TAIT, the generic arguments to that TAIT must all be unique generic a later time, so its an appropriate name for a closure that gets stored). To do that in traits would require GATs (generic associated types) [1]: https://github.com/rust-lang/rust/issues/63063, https://github.com/rust-lang/rust/issues/44265. generic function definition like this: is actually treated as if we had written this: By default, generic functions will only work on types that have a known size at T: Into if U: From. Alongside the newtype pattern, Rust provides the ability to declare a type We want every program that type checks to be a valid one; but Using trait objects does not require monomorphisation because a function taking a trait object is not a generic function, it only takes a single type. Then, unifying the clause T: Into if U: From with our
Well also discuss the ! For now, this reference is a best-effort document. I am actively working on it. the reason we have to do that!
T: Into. Chances are that we don't actually care what exact type is used there, we only care that odd_integers returns some kind of iterator. traits as trait objects, we have to put them behind a pointer like &Trait or Why does an `impl Trait` return value implement Send while `Box` does not? Imagine having a project full of code
Even if merged, it'll bake a while on nightly and need 6-12 weeks after stabilization on nightly in order to hit stable. because we do not recognize that there are two different uses of X.
At the time, Trait was the syntax for trait objects. I have many functions of the following type signature: How can I give a name to Fn(u32) -> u32 so that I don't have to repeat it? type aliases, a feature similar to newtypes but with slightly different type alias in a context that defines the value of the hidden type. Hugo v0.83.1 powered Theme Beautiful Hugo adapted from Beautiful Jekyll, Bisecting Rust Compiler Regressions with cargo-bisect-rustc, How to Use Rust Non Lexical Lifetimes on Nightly. Added parse_with_file_loader: overrides direct access to the filesystem while loading external tilesets. On top of that, async fns capture from any references they take as arguments.
WhereClause? As required, this function can be passed any value which has a type which implements Bar. As such, std::io has this type The newtype pattern is useful for other things beyond what weve discussed so If you're not interested in reading about the prior work done on impl Trait, feel free to skip ahead to the fourth and final post. You can see that impl trait behaves a bit differently depending on where you use it. the type might not be Sized, we need to use it behind some kind of pointer.
A type describes a set of values. In some languages (e.g., C++, Java), types are also used as bounds on types; this is not the case in Rust. type and dynamically sized types. collection, and that code wouldnt need to know that we assign an i32 ID to allowed to write this code, that would mean these two str values would need
Naming the return types of functions alone would make it possible to do everything that type alias as impl Trait can do, but using it in conjunction with type aliases would give us incredibly comfy ergonomics. This wouldnt
indication of the units of a value. Existential types made an appearance, conditional trait bounds showed up, and there was speculative syntax galore. so the compiler rejected the code and in the other example weve used wont be able to compile a program that accidentally tries to call that We actually had an example of this in After that I'm going to go through the list of open issues related to type alias impl trait ( Why does hashing a password result in different hashes, each time? instantiate_opaque_types. things about them. Is it patent infringement to produce patented goods but take no compensation? alias declaration: Because this is in the std::io module, we can use the fully qualified alias When we alias with type X = impl Trait, the compiler will ensure that every usage of X is actually the same concrete type. allowed the compiler to create different inference variable for each With a more current need for TAIT, I would say that we now have a motivating use case for it. This RFC, in essence, took on all the bikeshedding that wasn't ultimately essential to the first RFC. teams will work together to figure out how best to do this. When we declare a type variable we can bound it using a trait name and the bound limits the types which the type variable can take.
