[Rust Guide] 10.4. Trait Pt.2 - Traits as Parameters and Return Types, Trait Bounds

The error message you're encountering is due to the fact that Rust's type system requires functions with return types to be consistent and known at compile time. When using Box<dyn Trait> (or similar dynamic dispatch), you can return trait objects from a function, which allows for more flexibility in terms of what concrete types are returned.

In your case, since the function returns either a Tweet or a NewsArticle, both of which implement the Summary trait, but they have different concrete types, you need to use dynamic dispatch (i.e., returning a boxed trait object) instead of static dispatch (returning a specific type).

Here's how you can modify your code:

rust
 
1fn returns_summarizable(flag: bool) -> Box<dyn Summary> {
2    if flag {
3        Box::new(Tweet {
4            username: String::from("horse_ebooks"),
5            content: String::from(
6                "of course, as you probably already know, people",
7            ),
8            reply: false,
9            retweet: false,
10        })
11    } else {
12        Box::new(NewsArticle {
13            headline: String::from("Penguins win the Stanley Cup Championship!"),
14            location: String::from("Pitt
15
16[Read the full article at DEV Community](https://dev.to/someb1oody/rust-guide-104-trait-pt2-traits-as-parameters-and-return-types-trait-bounds-280a)
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