Errata print01-ch01-lst11-variance-explanation

_errata/print01-ch01-lst11-variance-explanation.md

@@ -0,0 +1,44 @@
+---
+chapter: 1
+page: 17
+kind: inaccuracy
+reporter: Yoav Orot
+date: 2025-02-17
+---
+
+In the first paragraph after the `NOTE` section, where the text describes
+listing 1-11, it says:
+
+> At 1, the compiler must determine what lifetime the lifetime parameter(s)
+> should be set to. If there are two lifetimes, `'a` is set to the to-be-
+> determined lifetime of the borrow of `s`, and `'b` is set to `'static` since
+> that’s the lifetime of the provided string `"hello"`. If there is just one
+> lifetime `'a`, the compiler infers that that lifetime must be `'static`.
+
+It should be noted that lifetime `'b` (or `'a` in the single lifetime scenario)
+is not set to `'static`, but to a coerced supertype of `'static` that is
+computed for the reference held by the `s` variable.
+
+Then, in the third paragraph after the `NOTE` section:
+
+> While `&'static str` can in general be shortened to any `&'a str` (`&'a T` is
+> covariant in `'a`), here it’s behind a `&mut T`, which is invariant in `T`.
+
+We should, as above, note that lifetime `'a` is not set to be `'static`, but to
+the supertype that was computed for the reference in `s`.
+
+If, in the single lifetime scenario, lifetime `'a` were actually set to
+`'static` (e.g. by writing `let mut s: &'static _ = "hello";`), listing 11 would
+not compile, even without the `println!("{}", s);` line, as the compiler would
+have complained that the `s` variable is dropped at the end of the function
+while still being borrowed.
+
+However, in this case, as part of Rust's lifetime subtyping, the lifetime of the
+borrowed `str` reference in the `s` variable is shortened only until the last
+usage of `s`, which is `println!("{}", s);`. This sets the lifetime of `'a` to
+include the aforementioned `println` line, which in turn leads to the mutable
+borrowing of the `s` variable (in `s: &mut s`) using the same lifetime duration.
+
+In other words, if `MutStr` had only one lifetime, instead of two, the `s`
+variable would have been mutably borrowed until the end of its own variable's
+lifetime.