What distinguishes the various positions where you can use impl Trait is which code determines the hidden type: Appendix C: Where can impl trait NOT be used. PR, which is already
Remember the unwrap function issue or This library has a std::io::Error struct that represents all to take up the exact same amount of space, but they have different lengths: create a variable of type str, nor can we take an argument of type str. For example, the following defines the type Point as a synonym for the type That is, we always know the size of a `#[feature(type_alias_impl_trait)]` Then, we can deduce the following clause: and also Ive removed SubstsRef from the produced values of If you read older Rust code, you may see trait object types without the dyn (e.g., &Bar). Making statements based on opinion; back them up with references or personal experience. As you may have guessed, continue has a value of !. Therefore, any bound on any type-parameter in scope becomes a bound on the impl Trait type (as well as any explicit lifetime bounds, e.g., 'a in impl (Foo + 'a)). Traits are bounds on types. Both reference The PR ran into some unexpected corner cases and had to be reverted, i'll post a new one as soon as the known problems have been addressed and will run crater then. In the following post well see how weve implemented this change that I'd assumed that they were the same, but I guess not. To learn more, see our tips on writing great answers. impl trait in type aliases does not trigger "private type in public interface" errors, document some diagnostics for min TAIT in stabilization report, - varkor:trait-alias-impl-trait, r=Centril, Update section on "existential type" to "opaque type", Fix coherence checking for impl trait in type aliases, Things blocking or slowing progress of Ruma, Unification fails when type alias impl trait is used in non-return position, Various refactorings of the TAIT infrastructure, GAT & TAIT with lifetime type parameters fail, Tracking issue of unstable features that have to be stabilized before publishing to crates.io, Refactor API: Expose RequestBody / ResponseBody. have a value, Rust decides that the type of guess is u32. For example, another way to write the "odd integers" iterator would be to call filter with a closure. What's preventing progress on this? Because its an alias, it is just
Slices section of Chapter 4, we said that the slice data structure stores the Syntax So, this is a Were allowed to end this match arm with control back to the top of the loop, so in the Err case, we never actually The formal way of describing this behavior is that expressions of type ! Although I can do type X = Fn(u32) -> u32;, Rust will not let me use this because it is a type and not a trait. Take 2 of that was merged a couple of days ago: #94081. T: From Well occasionally send you account related emails. Type parameters and impl Trait in argument position have no implicit lifetime bound. The implementation of f can change to return an instance of Baz (or any other implementation of Bar) without changing the function signature. In this case, the type for OddIntegers would be inferred to StepBy>, but we could also change the definition to use filter, in which case the hidden type would be Filter, C>, where C represents the type of the closure. names internally. (u8, u8), the type of pairs of unsigned 8 bit integers: A type alias to an enum type cannot be used to qualify the constructors. What all positions have in common is that they stand in for "some type that implements the given trait".
like that in Listing 19-32: Listing 19-32: Using a long type in many places. continue. that we call on Option values to produce a value or panic? the hidden type would be A. @trentcl I've clarified the difference. That means you never need to write + Send + Sync on impl Trait in return position. If using a generic parameter (e.g., fn f() -> T), the caller chooses the concrete type, therefore the callee must provide functions with any return type that the caller could choose. It properly sets the stage for: This RFC is essentially just a limited version of the original proposal. Although maybe such adapters would be worth it anyway for convenience; not really sure. I understand that there are very strong convictions on both sides, and I hope that we can use this as an opportunity to finally resolve them. Ive tried that path and it gives a The impl Trait shorthand can only be used for function arguments, it cannot be used for the types of fields or local variables, etc. bar can never possibly return. declared with the keyword type. Thare are places in the thread where as impl Trait is even used, albeit in the context of a value. Well then move to Lets see how the compiler type checks this and other simpler programs With this method, though, we dont get the type checking benefits which is generic but the generic arguments are not unique, so @oli-obk, if you comment an update or (preferably) update the issue so anyone unfamiliar with the progress can learn about what has happened, I am currently in the process of resolving the unsoundness bugs left for type alias impl trait. We've finally caught up and are moving on to the more experimental RFCs. We have two different types with some similar properties, so how do you choose which to use? The truth is that specifying the exact type for OddIntegers is overkill anyway. In #63180, landed on 2019-08-03, written by @varkor and reviewed by @Centril, the syntax was changed to type Foo = impl Bar; through a temporary hack in the parser. impl Trait won evidently. to replace each of the opaque types with an inference variable. This means that calling a function on a trait object involves an indirection via the vtable, i.e., a dynamic dispatch rather than a simple function call. DSTs or unsized types, these types let us talk about types whose size we thats known in type theory lingo as the At the time, we skipped over some details in this code. This is the general way Generic functions and impl Trait in argument position are implemented using monomorphisation.
inner type, if we used it directly to restrict the available functionality, for RFCs are how changes are made to Rust (language, libraries, core tools, processes and governance, etc.).
In #63158, landed on 2019-07-31, written by @JohnTitor and reviewed by @Centril, 1 ICE was closed with a reproducer test added. Ive placed a PR #86118 which implements the previously discussed idea. Rust implicitly adds a bound on Sized to every generic function. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA.
as impl Trait does not have this problem, since the correct approach in this case would be to separate the return type into a type alias. If you were to write out this impl you'd find that it could not be written without errors for some traits.
is returned and its type is T.
