Rust impl trait type alias

variant does not approach me. Perhaps there..

Rust impl trait type alias

A trait object is an opaque value of another type that implements a set of traits. The set of traits is made up of an object safe base trait plus any number of auto traits. Trait objects implement the base trait, its auto traits, and any supertraits of the base trait. Trait objects are written as the optional keyword dyn followed by a set of trait bounds, but with the following restrictions on the trait bounds. All traits except the first trait must be auto traits, there may not be more than one lifetime, and opt-out bounds e.

Sized are not allowed. Furthermore, paths to traits may be parenthesized. Edition Differences : In the edition, if the first bound of the trait object is a path that starts with ::then the dyn will be treated as a part of the path.

The first path can be put in parenthesis to get around this. Beginning in the edition, dyn is a true keyword and is not allowed in paths, so the parentheses are not necessary. Note: For clarity, it is recommended to always use the dyn keyword on your trait objects unless your codebase supports compiling with Rust 1.

Two trait object types alias each other if the base traits alias each other and if the sets of auto traits are the same and the lifetime bounds are the same. Due to the opaqueness of which concrete type the value is of, trait objects are dynamically sized types. Each instance of a pointer to a trait object includes:. The purpose of trait objects is to permit "late binding" of methods. Calling a method on a trait object results in virtual dispatch at runtime: that is, a function pointer is loaded from the trait object vtable and invoked indirectly.

The actual implementation for each vtable entry can vary on an object-by-object basis. In this example, the trait Printable occurs as a trait object in both the type signature of printand the cast expression in main.

Since a trait object can contain references, the lifetimes of those references need to be expressed as part of the trait object. There are defaults that allow this lifetime to usually be inferred with a sensible choice. Introduction 1.

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Notation 2. Lexical structure 2. Input format 2.

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Keywords 2. Identifiers 2. Comments 2. Whitespace 2. Tokens 2. Paths 3. Macros 3. Macros By Example 3. Procedural Macros 4. Crates and source files 5.GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.

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Sign in to your account. Tracking issues are used to record the overall progress of implementation. They are also uses as hubs connecting to other relevant issues, e. A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature. Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

It appears that the current implementation does not trigger any "private type in public interface" errors, for the following code playground :.

EDIT: Opened Inlanded onwritten by Centril and reviewed by varkor3 ICEs were closed with reproducer tests added.

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Inlanded onwritten by JohnTitor and reviewed by Centril1 ICE was closed with a reproducer test added. Concern: the defining implementation cannot currently be a struct field. In part this overlaps with Aaron 's concern of being unable to use the type within the defining scope except in defining uses.

Concern: it is not possible to define type aliases without trait bounds using the current syntax because type A; is not an existential type. Although unbounded existentials without even Drop are useless, it would be nice for them to be legal in the same way that a spurious let binding or unused struct field is legal. Unfortunately I see no solution to this using the RFC-proposed syntax.

Concern: scopes. I believe it was agreed that the details of an existential type should not cross API boundaries i. On the other hand, I believe it would be useful for the type details to be able to escape the defining scope within the same module, the same as non- pub types are accessible within the same module. This would allow struct literal macros to be significantly more useful. On the other hand, I believe it would be useful for the type details to be able to escape the defining scope.

Thanks Centril for reminding me of I'm still unsure whether type details should be available outside the defining scope the RFC is not really clear. Sized is an interesting type bound, but your test cases should really include the following currently passes :.

Done: I haven't found this bug reported somewhere thus I don't know if it's known. Nevermind, found the issue after searching for "existential type"I'm sorry for the noise It's not possible to use rustdoc as it's dropping the function bodies with.GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.

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Add the ability to create named existential types and support impl Trait in let, const, and static declarations. Thanks so much, cramertjfor taking this on! I wanted to follow up on two points that nikomatsakiseddybcramertj and I have been discussing but didn't make it in full detail in the RFC. First, the question of pursuing this feature as a next step, rather than adding direction support for fn -level impl Trait in traits: as we explored the design space, it turns out that the design for this "more advanced" feature is actually far more straightforward than for the "simpler", more sugary version.

In addition, there are some open questions around the sugary version that we could get insight into with more experience. I do want to emphasize that I personally am uncomfortable with the situation where you can use impl Trait in fn signatures, unless you are defining or implementing a trait. I think we should strive to support that ASAP.

rust impl trait type alias

But cramertj has convinced me that it's wisest to start with this RFC as the first step. The RFC strives to stay pretty high-level in terms of the specification, but it has some pretty significant implementation impact.

However, this is far less scary than it might sound at first. The idea we've discussed involves doing type inference as usual for each function, while treating instances of an impl Trait type alias as an inference variable.

In today's system, for type checking to succeed for a function, by the end of the process we must be able to fulfill all outstanding "obligations" basically: things to verify about the types, most commonly checking for trait implementations. In this setup, though, we may not have enough information within a single function to know for sure that all obligations have been fulfilled, since we may not know the full identity of the type alias.

There are a number of options for how to proceed, falling on a spectrum. Here are the two extremes:.

rust impl trait type alias

At the most conservative, we could force the type variable we introduced to be fully resolved by the end of type checking. That would mean each function using the alias, by itselfmust contain enough information to fully infer the concrete type for the alias.

rust impl trait type alias

In this approach, we'd still be able to require that all obligations are fulfilled by the end of type checking a function.A type alias defines a new name for an existing type.

Type aliases are declared with the keyword type. Every value has a single, specific type, but may implement several different traits, or be compatible with several different type constraints. For example, the following defines the type Point as a synonym for the type u8, u8the type of pairs of unsigned 8 bit integers:.

