Builder pattern for formatting strings from data: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6caf3a78d75d8ecbfec6a277b57829ab

builder.rs

fn main() {
    let topic = TopicBuilder::new("lift", "report")
        .with_subdevices(&["motor"])
        .build();
    println!("Topic: {:#?}", &topic);
}

#[derive(Debug)]
pub struct Topic {
    display: String,
    metadata: TopicMetadata,
}

impl std::fmt::Display for Topic {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.display)
    }
}

impl PartialEq for Topic {
    fn eq(&self, other: &Self) -> bool {
        self.metadata == other.metadata
    }
}

#[derive(Default, Debug, PartialEq, Clone)]
struct TopicMetadata {
    name: String,
    action: String,
    subdevices: Vec<String>,
    subposition: Option<String>,
}

#[derive(Debug)]
pub struct TopicBuilder {
    metadata: TopicMetadata,
}

impl TopicBuilder {
    pub fn new(name: &str, action: &str) -> Self {
        Self {
            metadata: TopicMetadata {
                name: name.to_string(),
                action: action.to_string(),
                ..Default::default()
            },
        }
    }

    pub fn with_subdevices(mut self, subdevices: &[&str]) -> Self {
        let subdevices: Vec<String> = subdevices.into_iter().map(|x| x.to_string()).collect();
        self.metadata.subdevices.extend_from_slice(&subdevices);
        self
    }

    pub fn build(self) -> Topic {
        let Self { metadata } = self;

        let mut sections = vec![metadata.name.to_string()];

        for device in &metadata.subdevices {
            sections.push(device.to_string());
        }

        if let Some(ref subposition) = metadata.subposition {
            sections.push(subposition.to_string());
        }

        sections.push(metadata.action.to_string());

        let display = sections.join("/");

        Topic { display, metadata }
    }
}