Rust declare first assign later pattern - rust

Question

I have a bi-directional grpc stream that acts as bridge to a kafka cluster. When the stream is first initialised, I was to create the kafka consumer and start using it.
To do so, I thought of initialising an empty consumer, waiting for the first input, then assigning a created consumer to an empty one. I tried to do so by following the pattern here.
https://doc.rust-lang.org/rust-by-example/variable_bindings/declare.html
Rust is throwing a possibly-unitialized variable error, is this because it is being initialised in an asynchronous stream?
use std::pin::Pin;
use futures::{Stream, StreamExt};
use kafka::consumer::{Consumer, FetchOffset, GroupOffsetStorage};
use tonic::transport::Server;
use tonic::{Request, Response, Status};
use bridge::kafka_stream_server::{KafkaStream, KafkaStreamServer};
use bridge::{KafkaResponse, PublishRequest};
pub mod bridge {
tonic::include_proto!("bridge"); // The string specified here must match the proto package name
}
#[derive(Default)]
pub struct KafkaStreamService {}
pub fn create_kafka_consumer(topic: String) -> Consumer {
Consumer::from_hosts(vec!["localhost:9092".to_owned()])
.with_topic(topic.to_owned())
.with_fallback_offset(FetchOffset::Latest)
.with_group("".to_owned())
.with_offset_storage(GroupOffsetStorage::Kafka)
.create()
.unwrap()
}
#[tonic::async_trait]
impl KafkaStream for KafkaStreamService {
type SubscribeStream =
Pin<Box<dyn Stream<Item = Result<KafkaResponse, Status>> + Send + Sync + 'static>>;
async fn subscribe(
&self,
request: Request<tonic::Streaming<PublishRequest>>,
) -> Result<Response<Self::SubscribeStream>, Status> {
println!("Initiated stream!");
let mut stream = request.into_inner();
let mut consumer_created_flag: bool = false;
let consumer: Consumer; //declared here
let output = async_stream::try_stream! {
while let Some(publication) = stream.next().await {
let message = publication?;
let topic = message.topic.clone();
if consumer_created_flag == false {
consumer = create_kafka_consumer(topic); //error occurs here
consumer_created_flag = true;
}
let reply = bridge::KafkaResponse {
content: format!("Hello {}!", "world"),
};
yield reply.clone();
}
};
Ok(Response::new(Box::pin(output) as Self::SubscribeStream))
}
}
#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
let addr = "[::1]:50051".parse().unwrap();
println!("KafkaService listening on: {}", addr);
let svc = KafkaStreamServer::new(KafkaStreamService::default());
Server::builder().add_service(svc).serve(addr).await?;
Ok(())
}
EDIT: verbose error as requested:
error[E0381]: use of possibly-uninitialized variable: `consumer`
--> src/server.rs:42:22
|
42 | let output = async_stream::try_stream! {
| ______________________^
43 | | while let Some(publication) = stream.next().await {
44 | | let message = publication?;
45 | | let topic = message.topic.clone();
46 | | if consumer_created_flag == false {
47 | | consumer = create_kafka_consumer(topic);
| | -------- use occurs due to use in generator
... |
54 | | }
55 | | };
| |_________^ use of possibly-uninitialized `consumer`
|
= note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

Answer 1

The declare first pattern only works with basic control flow (if, match, {}, etc). It falls apart when referenced or moved into another object, like an async block or a closure:
fn main() {
let val: i32;
let func = move || {
val = 5;
};
}
error[E0594]: cannot assign to `val`, as it is not declared as mutable
--> src/main.rs:4:9
|
2 | let val: i32;
| --- help: consider changing this to be mutable: `mut val`
3 | let func = move || {
4 | val = 5;
| ^^^^^^^ cannot assign
error[E0381]: use of possibly-uninitialized variable: `val`
--> src/main.rs:3:16
|
3 | let func = move || {
| ^^^^^^^ use of possibly-uninitialized `val`
4 | val = 5;
| --- use occurs due to use in closure
A potential fix is to move its declaration into the try_stream! macro:
let output = async_stream::try_stream! {
let mut consumer_created_flag: bool = false;
let consumer: Consumer;
while let Some(publication) = stream.next().await {
let message = publication?;
let topic = message.topic.clone();
if consumer_created_flag == false {
consumer = create_kafka_consumer(topic);
consumer_created_flag = true;
}
let reply = KafkaResponse {
content: format!("Hello {}!", "world"),
};
yield reply.clone();
}
};
However, this causes a new error because you're potentially assigning to it twice (the compiler doesn't know that consumer_created_flag is guarding it):
error[E0384]: cannot assign twice to immutable variable `consumer`
--> src\lib.rs:1348:21
|
44 | let consumer: Consumer; //declared here
| -------- help: make this binding mutable: `mut consumer`
...
49 | consumer = create_kafka_consumer(topic); //error occurs here
| ^^^^^^^^ cannot assign twice to immutable variable
Fortunately a quick fix is to simply make consumer mutable. And then the only thing the compiler complains about is that it is unused, but I figure there's a reason you've put it there.