RFC 2526: const-wildcard

lang (syntax | const)

• Feature Name: const_wildcard
• Start Date: 2018-08-18
• RFC PR: rust-lang/rfcs#2526
• Rust Issue: rust-lang/rust#54912

Summary

Allow assigning constants to _, as in const _: TYPE = VALUE, analogous to let _ = VALUE.

Motivation

The ability to ensure that code type checks while discarding the result is useful, especially in custom derives. For example, the following code will not compile if the type MyType doesn't implement the trait MyTrait:

const _FOO: () = {
    use std::marker::PhantomData;
    struct ImpelementsMyTrait<T: MyTrait>(PhantomData<T>);
    let _ = ImplementsMyTrait::<MyType>(PhantomData); // type checking error if MyType: !MyTrait
    ()
};

Unfortunately, this requires coming up with a unique identifier to assign to. This is error-prone because no matter what identifier is chosen, there's always a possibility that a user will have already used the same identifier in their code. If writing const _: () = { ... } were valid, then this would be a non-issue - the const _ could be repeated many times without conflicting with any other identifier in scope.

Guide-level explanation

Allow assigning to _ when defining a new constant. Just like let _, this doesn't introduce any new bindings, but still evaluates the rvalue at compile time like any other constant.

Reference-level explanation

The following changes are made to the language:

Grammar

The grammar of item_const is changed from:

item_const : CONST ident ':' ty '=' expr ';' ;

to:

item_const : CONST (ident | UNDERSCORE) ':' ty '=' expr ';' ;

Type checking

When type checking an associated const item, the token _ may not occur as the name of the item.

When type checking a const item not inside an impl item, the token _ is permitted as the name of such an item. When that token does occur, it is replaced with a freshly generated and unique identifier.

Drawbacks

The rules around constant identifiers are made somewhat more complicated, as is the compiler logic for handling them. A distinction is introduced between associated const items (inside impls) and non-associated const items.

Rationale and alternatives

Rationale

This would allow more ergonomic uses of a number of patterns used today:

• Ensuring that types have certain trait bounds in custom derives, as explained in the Motivation section.
• const_assert! and other macros in the static_assertions crate, which currently work only in a scope (so that they can use a let binding) or requires the user to specify a scope-unique name for a function which will be used to contain the expression that is the meat of the macro.

Eventually, we will likely want to support fully general pattern matching just like in let bindings (e.g., const (a, b): (u8, u8) = (1, 1)) to not have const _ be a special case in the language. However, this RFC leaves the details of such a design up to a future RFC.

Alternatives

• We could provide procedural macros with an API that fetches a new, globally-unique identifier.
• We could support anonymous modules (mod { ... } or mod _ { ... }).
• We could support anonymous top-level functions (fn _() { ... }).

Prior art

Go allows unnamed constants using the syntax const _ = .... It also allows top-level variable bindings which are evaluated at init time, before main is run - var _ = .... This latter syntax is often used to ensure that a particular type implements a particular interface, as in this example from the standard library:

var _ fmt.Formatter = &floatZero // *Float must implement fmt.Formatter

Unresolved questions

None.