For now, this reference is a best-effort document. We strive for validity and completeness, but are not yet there. In the future, the docs and lang teams will work together to figure out how best to do this. Until then, this is a best-effort attempt. If you find something wrong or missing, file an issue or send in a pull request.


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.