } else if profile == "release" {
+                assert!(cfg!(debug_assertions));
+            }
+        }
+    "#;
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2024"
+
+                [profile.dev.build-override]
+                debug-assertions = false
+
+                [profile.release.build-override]
+                debug-assertions = true
+            "#,
+        )
+        .file("src/lib.rs", r#""#)
+        .file("build.rs", build_rs)
+        .build();
+
+    p.cargo("check").run();
+    p.cargo("check --release").run();
+}