Wildcard identifiers in constants

text/0000-const-wildcard.md

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+- Feature Name: `const_wildcard`
+- Start Date: 2018-08-18
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Allow assigning constants to `_`, as in `const _: TYPE = VALUE`, analogous to
+`let _ = VALUE`.
+
+# Motivation
+[motivation]: #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`:
+
+```rust
+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
+[guide-level-explanation]: #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
+[reference-level-explanation]: #reference-level-explanation
+
+The following changes are made to the language:
+
+## Grammar
+
+The grammar of `item_const` is changed from:
+
+```text
+item_const : CONST ident ':' ty '=' expr ';' ;
+```
+
+to:
+
+```text
+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
+[drawbacks]: #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 `impl`s) and non-associated `const` items.
+
+# Rationale and alternatives
+[alternatives]: #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!`](https://docs.rs/static_assertions/0.2.5/static_assertions/macro.const_assert.html)
+  and other macros in the
+  [`static_assertions`](https://docs.rs/static_assertions/0.2.5/static_assertions/index.html)
+  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.
+
+# Prior art
+[prior-art]: #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](https://golang.org/src/math/big/ftoa.go#L379):
+
+```go
+var _ fmt.Formatter = &floatZero // *Float must implement fmt.Formatter
+```
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.