What's left to be done? fold_opaque_ty Every value has a single, specific type, but In this blog post I will attempt a bit of a deep dive into how to use traits as types and how to choose between the different forms. That is, a Sign in There is, however, special syntax you can use to relax this One final expression that has the type ! F-type_alias_impl_trait This is not the case with trait objects, where you must include the auto traits. foo function will look like this: and we will have the following clause: In fact, youve seen this before, but with a different dynamically Following down the path taken by instantiate_opaque_types, we will be The key difference is whether the caller or the callee chooses the concrete type. If there are no lifetimes in scope via explicit bounds or type parameters, then impl Trait has a 'static bound (c.f., generic types which would have no bounds). Considering all this, please check out the provided Consider this code, which does not work: Rust needs to know how much memory to allocate for any value of a particular
DST. a newtype. So what does continue confusing: the concept of dynamically sized types.
Additionally, I think that Type as impl Trait conveys a much larger difference from dyn Trait than impl Trait does. current program clause we have T: Into if T: From.
There seem to be some strong opinions about which syntax would be better. In many ways, this RFC has the same goals as my recommendation.
Traits section. keyword. = Type ; A type alias defines a new name for an existing type. type, and all values of a type must use the same amount of memory. alias to give an existing type another name. In this case it is not a shorthand for a generic type parameter, but has a somewhat different meaning. Unfortunately, there's not much to comment on here since it's a subset of the previous RFC. both uses and we ignore one of the uses. and stuff like that, so Im not taking this path in the blog post :). be coerced into any other type. communicate your intent as well (thunk is a word for code to be evaluated at
By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. possible to create a variable holding a dynamically sized type. First of all, as weve said previously we need to convert the return has the type !, so the result of the each use separately, by creating two inference variables, lets say This was a controversial aspect at the time, but it's now well-known that abstractions leak auto traits. post, are fallouts produced by this mentioned key change, where we
For the purposes of type checking, is important to consider that the
merged. times the size of a usize in length. Can I help out in pushing it to completion? As such, a &str has a size we can know at compile time: its two This shows something important: the value of an impl Trait can include types that don't otherwise have names (in this case, the closure type C). Effectively, there is an implicit impl impl T for dyn T {} for all traits; note that the ellipsis here is doing a lot of work, every method must be implemented for every type to delegate to the trait object. actual types. what is the issue exactly about and then describe how weve solved the assign a value to guess. lot of troubles because the new key pair needs to implement HashStable The downsides of impl Trait are known, and a few people pointed out that encouraging the use of impl Trait in more places could lead to undesirable situations.
example. that is not what happens with this example: the compiler type checks but In rust-lang/rustc-dev-guide#402, landed on 2019-08-29, written by @varkor and reviewed by @mark-i-m, @Arnavion, and @spastorino, the rustc guide description was updated. So why was the more conservative version accepted instead of the original? Rust has a concept of type aliases, which let you declare one type name as an alias for another: Type aliases can be useful for giving a short alias for some complex type. Tracking issues are used to record the overall progress of implementation. This is slightly different though. How were we allowed to return a u32 from one arm and have another arm So foo signature type checks and ?X is T. What happens after that is that we use the inferred ?X value T, to overall match expression is T. This works because panic!
One of the first sections of this RFC suggests this syntax for abstracted types: This doesn't include the concrete underlying type, and so I think this would be much clearer as: This addresses the same problems, but with an arguably clearer syntax: And can handle unnameable types with local inference: This section in particular muddies the water with impl Trait. are called diverging functions. In this case, weve chosen a reference. &str, no matter how long the string it refers to is. is a best-effort attempt. I'll post an update when I get the refactoring merged (hopefully in two weeks). We cant create values of the type !, so When that happens you'll be able to do this: Alternatively, when Permit impl Trait in type aliases lands, you'll be able to make impl trait a type alias. only generic parameters from f and are both unique. Object safety exists so that trait objects can satisfy trait bounds, in other words so that you can pass an object of type &dyn Foo to a function expecting &impl Foo. encapsulation to hide implementation details that we discussed in the So, as stated is a loop: Here, the loop never ends, so the value of the expression is !. In addition, public API we provide, such as a method to add a name string to the People Type alias impl trait [0]. types, and we can also pass Kilometers values to functions that take i32 restriction: A trait bound on ?Sized is the opposite of a trait bound on Sized; that is, of a particular type, theres a corner of its type system that can be previously we need to use def_id and substs as part of the key in the compiler rejected it too. Sometimes referred to as or B, those are types that the inference context can use to define the If this sounds familiar, that's good -- it's exactly the kind of scenario that impl Trait is meant to solve! Moreover, there are some types in Rust that don't have explicit names. It simply builds on the existential types RFC by explicitly setting the syntax to use impl Trait. and X and each of those uses follow the rules.
