I didn't ask
A::<'static>(&PROMOTED), I asked why
&i can be passed to the method that receive
I didn't ask
It's more like
A::<'_>::call(&b, &i);, but I'm not sure if this is explicitly documented anywhere.
(Also whenever I see "equivalent" in Rust documentation, I take it with a large grain of salt.)
If it is, It appears to me the lifetime in the method wouldn't have any restriction that we expect. For example, I would expect
b.call(&i); should restrict that the lifetime of
&i should outlive the lifetime in
A<'a> that is the type of
b, However, with the transformation form
A::<'_>::call(&b, &i);, the restriction will be gone since the call just need to infer the lifetime to make both arguments coerce to the common type.
In general Rust opts for practicality, and I think most programmers expect lifetimes to "just work" wherever possible.
So as a counter-point, here's a case where subtype coercion is possible but isn't automatically applied. If you haven't witnessed the situation and aren't experience enough to understand it should be allowed, you'd probably never figure out to manually force the coercion.
References as arguments have to automatically subtype or reborrow anyway in order for the language to be tolerable, so after thinking on it for a moment, this would be the relevant documentation.
Does it mean, for every method call, the call expression
receiver.method(...)will be transformed to
T::method(&receiver, ...) where
T is the general type if it was.
You haven't realized the covariance matters a lot.
The method signature is a contract that says kinda
call<'a, 'call>(&'call A<'a>, &'a i32), but
So consider a stricter contract
call_strict<'a>(a: &'a A<'a>, v: &'a i32) which is discouraged though. Rust Playground
If you disregard the point on covariance, the second line would also be a surprise for you:
let mut b: A<'static> = A::<'static>(&0); call_strict(&b, &i); // Note: this line is not `call_strict(&'static A<'static>, &'static i)` &mut b; // without the covariance after the instantiation, you couldn't do this
The code compiles fine.
The link shows it also works for
A::<'_>(&0) a type with inferred probably non-static lifetime.
Invariance. Consider to confine the power of covariance that would happen after the instantiation: Rust Playground
struct A<'a>(*mut &'a i32); let mut b: A<'static> = A::<'static>(&mut &0); b.call(&i); // error: `i` does not live long enough
This time, you're probably not stunned any more. Once you have a
A<'static>, the lifetime just stays
'static and everything seems easy to understand. Except for the following working case:
let mut b = A::<'_>(&mut &0); b.call(&i);
Oops, you haven't confined the power of covariance that has happened before the instantiation
let mut b = A::<'_>(&mut &0); b.call(&i); // ^ // ^-----------------↑ lifetimes are tied together according to the call method // but the compiler makes `&'long 0` shorter beforehand
The secret is the covariance on
A<'a>after the instantiation.
The secret is
b.call(&i) is not equivalent to
A::<'static>::call(&b, &i); even though it looks like we want to call method
call on the instance
b that has type
A<'static>, instead, the method call is equivalent to
A::<'_>::call(&b, &i), that is, the compiler will infer a lifetime for
'_ such that
'i: '_, then use the inferred lifetime
'_ to instantiate the method, then covariant happens after this instantiation.
Any transformation isn't happening on the AST level.
Some light testing seems to indicate that lifetimes are inferred but type and
const parameters are not.
The transformation is only used to think of how the lifetime will be inferred for a method call. In other words, in a method call expression, the lifetime parameter of the enclosing type in the method signature is not designated by the receiver expression, the lifetime instead should be re-inferred together with other arguments in order to find out the common lifetime they can be coerced to. Is this understanding right?
Seems that way. I think all lifetimes are replaced with inference variables and then checked against constraints.
Concrete lifetimes don't matter for method resolution either... effectively anyway. (I don't think there's a way to implement for a non-overlapping pair of lifetimes, technically, so it must be this way.)
However, this also remains an issue, the Rust Reference says:
For method calls, the receiver (
selfparameter) can only take advantage of unsized coercions.
In this case, the subtype coercion is expected to happen at the receiver position, how to interpret this point?
I've been confused by that phrasing forever. I dug into the history of the reference and didn't find any explanation in the PR.
(Then the next place the reference falls short is that only slice unsizing takes place for method resolution, and not say,
dyn Trait unsizing.)
Maybe, you may want to post an issue to Rust Reference to fix that error.
I linked to an existing issue (which I also commented on), but PRs have a better hit rate, so probably I should try that at some point. Although I've seen those stall out too sadly.
What Rust really needs is an actual spec.
What Rust really needs is an actual spec.
That would be better, however, Rust is a high-speed evolve language, so it may be too early to have a standard specification. Anyway, it would be meaningful if there was a book that can commonly interpret borrowing checker, lifetime, coercion, and some obscure concept.
which one, specifically?
The book is a primary book. I meant a book that interpret how borrowing checker work, how determine the variance for complex type, and so on.
Other than The Reference, I don't think there is one.
Also the Nomicon.
This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.