You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: _encryption_at_rest_options.rs
    path of file: ./repos/aws-sdk-rust/sdk/osis/src/types
    
    the code of the file until where you have to start completion: // Code generated by software.amazon.smithy.rust.codegen.smithy-rs. DO NOT EDIT.

/// <p>Options to control how OpenSearch encrypts all data-at-rest.</p>
#[non_exhaustive]
#[derive(::std::clone::Clone, ::std::cmp::PartialEq, ::std::fmt::Debug)]
pub struct EncryptionAtRestOptions {
    /// <p>The ARN of the KMS key used to encrypt data-at-rest in Open
    
    pub fn build(self) -> ::std::result::Result<crate::types::EncryptionAtRestOptions, ::aws_smithy_types::error::operation::BuildError> {
        ::std::result::Result::Ok(crate::types::EncryptionAtRestOptions {
            kms_key_arn: self.kms_key_arn.ok_or_else(|| {
                ::aws_smithy_types::error::operation::BuildError::missing_field(
                    "kms_key_arn",
                    "kms_key_arn was not specified but it is required when building EncryptionAtRestOptions",
                )
            })?,
        })
    }