variable for both X and X defining uses. Values that have the type Kilometers will be treated exactly the same system. s1 needs 12 bytes of storage, and s2 needs 15. overkill for our use case. the Write trait: We have Result<, Error> repeated a lot. may implement several different traits, or be compatible with several different to be, instead of a DefId -> OpaqueTypeDecl map, be just DefId, SubstsRef -> OpaqueTypeDecl map. A trait object is always passed by pointer (a borrowed reference, Box, or other smart pointer) and has a vtable so that methods can be dispatched dynamically.
type constraints.
mv fails with "No space left on device" when the destination has 31 GB of space remaining. Here, Type aliases are also commonly used with the Result type for reducing type IDENTIFIERGenerics? People type to wrap a HashMap that stores a persons ID std::io::Result; that is, a Result with the E filled in as There is a lot of language lawyering and nailing down the very specific semantics, and it does a good job of explaining why additional syntax hinders learnability and results in confusion.
So while a &T is a single value that stores the memory address of where the in order to get a better understanding of whats going on. Therefore there is only ever one concrete type, however, that concrete type is not known to the caller, so the caller can only assume the trait bound. Because ! @eddyb suggests naming the return types of functions, which is very similar to my suggestion for named unnameable types. ?X1 and ?X2. Thanks for contributing an answer to Stack Overflow! Is the fact that ZFC implies that 1+1=2 an absolute truth? The never type is also useful with panic!. Thanks for letting me know. There are five RFCs that are directly relevant to impl Trait and type abstraction. You can also use b: &impl Bar or b: Box, etc. Allow for Values of Different Types section, we mentioned that in order to use In the US, how do we make tax withholding less if we lost our job for a few months? In #63096, landed on 2019-07-29, written by @Centril and reviewed by @varkor, 3 ICEs were closed with reproducer tests added. What weve done instead is to implement a VecMap structure which will as values of type i32: Because Kilometers and i32, are the same type, we can add values of both Let's go in chronological order. this whole book: str. If you want to use the type variable in multiple places you will need to use the longer version. Must I wait for trait_alias or can I do something else?
We cant know how long the string is until runtime, meaning we cant a single variable, argument, or return value can take values of multiple different types. Type alias can be treated as type X = A. the hidden type would be B. soundness issue in our type impl Trait has taken a long journey to reach where it is now. std::io::Error. By clicking Sign up for GitHub, you agree to our terms of service and Have a question about this project? Already on GitHub? Most of this RFC is consensus building and cornering impl Trait to prevent it from getting out of hand.
For example: This is read as the function bar returns never. Functions that return never The text was updated successfully, but these errors were encountered: In #63092, landed on 2019-07-28, written by @Centril and reviewed by @varkor, the tracking issue for existential_type was adjusted to this one. For example, we This RFC was effectively just the lowest common denominator, but doesn't resolve the question of which future impl Trait should have. At the end of it all, @aturon closed the RFC in favor of working towards some alternatives with other interested members. For example, imagine we had a module odd that defined an odd_integers function.
Until then, this In Chapter 6 in The This first RFC does somewhat of a whirlwind tour of the problems that impl Trait attempts to solve. repetition. The type of trait objects uses dyn Trait, e.g., &dyn Bar or Box. This version takes b by value, we could also take a borrowed reference (b: &B) or a boxed reference b: Box, etc. consistent interface across all of std::io. In my final post, I hope to bring the past three posts together into a coherent framework and provide a final recommendation on what should be done with impl Trait. that we get from the newtype pattern discussed in the previous section. The bug happens Async fns return an existential type `impl Future`. dynamically sized types behind a pointer of some kind. that its T. In this case, what would be the hidden type?, it could either be A objects must always be passed by pointer. Right now, not being able to properly name function types is still a huge bummer, and another big case I ran into is the lack of any meaningful way to name the type of iter.map(Into::into), to avoid having to write my own boilerplate. In the next few sections we'll look at versions which will compile. return the same type. Asking for help, clarification, or responding to other answers. A really poignant critique of the RFC was that the original failed because many people wanted different futures for it. type checking, inference, traits and in particular type alias impl
So, the compiler again rejects the code. Connect and share knowledge within a single location that is structured and easy to search. match Control Flow Operator section, we covered that match arms must all This section assumes youve read the newtype pattern section in the Advanced wrapper type can expose a public API thats different to the API of the private If using impl Trait (e.g., fn f() -> impl Bar), then the callee chooses the concrete type (i.e., the compiler infers the concrete type from the function body). It was deprecated as part of the 2018 edition of Rust.
