Return Value Ignored

ID

kotlin.return_value_ignored

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

Kotlin

Tags

reliability

Description

Reports call sites whose return value is discarded when the called function is annotated with @CheckReturnValue (recognised across the common annotation packages that publish it, including javax.annotation and org.jetbrains.annotations). The annotation marks a function whose result must be consumed; ignoring it usually means the call was meant to mutate something but the API is immutable, or a developer forgot to use the return value.

Rationale

@CheckReturnValue
fun produce(): Int = 42

fun caller() {
    produce()          // result thrown away — the computation has no effect
}

@CheckReturnValue is a deliberate contract: "do not call me for side effects". When the rule fires it almost always points at a real bug, because the only reason to call a side-effect-free function is to use its result.

Remediation

@CheckReturnValue
fun produce(): Int = 42

fun caller() {
    val v = produce()        // consume the result
    println(produce())       // or pass it onwards
}

Either consume the return value (assign, return, pass as argument, or use it in an expression) or — if the call truly should be fire-and-forget — remove the @CheckReturnValue annotation from the function declaration.

References