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fix: ignore implicit std dependencies in unused-crate-dependencies lint #16677

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Merged
epage merged 2 commits into from
Mar 6, 2026
+123 −58
Merged

fix: ignore implicit std dependencies in unused-crate-dependencies lint #16677

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e8db17f
test to reproduce unused_crate_deps warning with -Zbuild-std
biscuitrescue Mar 5, 2026
71f88a1
suppress unused_crate_deps for implicit build-std deps
biscuitrescue Mar 5, 2026
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117 changes: 62 additions & 55 deletions src/cargo/core/compiler/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1874,68 +1874,75 @@ pub fn extern_args(
let no_embed_metadata = build_runner.bcx.gctx.cli_unstable().no_embed_metadata;

// Closure to add one dependency to `result`.
let mut link_to =
|dep: &UnitDep, extern_crate_name: InternedString, noprelude: bool| -> CargoResult<()> {
let mut value = OsString::new();
let mut opts = Vec::new();
let is_public_dependency_enabled = unit
.pkg
.manifest()
.unstable_features()
.require(Feature::public_dependency())
.is_ok()
|| build_runner.bcx.gctx.cli_unstable().public_dependency;
if !dep.public && unit.target.is_lib() && is_public_dependency_enabled {
opts.push("priv");
*unstable_opts = true;
}
if noprelude {
opts.push("noprelude");
*unstable_opts = true;
}
if !opts.is_empty() {
value.push(opts.join(","));
value.push(":");
}
value.push(extern_crate_name.as_str());
value.push("=");

let mut pass = |file| {
let mut value = value.clone();
value.push(file);
result.push(OsString::from("--extern"));
result.push(value);
};
let mut link_to = |dep: &UnitDep,
extern_crate_name: InternedString,
noprelude: bool,
nounused: bool|
-> CargoResult<()> {
let mut value = OsString::new();
let mut opts = Vec::new();
let is_public_dependency_enabled = unit
.pkg
.manifest()
.unstable_features()
.require(Feature::public_dependency())
.is_ok()
|| build_runner.bcx.gctx.cli_unstable().public_dependency;
if !dep.public && unit.target.is_lib() && is_public_dependency_enabled {
opts.push("priv");
*unstable_opts = true;
}
if noprelude {
opts.push("noprelude");
*unstable_opts = true;
}
if nounused {
opts.push("nounused");
*unstable_opts = true;
}
Comment on lines +1899 to +1902
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@epage epage Feb 26, 2026

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What does the path to stabilizing this look like?

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@epage epage Feb 26, 2026

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I also wonder if we should start tracking all of the different unstable features of rustc build-std is relying on to not get surprised when we stabilize it.

CC @davidtwco

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@biscuitrescue biscuitrescue Feb 28, 2026

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What does the path to stabilizing this look like?

It's tied to stabilising -Zbuilt-std itself. This should become default behaviour with no flag needed, since it corrects a warning given by cargo's implicitily injected dependencies.

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@epage epage Feb 28, 2026

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So we'll eventually remove the use of nounused?

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@biscuitrescue biscuitrescue Feb 28, 2026

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yes. i feel that when -Zbuild-std stabilises, cargo shouldnt inject shouldnt be injecting crates that trigger linting errors that the user didnt ask for. I think nounused should be a temporary workaround till that isnt the case, unless there's a flaw in this thinking that im not seeing?

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@epage epage Mar 2, 2026

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How would that work? Especially the build-std config variant.

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@biscuitrescue biscuitrescue Mar 3, 2026

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I was wondering if RFC 3875 could have a way of doing this? It talks of users explicitly declaring what std lib crates to inject. That, in addition to #16675, which aims to add builtin and opaque deps. Wouldn't builtin by default replace nounused?

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@epage epage Mar 3, 2026

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Where explicit dependencies are used. Build-std will be stabilized without that with rust-lang/rfcs#3874

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@biscuitrescue biscuitrescue Mar 5, 2026

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okay so nounused would need to be stabilised too. im sorry i missed this. The path to stabilisation would be through Rust #98400, but that is itself blocked by Rust #98405 harmonise --extern syntax with command line syntax used for native library modifiers.
Once Rust #98400 unblocks, Cargo could remove unstable_opts = true for nounused?

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@epage epage Mar 5, 2026

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Thanks for that! I've noted this in the PR to draw attention to it.

if !opts.is_empty() {
value.push(opts.join(","));
value.push(":");
}
value.push(extern_crate_name.as_str());
value.push("=");

let mut pass = |file| {
let mut value = value.clone();
value.push(file);
result.push(OsString::from("--extern"));
result.push(value);
};

let outputs = build_runner.outputs(&dep.unit)?;
let outputs = build_runner.outputs(&dep.unit)?;

if build_runner.only_requires_rmeta(unit, &dep.unit) || dep.unit.mode.is_check() {
// Example: rlib dependency for an rlib, rmeta is all that is required.
let output = outputs
.iter()
.find(|output| output.flavor == FileFlavor::Rmeta)
.expect("failed to find rmeta dep for pipelined dep");
pass(&output.path);
} else {
// Example: a bin needs `rlib` for dependencies, it cannot use rmeta.
for output in outputs.iter() {
if output.flavor == FileFlavor::Linkable {
pass(&output.path);
}
// If we use -Zembed-metadata=no, we also need to pass the path to the
// corresponding .rmeta file to the linkable artifact, because the
// normal dependency (rlib) doesn't contain the full metadata.
else if no_embed_metadata && output.flavor == FileFlavor::Rmeta {
pass(&output.path);
}
if build_runner.only_requires_rmeta(unit, &dep.unit) || dep.unit.mode.is_check() {
// Example: rlib dependency for an rlib, rmeta is all that is required.
let output = outputs
.iter()
.find(|output| output.flavor == FileFlavor::Rmeta)
.expect("failed to find rmeta dep for pipelined dep");
pass(&output.path);
} else {
// Example: a bin needs `rlib` for dependencies, it cannot use rmeta.
for output in outputs.iter() {
if output.flavor == FileFlavor::Linkable {
pass(&output.path);
}
// If we use -Zembed-metadata=no, we also need to pass the path to the
// corresponding .rmeta file to the linkable artifact, because the
// normal dependency (rlib) doesn't contain the full metadata.
else if no_embed_metadata && output.flavor == FileFlavor::Rmeta {
pass(&output.path);
}
}
Ok(())
};
}
Ok(())
};

for dep in deps {
if dep.unit.target.is_linkable() && !dep.unit.mode.is_doc() {
link_to(dep, dep.extern_crate_name, dep.noprelude)?;
link_to(dep, dep.extern_crate_name, dep.noprelude, dep.nounused)?;
}
}
if unit.target.proc_macro() {
Expand Down
2 changes: 2 additions & 0 deletions src/cargo/core/compiler/unit_dependencies.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -206,6 +206,7 @@ fn attach_std_deps(
// TODO: Does this `public` make sense?
public: true,
noprelude: true,
nounused: true,
}));
found = true;
}
Expand Down Expand Up @@ -911,6 +912,7 @@ fn new_unit_dep_with_profile(
dep_name,
public,
noprelude: false,
nounused: false,
})
}

Expand Down
16 changes: 13 additions & 3 deletions src/cargo/core/compiler/unit_graph.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,8 @@ pub struct UnitDep {
pub public: bool,
/// If `true`, the dependency should not be added to Rust's prelude.
pub noprelude: bool,
/// If `true`, the dependency will not trigger Rust lint.
pub nounused: bool,
}

const VERSION: u32 = 1;
Expand Down Expand Up @@ -74,6 +76,9 @@ struct SerializedUnitDep {
// This is only set on nightly since it is unstable.
#[serde(skip_serializing_if = "Option::is_none")]
noprelude: Option<bool>,
// This is only set on nightly since it is unstable.
#[serde(skip_serializing_if = "Option::is_none")]
nounused: Option<bool>,
// Intentionally not including `unit_for` because it is a low-level
// internal detail that is mostly used for building the graph.
}
Expand Down Expand Up @@ -101,16 +106,21 @@ pub fn emit_serialized_unit_graph(
.iter()
.map(|unit_dep| {
// https://github.com/rust-lang/rust/issues/64260 when stabilized.
let (public, noprelude) = if gctx.nightly_features_allowed {
(Some(unit_dep.public), Some(unit_dep.noprelude))
let (public, noprelude, nounused) = if gctx.nightly_features_allowed {
(
Some(unit_dep.public),
Some(unit_dep.noprelude),
Some(unit_dep.nounused),
)
} else {
(None, None)
(None, None, None)
};
SerializedUnitDep {
index: indices[&unit_dep.unit],
extern_crate_name: unit_dep.extern_crate_name,
public,
noprelude,
nounused,
}
})
.collect();
Expand Down
43 changes: 43 additions & 0 deletions tests/build-std/main.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -438,6 +438,49 @@ fn test_proc_macro() {
.run();
}

#[cargo_test(build_std_real)]
fn build_std_does_not_warn_about_implicit_std_deps() {
let p = project()
.file(
"Cargo.toml",
r#"
[package]
name = "buildstd_test"
version = "0.1.0"
edition = "2021"

[dependencies]
bar = { path = "bar" }
"#,
)
.file("src/main.rs", "fn main() {}")
.file(
"bar/Cargo.toml",
r#"
[package]
name = "bar"
version = "0.1.0"
edition = "2021"
"#,
)
.file("bar/src/lib.rs", "")
.build();

p.cargo("build")
.build_std()
.target_host()
.env("RUSTFLAGS", "-W unused-crate-dependencies")
.with_stderr_data(
str![[r#"
[WARNING] extern crate `bar` is unused in crate `buildstd_test`
[WARNING] `buildstd_test` (bin "buildstd_test") generated 1 warning
...
"#]]
.unordered(),
)
.run();
}

#[cargo_test(build_std_real)]
fn default_features_still_included_with_extra_build_std_features() {
// This is a regression test to ensure when adding extra `build-std-features`,
Expand Down
3 changes: 3 additions & 0 deletions tests/testsuite/unit_graph.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -61,6 +61,7 @@ fn simple() {
"extern_crate_name": "b",
"index": 1,
"noprelude": false,
"nounused": false,
"public": false
}
],
Expand Down Expand Up @@ -106,6 +107,7 @@ fn simple() {
"extern_crate_name": "c",
"index": 2,
"noprelude": false,
"nounused": false,
"public": false
}
],
Expand Down Expand Up @@ -189,6 +191,7 @@ fn simple() {
"extern_crate_name": "a",
"index": 0,
"noprelude": false,
"nounused": false,
"public": false
}
],
Expand Down