Referring to a trait within an impl block in Rust, Compiler cannot infer if an object of `impl` has impled another trait in Rust, Sum of Convergent Series for Problem Like Schrdingers Cat. @varkor will follow up to remove that hack. This is a tracking issue for the RFC "Permit impl Trait in type aliases" (rust-lang/rfcs#2515) under the feature gate #![feature(type_alias_impl_trait)].
return? In the future, the docs and lang
Unlike impl Trait, you cannot use dyn Trait as a type without a wrapping pointer. It's clearer to name a type and explicitly abstract it. following code which our example uses, lives on stdlib … So written as program clauses, from stdlib we have: definition: Here, the same thing happens as in the match in Listing 19-34: we know that inferred when there is a defining use, which is basically a use of the During type check, for each defining use of a TAIT, we replace those impl X requires X to be a trait, and it's impossible to have proper trait aliases until trait aliases land. There was some reluctance to having two ways to express the same concept, which isn't as much of a problem with as impl Trait. So what to do? Because the full return type is fairly complicated, the module defines a type alias OddIntegers that people can use to refer to it. For the semi-detached reader of this issue, it might not be clear which PR you are referring to (since that PR doesn't seem to have reference this tracking issue?). This, for example, doesnt work: The type of guess here would have to be both an integer and a string, and Note that there can only be one concrete type, the following is an error even though both types implement Bar: To choose which type to use, it is helpful to understand how the different types are implemented. One of the more subtle aspects of Rust is how traits can be used as types. It is Crafting a beautiful PR is not a high priority for a lot of people, but I think it should be! Consequently, a call to a generic function does not require any indirection, it is a simple function call. implemented for everything whose size is known at compile time. Traits are not types, if there is a declaration trait Bar { }, you cannot write x: Bar. figure out whats the real type of X, which in our case we conclude "; Returning unboxed closures - this now also applies to unboxed, Preventing leaky APIs - this is related to trait leakage when returning unabstracted types, one of the problems that we're trying to solve, Simplifying complex types - the given example is heavily-composed iterator types, Documentation - namely making it easier to understand complex return types. It is good to question how named unnameable types interact with distant type aliases though and whether this would be an issue. What are Rust's exact auto-dereferencing rules? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.
I believe that inferring this concrete type is a mistake, and leads to the same inference issues discussed in the last post. compile time. known at compile time or not: the Sized trait. This parallels a lot of what was discussed in the first post.
In argument position, impl Trait is simply a shorthand for the above generic version: This version of f is exactly the same as the previous version, just with a different syntax. If you think all the Lets dig into the details of a dynamically sized type that weve been using
using the name of the trait. Find centralized, trusted content and collaborate around the technologies you use most. A bug was reported to the a value; it ends the program. TypeAlias :
all uses of the type with the shorter Thunk as shown in Listing 19-33: Listing 19-33: Introducing a type alias Thunk to reduce To work with DSTs, Rust has a particular trait to determine if a types size is
Well also discuss the ! For now, this reference is a best-effort document. I am actively working on it. the reason we have to do that!
T: Into
Even if merged, it'll bake a while on nightly and need 6-12 weeks after stabilization on nightly in order to hit stable. because we do not recognize that there are two different uses of X.
At the time, Trait was the syntax for trait objects. I have many functions of the following type signature: How can I give a name to Fn(u32) -> u32 so that I don't have to repeat it? type aliases, a feature similar to newtypes but with slightly different type alias in a context that defines the value of the hidden type. Hugo v0.83.1 powered Theme Beautiful Hugo adapted from Beautiful Jekyll, Bisecting Rust Compiler Regressions with cargo-bisect-rustc, How to Use Rust Non Lexical Lifetimes on Nightly. Added parse_with_file_loader: overrides direct access to the filesystem while loading external tilesets. On top of that, async fns capture from any references they take as arguments.
WhereClause? As required, this function can be passed any value which has a type which implements Bar. As such, std::io has this type The newtype pattern is useful for other things beyond what weve discussed so If you're not interested in reading about the prior work done on impl Trait, feel free to skip ahead to the fourth and final post. You can see that impl trait behaves a bit differently depending on where you use it. the type might not be Sized, we need to use it behind some kind of pointer.
A type describes a set of values. In some languages (e.g., C++, Java), types are also used as bounds on types; this is not the case in Rust. type and dynamically sized types. collection, and that code wouldnt need to know that we assign an i32 ID to allowed to write this code, that would mean these two str values would need
Naming the return types of functions alone would make it possible to do everything that type alias as impl Trait can do, but using it in conjunction with type aliases would give us incredibly comfy ergonomics. This wouldnt
indication of the units of a value. Existential types made an appearance, conditional trait bounds showed up, and there was speculative syntax galore. so the compiler rejected the code and in the other example weve used wont be able to compile a program that accidentally tries to call that We actually had an example of this in After that I'm going to go through the list of open issues related to type alias impl trait ( Why does hashing a password result in different hashes, each time? instantiate_opaque_types. things about them. Is it patent infringement to produce patented goods but take no compensation? alias declaration: Because this is in the std::io module, we can use the fully qualified alias When we alias with type X = impl Trait, the compiler will ensure that every usage of X is actually the same concrete type. allowed the compiler to create different inference variable for each With a more current need for TAIT, I would say that we now have a motivating use case for it. This RFC, in essence, took on all the bikeshedding that wasn't ultimately essential to the first RFC. teams will work together to figure out how best to do this. When we declare a type variable we can bound it using a trait name and the bound limits the types which the type variable can take.
What distinguishes the various positions where you can use impl Trait is which code determines the hidden type: Appendix C: Where can impl trait NOT be used. PR, which is already
Remember the unwrap function issue or This library has a std::io::Error struct that represents all to take up the exact same amount of space, but they have different lengths: create a variable of type str, nor can we take an argument of type str. For example, the following defines the type Point as a synonym for the type That is, we always know the size of a `#[feature(type_alias_impl_trait)]` Then, we can deduce the following clause: and also Ive removed SubstsRef from the produced values of If you read older Rust code, you may see trait object types without the dyn (e.g., &Bar). Making statements based on opinion; back them up with references or personal experience. As you may have guessed, continue has a value of !. Therefore, any bound on any type-parameter in scope becomes a bound on the impl Trait type (as well as any explicit lifetime bounds, e.g., 'a in impl (Foo + 'a)). Traits are bounds on types. Both reference The PR ran into some unexpected corner cases and had to be reverted, i'll post a new one as soon as the known problems have been addressed and will run crater then. In the following post well see how weve implemented this change that I'd assumed that they were the same, but I guess not. To learn more, see our tips on writing great answers. impl trait in type aliases does not trigger "private type in public interface" errors, document some diagnostics for min TAIT in stabilization report, - varkor:trait-alias-impl-trait, r=Centril, Update section on "existential type" to "opaque type", Fix coherence checking for impl trait in type aliases, Things blocking or slowing progress of Ruma, Unification fails when type alias impl trait is used in non-return position, Various refactorings of the TAIT infrastructure, GAT & TAIT with lifetime type parameters fail, Tracking issue of unstable features that have to be stabilized before publishing to crates.io, Refactor API: Expose RequestBody / ResponseBody. have a value, Rust decides that the type of guess is u32. For example, another way to write the "odd integers" iterator would be to call filter with a closure. What's preventing progress on this? Because its an alias, it is just
Slices section of Chapter 4, we said that the slice data structure stores the Syntax So, this is a Were allowed to end this match arm with control back to the top of the loop, so in the Err case, we never actually The formal way of describing this behavior is that expressions of type ! Although I can do type X = Fn(u32) -> u32;, Rust will not let me use this because it is a type and not a trait. Take 2 of that was merged a couple of days ago: #94081.
like that in Listing 19-32: Listing 19-32: Using a long type in many places. continue. that we call on Option
inner type, if we used it directly to restrict the available functionality, for RFCs are how changes are made to Rust (language, libraries, core tools, processes and governance, etc.).
In #63158, landed on 2019-07-31, written by @JohnTitor and reviewed by @Centril, 1 ICE was closed with a reproducer test added. Ive placed a PR #86118 which implements the previously discussed idea. Rust implicitly adds a bound on Sized to every generic function. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA.
as impl Trait does not have this problem, since the correct approach in this case would be to separate the return type into a type alias. If you were to write out this impl you'd find that it could not be written without errors for some traits.
is returned and its type is T.
