XML serializer fails Clippy when serializing Strings

[david-perez]

It can serialize Option<String>, which is why we never realized this basic shortcoming (only the client SDKs heavily exercise the restXml protocol in CI).

The problem manifests itself easily when generating a server SDK, since required is honored as non-Optional String.

Model:

$version: "2.0"

namespace com.amazonaws.simple

use aws.protocols#restXml

@restXml
service SimpleService {
    version: "2022-01-01",
    operations: [
        SimpleOperation,
    ],
}

@http(uri: "/simple-operation", method: "GET")
operation SimpleOperation {
    output: SimpleOperationOutput,
}

structure SimpleOperationOutput {
    @required
    requiredString: String,
}

Generated code (input.required_string is of type String):

pub fn serialize_structure_crate_output_simple_operation_output(
    input: &crate::output::SimpleOperationOutput,
    writer: aws_smithy_xml::encode::ElWriter,
) -> Result<(), aws_smithy_http::operation::SerializationError> {
    #[allow(unused_mut)]
    let mut scope = writer.finish();
    {
        let mut inner_writer = scope.start_el("requiredString").finish();
        inner_writer.data(&input.required_string.as_ref());
    }
    scope.finish();
    Ok(())
}

Clippy output:

error: this expression creates a reference which is immediately dereferenced by the compiler
  --> simple/rust-server-codegen/src/xml_ser.rs:10:27
   |
10 |         inner_writer.data(&input.required_string.as_ref());
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: change this to: `input.required_string.as_ref()`
   |
   = note: `-D clippy::needless-borrow` implied by `-D warnings`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

Relevant code:

https://github.com/awslabs/smithy-rs/blob/e78da5598a2d94d23fd18e30492a20ce3603f844/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/serialize/XmlBindingTraitSerializerGenerator.kt#L292-L292