E0013 — Missing return on a code path

A function with a non-void declared return type has a path that falls off the end without returning a value.

---

What triggers it

Control-flow analysis over the function body finds at least one branch (commonly the else-less side of an if) that reaches the closing } without executing a return.

Minimal example

fn sign(int n) -> int {
    if n > 0 {
        return 1;
    }
}

Output:

scratch.rz:1:1: error[E0013]: missing return in function `sign`
fn sign(int n) -> int {
^^^^^^^^^^^^^^^^^^^^^^^

Fix

Cover every path with an explicit return, including the fall-through / else case.

fn sign(int n) -> int {
    if n > 0 {
        return 1;
    }
    return -1;
}

Source

Emitted from the typechecker’s return-path exhaustiveness check in resilient/src/typechecker.rs.