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Subtyping

Subtyping is implicit and can occur at any stage in type checking or inference. Subtyping in Rust is very restricted and occurs only due to variance with respect to lifetimes and between types with higher ranked lifetimes. If we were to erase lifetimes from types, then the only subtyping would be due to type equality.

Consider the following example: string literals always have 'static lifetime. Nevertheless, we can assign s to t:

# #![allow(unused_variables)]
#fn main() {
fn bar<'a>() {
    let s: &'static str = "hi";
    let t: &'a str = s;
}

#}

Since 'static "lives longer" than 'a, &'static str is a subtype of &'a str.

© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/reference/subtyping.html