Lint Levels & Attributes

Rust lints can be allowed, warned, denied, or forbidden at crate, module, or item level. Cargo.toml [lints] tables centralize policy for rustc and clippy.

Recipe

[lints.rust]
unsafe_code = "forbid"
missing_docs = "warn"
 
[lints.clippy]
all = "warn"
pedantic = "allow"

#[deny(clippy::unwrap_used)]
fn production_path() { }

When to reach for this: Crate-wide safety policy, tightening warnings-as-errors, or targeted exceptions.

Working Example

#![deny(unsafe_code)]
#![warn(missing_docs)]
 
mod api {
    #![allow(clippy::module_name_repetitions)]
 
    /// Creates a user.
    pub fn create_user() { }
}

# Cargo.toml alternative to inner attributes
[lints.rust]
unsafe_code = "forbid"

What this demonstrates:

Crate attributes apply to entire compilation unit
Inner #![...] on modules scope subtree
forbid cannot be overridden downstream (strongest)
Manifest [lints] preferred for workspace consistency

Deep Dive

Levels

Level	Behavior
`allow`	Silent
`warn`	Warning, build succeeds
`deny`	Error
`forbid`	Error; cannot `allow` in children

rustc vs clippy

[lints.rust]
unused_must_use = "deny"
 
[lints.clippy]
unwrap_used = "deny"   # apps: deny; libs: warn

Libraries often warn on unwrap in public API
Applications may deny unwrap_used in non-test code

Gotchas

#![deny(warnings)] without policy - breaks on new Rust warnings after upgrade. Fix: pin toolchain; fix warnings promptly on bump.
forbid then allow in submodule - compile error. Fix: use deny if children need exceptions.
Duplicate attributes and manifest - confusing precedence. Fix: pick one source of truth per lint.
Allowing clippy::all - disables valuable checks. Fix: allow specific lints only.
Test code same lint level - unwrap in tests is fine. Fix: #[cfg(test)] mod tests { #![allow(clippy::unwrap_used)] }.

Alternatives

Alternative	Use When	Don't Use When
CI `-D warnings` only	No manifest support needed	Per-crate policy differences
`cargo clippy -- -W lint`	Experiment locally	Permanent policy
`#[expect(lint)]` (nightly)	Suppress with intent tracking	Stable MSRV 1.97 teams use `allow`

FAQs

deny(warnings) in lib vs bin?

Libs often warn; bins/services deny for production rigor.

How lint inheritance works in workspace?

Each crate has own [lints]; share via copy or workspace lint table future patterns - document in CONTRIBUTING.

missing_docs on private items?

Usually public items only; configure missing_docs on pub via clippy/rustc docs.

Can CI override manifest?

RUSTFLAGS and clippy args add warnings; manifest still applies as base.

forbid unsafe in workspace?

Set on domain crates; allow single ffi crate with documented safety reviews.

How allow for one line?

#[allow(clippy::lint)] on fn or statement.

What breaks on edition bump?

New rustc lints may warn; run cargo fix and clippy after toolchain upgrade.

deny in build.rs?

Separate compilation unit; set lints in build.rs crate attributes if needed.

Clippy lint groups?

clippy::all, clippy::pedantic - enable groups then allow noisy subsets.

Document lint policy?

ARCHITECTURE.md or CONTRIBUTING: unsafe, unwrap, missing_docs expectations.

Clippy - clippy-specific lints
Documentation Lints - missing_docs
Configuring Clippy - thresholds
Linting in CI - -D warnings

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Lint Levels & Attributes

Rust lints can be allowed, warned, denied, or forbidden at crate, module, or item level. Cargo.toml [lints] tables centralize policy for rustc and clippy.

Recipe

[lints.rust]
unsafe_code = "forbid"
missing_docs = "warn"
 
[lints.clippy]
all = "warn"
pedantic = "allow"

#[deny(clippy::unwrap_used)]
fn production_path() { }

When to reach for this: Crate-wide safety policy, tightening warnings-as-errors, or targeted exceptions.

Working Example

#![deny(unsafe_code)]
#![warn(missing_docs)]
 
mod api {
    #![allow(clippy::module_name_repetitions)]
 
    /// Creates a user.
    pub fn create_user() { }
}

# Cargo.toml alternative to inner attributes
[lints.rust]
unsafe_code = "forbid"

What this demonstrates:

Crate attributes apply to entire compilation unit
Inner #![...] on modules scope subtree
forbid cannot be overridden downstream (strongest)
Manifest [lints] preferred for workspace consistency

Deep Dive

Levels

Level	Behavior
`allow`	Silent
`warn`	Warning, build succeeds
`deny`	Error
`forbid`	Error; cannot `allow` in children

rustc vs clippy

[lints.rust]
unused_must_use = "deny"
 
[lints.clippy]
unwrap_used = "deny"   # apps: deny; libs: warn

Libraries often warn on unwrap in public API
Applications may deny unwrap_used in non-test code

Gotchas

#![deny(warnings)] without policy - breaks on new Rust warnings after upgrade. Fix: pin toolchain; fix warnings promptly on bump.
forbid then allow in submodule - compile error. Fix: use deny if children need exceptions.
Duplicate attributes and manifest - confusing precedence. Fix: pick one source of truth per lint.
Allowing clippy::all - disables valuable checks. Fix: allow specific lints only.
Test code same lint level - unwrap in tests is fine. Fix: #[cfg(test)] mod tests { #![allow(clippy::unwrap_used)] }.

Alternatives

Alternative	Use When	Don't Use When
CI `-D warnings` only	No manifest support needed	Per-crate policy differences
`cargo clippy -- -W lint`	Experiment locally	Permanent policy
`#[expect(lint)]` (nightly)	Suppress with intent tracking	Stable MSRV 1.97 teams use `allow`

FAQs

deny(warnings) in lib vs bin?

Libs often warn; bins/services deny for production rigor.

How lint inheritance works in workspace?

Each crate has own [lints]; share via copy or workspace lint table future patterns - document in CONTRIBUTING.

missing_docs on private items?

Usually public items only; configure missing_docs on pub via clippy/rustc docs.

Can CI override manifest?

RUSTFLAGS and clippy args add warnings; manifest still applies as base.

forbid unsafe in workspace?

Set on domain crates; allow single ffi crate with documented safety reviews.

How allow for one line?

#[allow(clippy::lint)] on fn or statement.

What breaks on edition bump?

New rustc lints may warn; run cargo fix and clippy after toolchain upgrade.

deny in build.rs?

Separate compilation unit; set lints in build.rs crate attributes if needed.

Clippy lint groups?

clippy::all, clippy::pedantic - enable groups then allow noisy subsets.

Document lint policy?

ARCHITECTURE.md or CONTRIBUTING: unsafe, unwrap, missing_docs expectations.

Clippy - clippy-specific lints
Documentation Lints - missing_docs
Configuring Clippy - thresholds
Linting in CI - -D warnings

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Lint Levels & Attributes

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Lint Levels & Attributes