E3028

Error · Type

E3028 — A function with a non-void return type whose body can complete without returning. Reported here rather than left to codegen, where it surfaced as an MLIR verifier message naming an operation, with no source location.

What the compiler reports

E3028] at {{[1-9][0-9]*}}:{{[0-9]+}}: 'falls_off_the_end' returns i32 but its body can finish without returning a value

The fragments the test suite holds the compiler to. An ellipsis marks text the test does not constrain; the emitted message also carries a source location.

A program that triggers it

//
// A function that promises a value and can finish without producing one. This used to reach
// codegen: the emitter left a block with no terminator and the MLIR verifier reported it as
// `block with no terminator, has %1 = "arith.addi"(...)` -- an operation name, no source
// location, and no statement of what was wrong. For what is probably the most common mistake a
// newcomer makes.
//
// The location is the last statement, which is where the missing `return` belongs.
//
fn falls_off_the_end(x : i32) -> i32 {
  let y : i32 = x + 1;
}

fn empty(x : i32) -> i32 {
}

// One branch returning is not enough -- the other path reaches the end.
fn only_one_branch(x : i32) -> i32 {
  if x > 0 {
    return 1;
  }
}

fn main() -> i32 {
  return 0;
}

From tests/frontend/fail/missing_return.vx, which asserts this diagnostic on every commit.

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