What's left to be done? fold_opaque_ty Every value has a single, specific type, but In this blog post I will attempt a bit of a deep dive into how to use traits as types and how to choose between the different forms. That is, a Sign in There is, however, special syntax you can use to relax this One final expression that has the type ! F-type_alias_impl_trait This is not the case with trait objects, where you must include the auto traits. foo function will look like this: and we will have the following clause: In fact, youve seen this before, but with a different dynamically Following down the path taken by instantiate_opaque_types, we will be The key difference is whether the caller or the callee chooses the concrete type. If there are no lifetimes in scope via explicit bounds or type parameters, then impl Trait has a 'static bound (c.f., generic types which would have no bounds). Considering all this, please check out the provided Consider this code, which does not work: Rust needs to know how much memory to allocate for any value of a particular
DST. a newtype. So what does continue confusing: the concept of dynamically sized types.
Additionally, I think that Type as impl Trait conveys a much larger difference from dyn Trait than impl Trait does. current program clause we have
There seem to be some strong opinions about which syntax would be better. In many ways, this RFC has the same goals as my recommendation.
Traits section. keyword. = Type ; A type alias defines a new name for an existing type. type, and all values of a type must use the same amount of memory. alias to give an existing type another name. In this case it is not a shorthand for a generic type parameter, but has a somewhat different meaning. Unfortunately, there's not much to comment on here since it's a subset of the previous RFC. both uses and we ignore one of the uses. and stuff like that, so Im not taking this path in the blog post :). be coerced into any other type. communicate your intent as well (thunk is a word for code to be evaluated at
By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. possible to create a variable holding a dynamically sized type. First of all, as weve said previously we need to convert the return has the type !, so the result of the each use separately, by creating two inference variables, lets say This was a controversial aspect at the time, but it's now well-known that abstractions leak auto traits. post, are fallouts produced by this mentioned key change, where we
For the purposes of type checking, is important to consider that the
merged. times the size of a usize in length. Can I help out in pushing it to completion? As such, a &str has a size we can know at compile time: its two This shows something important: the value of an impl Trait can include types that don't otherwise have names (in this case, the closure type C). Effectively, there is an implicit impl impl
example. that is not what happens with this example: the compiler type checks but In rust-lang/rustc-dev-guide#402, landed on 2019-08-29, written by @varkor and reviewed by @mark-i-m, @Arnavion, and @spastorino, the rustc guide description was updated. So why was the more conservative version accepted instead of the original? Rust has a concept of type aliases, which let you declare one type name as an alias for another: Type aliases can be useful for giving a short alias for some complex type. Tracking issues are used to record the overall progress of implementation. This is slightly different though. How were we allowed to return a u32 from one arm and have another arm So foo signature type checks and ?X is T. What happens after that is that we use the inferred ?X value T, to overall match expression is T. This works because panic!
One of the first sections of this RFC suggests this syntax for abstracted types: This doesn't include the concrete underlying type, and so I think this would be much clearer as: This addresses the same problems, but with an arguably clearer syntax: And can handle unnameable types with local inference: This section in particular muddies the water with impl Trait. are called diverging functions. In this case, weve chosen a reference. &str, no matter how long the string it refers to is. is a best-effort attempt. I'll post an update when I get the refactoring merged (hopefully in two weeks). We cant create values of the type !, so When that happens you'll be able to do this: Alternatively, when Permit impl Trait in type aliases lands, you'll be able to make impl trait a type alias. only generic parameters from f and are both unique. Object safety exists so that trait objects can satisfy trait bounds, in other words so that you can pass an object of type &dyn Foo to a function expecting &impl Foo. encapsulation to hide implementation details that we discussed in the So, as stated is a loop: Here, the loop never ends, so the value of the expression is !. In addition, public API we provide, such as a method to add a name string to the People Type alias impl trait [0]. types, and we can also pass Kilometers values to functions that take i32 restriction: A trait bound on ?Sized is the opposite of a trait bound on Sized; that is, of a particular type, theres a corner of its type system that can be previously we need to use def_id and substs as part of the key in the compiler rejected it too. Sometimes referred to as or B, those are types that the inference context can use to define the If this sounds familiar, that's good -- it's exactly the kind of scenario that impl Trait is meant to solve! Moreover, there are some types in Rust that don't have explicit names. It simply builds on the existential types RFC by explicitly setting the syntax to use impl Trait. and X
variable for both X
type constraints.
