Sometimes when writing a function with multiple input lifetimes, I encounter an error like:
error[E0623]: lifetime mismatch
--> src/main.rs:4:27
|
3 | fn use_ref_ref<'a, 'b>(_ra: &'a Box<()>, _rb: &'a mut &'b mut ()) {
| ----------- ----------
| |
| these two types are declared with different lifetimes...
4 | use_same_ref_ref(_ra, _rb);
| ^^^ ...but data from `_ra` flows into `_rb` here
Note the phrasing: "...but data from _ra flows into _rb here". The code to produce that looks like:
fn use_same_ref_ref<'c>(_ra: &'c Box<()>, _rb: &'c mut &'c mut ()) {}
fn use_ref_ref<'a, 'b>(_ra: &'a Box<()>, _rb: &'a mut &'b mut ()) {
use_same_ref_ref(_ra, _rb);
}
fn main() {
let mut ra = Box::new(());
let mut rb = &mut ();
use_ref_ref(&mut ra, &mut rb);
}
The underlying reason for this error is explained extremely well in Why does linking lifetimes matter only with mutable references? I am not asking about the reason this is an error! I would like to know why the error message talks about data "flowing" from one reference into another, and how to interpret that.
The first few times I read it I found it extremely confusing. It even occurs with a single reference (as per the linked question) which is even more confusing ("data from x flows into x" - well, sure it does, who cares?).
What is the link between the compiler's "flowing" explanation and the aliasing constraints being enforced here?
