return

The return statement exits a function, optionally with a value.

Surface Syntax#

Return a value:

return <expr>;

Return from a void function:

return;

Semantics#

When a return statement executes:

• the current function terminates immediately, and
• control transfers back to the caller,
• carrying a return value if the function’s result type is non-void.

No statements after a return in the same control-flow path are executed.

Type Checking Rules#

The checker enforces:

• return is only valid inside a function body (otherwise E2009).
• In a function with non-void result type R, return must provide an expression whose type is R (otherwise E2009).
• In a void function, return; is permitted and return <expr>; is rejected (E2009).
• In a function with non-void result, falling off the end of the function body is a compile-time error (diagnostics, E2010).

Examples#

Returning from main#

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

Early return#

fn main () -> int {
  let x: int = 1;
  if x == 0 {
    return 0;
  }
  return 1;
}

return; in a void function#

struct Counter {
  value: int,
}

impl Counter {
  fn inc (mut self: &Counter) -> void {
    self.value += 1;
    return;
  }
}

Notes#

Supported forms:

• return <expr>; from non-void functions, with type checking.
• return; from void functions.
• Missing return in a non-void function is rejected (E2010).

examples: