This program demonstrates the setup I'm using to explain the weird behaviour I'm getting

weird_behaviour.rs

fn parse_function_call(tokens: &[Token]) -> Result<ASTNode, SyntaxError> {
  if /* Check if syntax is valid */ {
    return Err(SyntaxError::InvalidToken);
  }

  // ...
}

fn parse_mathematical_expression(tokens: &[Token]) -> Result<ASTNode, SyntaxError> {
  if /* Check if syntax is valid */ {
    return Err(SyntaxError::InvalidSyntax);
  }

  // ...
}

pub fn parse(tokens: &[Token]) -> Result<Box<ASTNode>, SyntaxError> {
  if let Ok(call) = parse_function_call(tokens) {
    return Ok(Box::new(call));
  }

  if let Ok(expr) = parse_mathematical_expression(tokens) {
    return Ok(Box::new(expr));
  }

  Err(SyntaxError::UnexpectedToken(tokens.get(0).unwrap()))
}