Introduction 1. Notation 2. Lexical structure 2. Input format 2. Keywords 2.

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Identifiers 2. Comments 2. Whitespace 2. Tokens 2. Paths 3. Macros 3. Macros By Example 3. Procedural Macros 4. Crates and source files 5. Conditional compilation 6. Items 6. Modules 6. Extern crates 6. Use declarations 6. Functions 6.GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.

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Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Already on GitHub? Sign in to your account. Allow impl Trait to be used in type aliases and associated traits, resolving the open question in RFC as to the concrete syntax for existential type. This makes it possible to write type aliases of the form:. Tracking issue. Thanks to rpjohnst and Centril for their ideas, discussion and feedback. My biggest issue with this restriction is that it makes impl Trait inconsistent between type aliases and everywhere else excluding the already inconsistent argument position.

With existential type there was a simple rule that could be applied to impl Trait in every position except argument position: it introduces a new anonymous existential type in the current context. I'm not quite sure I follow your point. This restriction is simply a syntactic one: it is simply intended to sidestep the question of what:. I mean that with existential types it's possible to have a single very simple rule for desugaring impl Trait that covers both:.

Ah, I see. Yes, you do have to have a single type alias as a base case if you're using the impl Trait type aliases to desugar. In practice, the desugaring of existential type is effectively replaced by the type alias.

That is, the existential type design was originally intended to act as a desugaring for return-position and variable-binding e. Using type aliases fills that role: but you can't use it to desugar itself. In practice, I don't think this is important, as there's no practical difference between the existential type itself and its alias. Why can't it be used to desugar, varkor?

Because if you try to desugar each occurrence of impl Trait you would end up trying to desugar:. So technically where clauses would have to be added to make this equivalent to the existential type syntax.GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.

Add the ability to create named existential types and support impl Trait in letconstand static declarations. In its current form, the feature makes it possible for functions to return unnameable or complex types such as closures and iterator combinators. However, the current feature has some severe limitations. Right now, it isn't possible to return an impl Trait type from a trait implementation.

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This is a huge restriction which this RFC fixes by making it possible to create a named existential type:. Separately, this RFC adds the ability to store an impl Trait type in a letconst or static.

This makes const and static declarations more concise, and makes it possible to store types such as closures or iterator combinators in const s and static s. In a future world where const fn has been expanded to trait functions, one could imagine iterator constants such as this:.

That RFC proposes to let users omit the types of const s and statics s. However, in some cases, completely omitting the types of const and static items could make it harder to tell what sort of value is being stored in a const or static. Allowing impl Trait in const s and static s would resolve the unnameable type issue while still allowing users to provide some information about the type. Declaring a variable of type impl Trait will hide its concrete type.

This is useful for declaring a value which implements a trait, but whose concrete type might change later on. Finally, note that impl Trait let declarations hide the concrete types of local variables:.

At first glance, this behavior doesn't seem particularly useful. Indeed, impl Trait in let bindings exists mostly for consistency with const s and static s. However, it can be useful for documenting the specific ways in which a variable is used. It can also be used to provide better error messages for complex, nested types:. Rust allows users to declare existential type s. An existential type allows you to give a name to a type without revealing exactly what type is being used.

rust impl trait type alias

In the example above, Foo refers to i32similar to a type alias. However, unlike a normal type alias, the concrete type of Foo is hidden outside of the module. Outside the module, the only thing that is known about Foo is that it implements the traits that appear in its declaration e. Debug in existential type Foo: Debug. If a user outside the module tries to use a Foo as an i32they will see an error:. This makes it possible to write modules that hide their concrete types from the outside world, allowing them to change implementation details without affecting consumers of their API.

This aids the function's ability to locally infer the concrete type of Foo. One particularly noteworthy use of existential types is in trait implementations. With this feature, we can declare associated types as follows:.

Similarly to impl Trait under RFCexistential type implicitly captures all generic type parameters in scope. In practice, this means that existential associated types may contain generic parameters from their impl:. The rules for impl Trait values in letconstand static declarations work mostly the same as impl Trait return values as specified in RFC These values hide their concrete type and can only be used as a value which is known to implement the specified traits.

They inherit any type parameters in scope.Opaque types are syntax to declare an opaque type alias that only exposes a specific set of traits as their interface; the concrete type in the background is inferred from a certain set of use sites of the opaque type. This declares an opaque type named Fooof which the only information is that it implements Bar. Therefore, any of Bar 's interface can be used on a Foobut nothing else regardless of whether it implements any other traits.

Since there needs to be a concrete background type, you can currently express that type by using the opaque type in a "defining use site". Currently only the return value of a function can be a defining use site of an opaque type and only if the return type of that function contains the opaque type. The defining use of an opaque type can be any code within the parent of the opaque type definition.

This includes any siblings of the opaque type and all children of the siblings. The initiative for "not causing fatal brain damage to developers due to accidentally running infinite loops in their brain while trying to comprehend what the type system is doing" has decided to disallow children of opaque types to be defining use sites. Associated opaque types can be defined by any other associated item on the same trait impl or a child of these associated items.

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For instance:. About this guide 1. Light default Rust Coal Navy Ayu. Opaque types type alias impl Trait Opaque types are syntax to declare an opaque type alias that only exposes a specific set of traits as their interface; the concrete type in the background is inferred from a certain set of use sites of the opaque type.

Defining use site s Currently only the return value of a function can be a defining use site of an opaque type and only if the return type of that function contains the opaque type. Associated opaque types Associated opaque types can be defined by any other associated item on the same trait impl or a child of these associated items.

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