clippy::must_use_candidate: Checks for public functions that have no #[must_use] attribute, but return something not already marked must-use, have no mutable arg and mutate no statics

// this could be annotated with `#[must_use]`.
pub fn id<T>(t: T) -> T { t }

[lints.clippy]
must_use_candidate = "warn"

#![warn(clippy::must_use_candidate)]

cargo clippy -- -W clippy::must-use-candidate

Clippy's own documentation

What it does

Checks for public functions that have no #[must_use] attribute, but return something not already marked must-use, have no mutable arg and mutate no statics.

Why is this bad?

Not bad at all, this lint just shows places where you could add the attribute.

Known problems

The lint only checks the arguments for mutable types without looking if they are actually changed. On the other hand, it also ignores a broad range of potentially interesting side effects, because we cannot decide whether the programmer intends the function to be called for the side effect or the result. Expect many false positives. At least we don’t lint if the result type is unit or already #[must_use].

Examples

// this could be annotated with `#[must_use]`.
pub fn id<T>(t: T) -> T { t }

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

Related lints

• too_many_linesChecks for functions with a large amount of lines
• missing_errors_docChecks the doc comments of publicly visible functions that return a Result type and warns if there is no # Errors section
• uninlined_format_argsDetect when a variable is not inlined in a format string, and suggests to inline it
• missing_fields_in_debugmanual `Debug` impl does not include all fields
• missing_panics_docChecks the doc comments of publicly visible functions that may panic and warns if there is no # Panics section
• mut_mutmultiple successive mutable borrows
• naive_bytecountChecks for naive byte counts
• needless_bitwise_booluse of bitwise operator instead of lazy operator between booleans

All 847 Clippy lints · Rust compiler errors · Cargo.toml checker