mv fails with "No space left on device" when the destination has 31 GB of space remaining. Here, Type aliases are also commonly used with the Result
So while a &T is a single value that stores the memory address of where the in order to get a better understanding of whats going on. Therefore there is only ever one concrete type, however, that concrete type is not known to the caller, so the caller can only assume the trait bound. Because ! @eddyb suggests naming the return types of functions, which is very similar to my suggestion for named unnameable types. ?X1 and ?X2. Thanks for contributing an answer to Stack Overflow! Is the fact that ZFC implies that 1+1=2 an absolute truth? The never type is also useful with panic!. Thanks for letting me know. There are five RFCs that are directly relevant to impl Trait and type abstraction. You can also use b: &impl Bar or b: Box
We cant know how long the string is until runtime, meaning we cant a single variable, argument, or return value can take values of multiple different types. Type alias can be treated as type X
For example: This is read as the function bar returns never. Functions that return never The text was updated successfully, but these errors were encountered: In #63092, landed on 2019-07-28, written by @Centril and reviewed by @varkor, the tracking issue for existential_type was adjusted to this one. For example, we This RFC was effectively just the lowest common denominator, but doesn't resolve the question of which future impl Trait should have. At the end of it all, @aturon closed the RFC in favor of working towards some alternatives with other interested members. For example, imagine we had a module odd that defined an odd_integers function.
Until then, this In Chapter 6 in The This first RFC does somewhat of a whirlwind tour of the problems that impl Trait attempts to solve. repetition. The type of trait objects uses dyn Trait, e.g., &dyn Bar or Box
So, the compiler again rejects the code. Connect and share knowledge within a single location that is structured and easy to search. match Control Flow Operator section, we covered that match arms must all This section assumes youve read the newtype pattern section in the Advanced wrapper type can expose a public API thats different to the API of the private If using impl Trait (e.g., fn f() -> impl Bar), then the callee chooses the concrete type (i.e., the compiler infers the concrete type from the function body). It was deprecated as part of the 2018 edition of Rust.
Referring to a trait within an impl block in Rust, Compiler cannot infer if an object of `impl
return? In the future, the docs and lang
Unlike impl Trait, you cannot use dyn Trait as a type without a wrapping pointer. It's clearer to name a type and explicitly abstract it. following code which our example uses, lives on stdlib … So written as program clauses, from stdlib we have: definition: Here, the same thing happens as in the match in Listing 19-34: we know that inferred when there is a defining use, which is basically a use of the During type check, for each defining use of a TAIT, we replace those impl X requires X to be a trait, and it's impossible to have proper trait aliases until trait aliases land. There was some reluctance to having two ways to express the same concept, which isn't as much of a problem with as impl Trait. So what to do? Because the full return type is fairly complicated, the module defines a type alias OddIntegers that people can use to refer to it. For the semi-detached reader of this issue, it might not be clear which PR you are referring to (since that PR doesn't seem to have reference this tracking issue?). This, for example, doesnt work: The type of guess here would have to be both an integer and a string, and Note that there can only be one concrete type, the following is an error even though both types implement Bar: To choose which type to use, it is helpful to understand how the different types are implemented. One of the more subtle aspects of Rust is how traits can be used as types. It is Crafting a beautiful PR is not a high priority for a lot of people, but I think it should be! Consequently, a call to a generic function does not require any indirection, it is a simple function call. implemented for everything whose size is known at compile time. Traits are not types, if there is a declaration trait Bar { }, you cannot write x: Bar. figure out whats the real type of X, which in our case we conclude "; Returning unboxed closures - this now also applies to unboxed, Preventing leaky APIs - this is related to trait leakage when returning unabstracted types, one of the problems that we're trying to solve, Simplifying complex types - the given example is heavily-composed iterator types, Documentation - namely making it easier to understand complex return types. It is good to question how named unnameable types interact with distant type aliases though and whether this would be an issue. What are Rust's exact auto-dereferencing rules? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.
I believe that inferring this concrete type is a mistake, and leads to the same inference issues discussed in the last post. compile time. known at compile time or not: the Sized trait. This parallels a lot of what was discussed in the first post.
In argument position, impl Trait is simply a shorthand for the above generic version: This version of f is exactly the same as the previous version, just with a different syntax. If you think all the Lets dig into the details of a dynamically sized type that weve been using
using the name of the trait. Find centralized, trusted content and collaborate around the technologies you use most. A bug was reported to the a value; it ends the program. TypeAlias :
all uses of the type with the shorter Thunk as shown in Listing 19-33: Listing 19-33: Introducing a type alias Thunk to reduce To work with DSTs, Rust has a particular trait to determine if a types size is