Document variance #186
Document variance #186
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I don't know nearly enough about this to be reviewing it. |
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cc @gankro @nikomatsakis @arielb1 ? |
src/subtyping.md
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| Since `'static` outlives `'a`, `&'static str` is a subtype of `&'a str`. | ||
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| Subtyping also exists for [higher-ranked types]. Replacing a higher ranked | ||
| lifetime with a concrete lifetime produces a subtype. |
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Nit: I don't believe the documentation you linked specifies what a "higher ranked type" is. Not entirely sure what you mean here. Literally just function pointers?
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Function pointers and trait objects
src/subtyping.md
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| // Error: in type `&'static &'a i32`, reference has a longer lifetime than | ||
| // the data it references | ||
| } | ||
| ``` |
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Unclear to me why this merits discussion (is this meaningfully different from adding a note that "the expression must be parse"?)
src/subtyping.md
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| * `std::cell::UnsafeCell<T>` is invariant in `T`. | ||
| * `std::marker::PhantomData<T>` is covariant in `T`. | ||
| * Trait objects are invariant in their type parameters and associated types and | ||
| covariant in their [lifetime bound]. |
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I think this would be better written as a table, but idk
src/subtyping.md
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| The variance of `struct`, `enum`, `union` and tuple types is decided by looking | ||
| at the variance of the types of their fields. If a type parameter is used more | ||
| than once then the more restrictive variance is used. For example the following | ||
| struct is covariant in `'a` and `T` and invariant in `'b` and `U`. |
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This is a bit unclear (and either very subtle or just actually wrong for co+contra). There's basically two interesting cases for multi-occurence:
- If everything agrees on the variance, then that variance is used
- If two different variances would apply, it's invariant
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| | `fn(T) -> ()` | | contravariant | | ||
| | `std::cell::UnsafeCell<T>` | | invariant | | ||
| | `std::marker::PhantomData<T>` | | covariant | | ||
| | `Trait<T> + 'a` | covariant | invariant | |
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Thing I don't know but someone should check: is &'a mut Trait<T> + 'a actually invariant in 'a? (this is implied my the entries for &mut T and Trait<T> + 'a, but I would almost expect the compiler to magic this away.
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It looks like it is invariant, but there is a coercion from &mut (Trait + 'a) to &mut Trait + 'b where 'a: 'b. The same as &mut (Trait + Send) and &mut Trait.
trait A {}
// This compiles (also with `*mut`, or `Box<RefCell>`)
fn coercion<'a, 'b>(x: &'b mut (A + 'static)) -> &'b mut (A + 'a) {
x
}
// This does not
fn not_subtype<'a, 'b, 'c>(x: &'c &'b mut (A + 'static)) -> &'c &'b mut (A + 'a) {
x
}
src/subtyping.md
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| Since `'static` "lives longer" than `'a`, `&'static str` is a subtype of | ||
| `&'a str`. | ||
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| Since `'static` outlives `'a`, `&'static str` is a subtype of `&'a str`. |
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If it's not declared above, could we clarify that 'a here is a standin for "any generic lifetime in scope" (or maybe just "some lifetime"?)
src/subtyping.md
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| Since `'static` outlives `'a`, `&'static str` is a subtype of `&'a str`. | ||
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| Subtyping also exists for [higher-ranked] function pointer and trait object | ||
| types. Replacing a higher ranked lifetime with a concrete lifetime produces a |
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I would rewrite this to "function pointers and trait objects" (current way reads awkward)
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Is the notion of a concrete lifetime really well-defined and/or clear here?
src/subtyping.md
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| Subtyping also exists for [higher-ranked] function pointer and trait object | ||
| types. Replacing a higher ranked lifetime with a concrete lifetime produces a | ||
| subtype. |
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This is an interesting phrasing, I'm not sure if it makes sense to most.
src/subtyping.md
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| // Explicitly f: &(for<'a> Fn(&'a i32) -> &'a i32). | ||
| let f: &(Fn(&i32) -> &i32) = &(|x| x); | ||
| let g: &(Fn(&'static i32) -> &'static i32) = f; | ||
| } |
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Unless I'm mistaken, doctest implicitly wraps the example in main, so you can get rid of these distracting functions.
Also consider renaming f and g to "base" and "subtype" or something more self-descriptive.
src/subtyping.md
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| y: *const T, | ||
| z: UnsafeCell<&'b f64>, | ||
| w: *mut U, | ||
| } |
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This example would benefit from inline comments or outline discussion (I originally missed that U was used twice).
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@gankro Wanna give this a re-review? (You're allowed to say no) |
| Since `'static` outlives the lifetime parameter `'a`, `&'static str` is a | ||
| subtype of `&'a str`. | ||
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| [Higher-ranked] [function pointers] and [trait objects] have another |
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why is this escaped space necessary? otherwise markdown parses it as a reference?
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Markdown would read it as a link with text of "[Higher ranked]" and a link reference to "function pointers" instead of as two separate links.
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Rebase this on master and leave a note saying you've done so and I'll get it merged. |
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rebased, but mdbook isn't building on travis. |
First of a few PRs to improve the documentation of lifetimes in the reference. Based on the documentation in the Nomicon.
cc #9 (rust-lang/rfcs#738)