Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::fn_params_excessive_bools: Checks for excessive use of bools in function declarations

What it meansChecks for excessive use of bools in function declarations. It is in the pedantic group and off by default.

Does it matter?

Boolean parameters obscure meaning at the call site. The reader has to remember what each true or false means and which position each flag belongs in, with no help from the type system. The best replacement depends on what role the bool plays: Use a two-variant enum when one parameter selects a mode. Split one function into two named operations when the choices are distinct actions. Group multiple related flags into a struct or options type when they travel together. Move the setting onto self or an existing config/context parameter when it is really part of the surrounding state. This optimizes for readability of the calling code, which future readers encounter more often than the function declaration. The max-fn-params-bools configuration accepts 0, which lints on any function with a bool parameter.

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

fn render_document(show_hidden: bool, is_draft: bool) { ... }

Write this instead

enum Visibility {
    ShowHidden,
    HideHidden,
}

enum DocumentKind {
    Draft,
    Final,
}

fn render_document(visibility: Visibility, kind: DocumentKind) { ... }

// or split the operation when the choices are distinct
fn render_draft() { ... }
fn render_final() { ... }

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
fn_params_excessive_bools = "warn"

In code

#![warn(clippy::fn_params_excessive_bools)]

One run

cargo clippy -- -W clippy::fn-params-excessive-bools

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Settings in clippy.toml

max-fn-params-bools: The maximum number of bool parameters a function can have. Use 0 to lint on any function with a bool parameter.

Pedanticgroup

allowdefault level

nofixed by cargo clippy --fix

1.43.0added in Rust

Clippy's own documentation

What it does

Checks for excessive use of bools in function declarations.

Why is this bad?

Boolean parameters obscure meaning at the call site. The reader has to remember what each true or false means and which position each flag belongs in, with no help from the type system.

The best replacement depends on what role the bool plays:

Use a two-variant enum when one parameter selects a mode.
Split one function into two named operations when the choices are distinct actions.
Group multiple related flags into a struct or options type when they travel together.
Move the setting onto self or an existing config/context parameter when it is really part of the surrounding state.

This optimizes for readability of the calling code, which future readers encounter more often than the function declaration.

The max-fn-params-bools configuration accepts 0, which lints on any function with a bool parameter.

Example

fn render_document(show_hidden: bool, is_draft: bool) { ... }

Use instead:

enum Visibility {
    ShowHidden,
    HideHidden,
}

enum DocumentKind {
    Draft,
    Final,
}

fn render_document(visibility: Visibility, kind: DocumentKind) { ... }

// or split the operation when the choices are distinct
fn render_draft() { ... }
fn render_final() { ... }

Configuration

max-fn-params-bools: The maximum number of bool parameters a function can have. Use 0 to lint on any function with a bool parameter.

(default: 3)

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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