`cargo bench --bench x` builds unrelated build targets

[bdonlan]

Problem

Cargo allows you to specify a single build target for cargo run and cargo bench as in:

• cargo run --bin foo
• cargo bench --bench bar

For cargo run, only the specified target is built. However, for cargo bench, all benchmark and bin targets are built before executing the benchmark. For applications with heavyweight bins, this can slow down the build quite a bit.

Steps

1. Create a project with a benchmark target
2. Run cargo bench --bench benchmark-target-name
3. Observe that other bins are built as well

Possible Solution(s)

No response

Notes

No response

Version

% cargo version --verbose
cargo 1.87.0 (99624be96 2025-05-06)
release: 1.87.0
commit-hash: 99624be96e9d213b0e9b1e36451271f24e4a41d8
commit-date: 2025-05-06
host: x86_64-unknown-linux-gnu
libgit2: 1.9.0 (sys:0.20.0 vendored)
libcurl: 8.12.1-DEV (sys:0.4.80+curl-8.12.1 vendored ssl:OpenSSL/3.4.1)
ssl: OpenSSL 3.4.1 11 Feb 2025
os: Amazon Linux AMI 2.0.0 [64-bit]

[epage]

To avoid ambiguity, could you provide a reproduction case to highlight what all additional targets are affected?

[epage]

Possibly related: #12980

[bdonlan]

Here's a simple repro:

1. Run cargo new --bin
2. mkdir src/bin benches
3. echo 'fn main() { const { panic!() } }' > src/bin/explode.rs
4. echo 'fn main() {}' benches/x.rs
5. Append the following to Cargo.toml:

[[bench]]
name = "x"
path = "benches/x.rs"
harness = false

6. Run cargo bench --bench x

Cargo will fail to run the benchmark due to the failure to build src/bin/explode.rs.

[epage]

Are bin targets the only kind you are concerned about?

[bdonlan]

Test targets are an issue for us as well (our unit tests are a bit slow to build, particularly in a release profile). A more general description would be: I want to be able to build and/or execute just a specific benchmark target (and the libs it depends on).

[epage]

I created a project with most types of build targets.

I tried to demonstrate whats happening with cargo read-manifest or cargo metadata but neither reports bench.

Looking at the code, production build crates are given the opportunity for white box tests and benches. That does seem wasteful without stabilized bench as there aren't dual test/bench harnesses at this time though we are working towards having one.

cargo/src/cargo/core/manifest.rs

Lines 818 to 874 in 7388d86

	fn new(src_path: TargetSourcePath, edition: Edition) -> Target {
	Target {
	inner: Arc::new(TargetInner {
	kind: TargetKind::Bin,
	name: String::new(),
	name_inferred: false,
	bin_name: None,
	src_path,
	required_features: None,
	doc: false,
	doctest: false,
	harness: true,
	for_host: false,
	proc_macro: false,
	doc_scrape_examples: RustdocScrapeExamples::Unset,
	edition,
	tested: true,
	benched: true,
	}),
	}
	}
	fn with_path(src_path: PathBuf, edition: Edition) -> Target {
	Target::new(TargetSourcePath::from(src_path), edition)
	}
	pub fn lib_target(
	name: &str,
	crate_targets: Vec<CrateType>,
	src_path: PathBuf,
	edition: Edition,
	) -> Target {
	let mut target = Target::with_path(src_path, edition);
	target
	.set_kind(TargetKind::Lib(crate_targets))
	.set_name(name)
	.set_doctest(true)
	.set_doc(true);
	target
	}
	pub fn bin_target(
	name: &str,
	bin_name: Option<String>,
	src_path: PathBuf,
	required_features: Option<Vec<String>>,
	edition: Edition,
	) -> Target {
	let mut target = Target::with_path(src_path, edition);
	target
	.set_kind(TargetKind::Bin)
	.set_name(name)
	.set_binary_name(bin_name)
	.set_required_features(required_features)
	.set_doc(true);
	target
	}

While the docs only mention binaries being built for integration tests, they also get built for integration benches:

cargo/src/cargo/core/compiler/unit_dependencies.rs

Lines 357 to 361 in 7388d86

	// If any integration tests/benches are being run, make sure that
	// binaries are built as well.
	if !unit.mode.is_check()
	&& unit.mode.is_any_test()
	&& (unit.target.is_test() || unit.target.is_bench())

While the docs could be improved, control over building of the binaries is being discussed in #12980.