Extend rust reference with a section about subtyping
@@ -3599,6 +3599,26 @@ The notation `&self` is a shorthand for `self: &Self`. In this case,
in the impl, `Self` refers to the value of type `String` that is the
receiver for a call to the method `make_string`.
+## 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`:
+
+```
+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`.
+
# Special traits
Several traits define special evaluation behavior.
