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: _custom_extension.rs
    path of file: ./repos/aws-sdk-rust/sdk/acmpca/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></p>
/// <p>Specifies the X.509 extension information for a certificate.</p>
/// <p>Extensions present in <code>CustomExtensions</code> follow the <code>ApiPassthrough</code> <a href="https://docs.aws.amazon.com/privateca/latest/userguide/UsingTemplates.html#template-order-of-operations">template rules</a>.</p>
#[non_exhaustive]
#[derive(::std::clone::Clone, ::std::cmp::PartialEq, ::std::fmt::Debug)]
pub struct CustomExtension {
    /// <p></p>
    /// <p>Specifies the object identifier (OID) of the X.509 extension. For more information, see the <a href="https://oidref.com/2.5.29">Global OID reference database.</a></p>
    pub object_identifier: ::std::string::String,
    /// <p></p>
  
    
    pub fn build(self) -> ::std::result::Result<crate::types::CustomExtension, ::aws_smithy_types::error::operation::BuildError> {
        ::std::result::Result::Ok(crate::types::CustomExtension {
            object_identifier: self.object_identifier.ok_or_else(|| {
                ::aws_smithy_types::error::operation::BuildError::missing_field(
                    "object_identifier",
                    "object_identifier was not specified but it is required when building CustomExtension",
                )
            })?,
            value: self.value.ok_or_else(|| {
                ::aws_smithy_types::error::operation::BuildError::missing_field(
                    "value",
                    "value was not specified but it is required when building CustomExtension",
                )
            })?,
            critical: self.critical,
        })
    }