Enable CARGO_CFG_DEBUG_ASSERTIONS in build scripts based on profile #16160

moabo3li · 2025-10-27T07:28:17Z

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

rustbot · 2025-10-27T07:28:22Z

r? @ehuss

rustbot has assigned @ehuss.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

crates/build-rs/src/input.rs

src/cargo/core/compiler/custom_build.rs

src/cargo/core/profiles.rs

src/cargo/core/compiler/custom_build.rs

tests/testsuite/build_script_env.rs

tests/testsuite/build.rs

rustbot · 2025-10-28T13:20:05Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

…ting fix(test): update test to reflect CARGO_CFG_DEBUG_ASSERTIONS is now passed to build scripts chore(deps): update build-rs to version 0.3.3 in Cargo.lock and Cargo.toml

…verride

…le override

…e for debug and release profiles

epage · 2025-10-28T18:50:42Z

tests/testsuite/build_script_env.rs

+#[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();
+}


Why is this checking dev and release? The test is only relevant for the profile build-override is being used on

The test is checking both to verify that build-override correctly affects build script compilation in both profiles. It's showing the override works bidirectionally:
You can disable debug assertions for build scripts in dev (normally they're on)
You can enable debug assertions for build scripts in release (normally they're off)

Oh right, for some reason I had missed the use of build-override in both.

We don't really need tests for each of these two profiles. Once we test things in one profile, its sufficient for all.

…emove reference to debug_assertions

epage · 2025-10-28T20:07:13Z

tests/testsuite/build_script_env.rs

test(add): add test for CARGO_CFG_DEBUG_ASSERTIONS in dev profile

For future reference, in Conventional commits, add is the scope, or what this applies to. In this case you are sayign you are doing tests for cargo add.

epage

While some nits, this is good enough to go in. Thanks for your work on this!

View changes since this review

epage · 2025-10-28T20:09:18Z

@rfcbot fcp merge

Like with #14902, this adds an artificial CARGO_CFG to build scripts for --cfg debug_assertions. For more background, see #15760 (comment)

rust-rfcbot · 2025-10-28T20:09:20Z

Team member @epage has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

rustbot assigned ehuss Oct 27, 2025

rustbot added A-build-scripts Area: build.rs scripts A-profiles Area: profiles S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 27, 2025

moabo3li force-pushed the debug_assertion_handling branch from 76b11fa to 72144ac Compare October 27, 2025 07:40