extern types

text/0000-extern-types.md

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+- Feature Name: extern_types
+- Start Date: 2017-01-18
+- RFC PR: 
+- Rust Issue: 
+
+# Summary
+[summary]: #summary
+
+Add an `extern type` syntax for declaring types which are opaque to Rust's type
+system.
+
+# Motivation
+[motivation]: #motivation
+
+When interacting with external libraries we often need to be able to handle pointers to data that we don't know the size or layout of.
+
+In C it's possible to declare a type but not define it.
+These incomplete types can only be used behind pointers, a compilation error will result if the user tries to use them in such a way that the compiler would need to know their layout.
+
+In Rust, we don't have this feature. Instead, a couple of problematic hacks are used in its place.
+
+One is, we define the type as an uninhabited type. eg.
+
+```rust
+enum MyFfiType {}
+```
+
+Another is, we define the type with a private field and no methods to construct it.
+
+```rust
+struct MyFfiType {
+    _priv: (),
+}
+```
+
+The point of both these constructions is to prevent the user from being able to create or deal directly with instances of the type.
+Neither of these types accurately reflect the reality of the situation.
+The first definition is logically problematic as it defines a type which can never exist.
+This means that references to the type can also—logically—never exist and raw pointers to the type are guaranteed to be
+invalid.
+The second definition says that the type is a ZST, that we can store it on the stack and that we can call `ptr::read`, `mem::size_of` etc. on it.
+None of this is of course valid.
+
+The controversies on how to represent foreign types even extend to the standard library too; see the discussion in the [libc_types RFC PR](https://github.com/rust-lang/rfcs/pull/1783).
+
+This RFC instead proposes a way to directly express that a type exists but is unknown to Rust.
+
+Finally, In the 2017 roadmap, [integration with other languages](https://github.com/rust-lang/rfcs/blob/master/text/1774-roadmap-2017.md#integration-with-other-languages), is listed as a priority.
+Just like unions, this is an unsafe feature necessary for dealing with legacy code in a correct and understandable manner.
+
+# Detailed design
+[design]: #detailed-design
+
+Add a new kind of type declaration, an extern type:
+
+```rust
+extern {
+    type Foo;
+}
+```
+
+These types are FFI-safe. They are also DSTs, meaning that they do not implement `Sized`. Being DSTs, they cannot be kept on the stack, can only be accessed through pointers and references and cannot be moved from.
+
+In Rust, pointers to DSTs carry metadata about the object being pointed to.
+For strings and slices this is the length of the buffer, for trait objects this is the object's vtable.
+For extern types the metadata is simply `()`.
+This means that a pointer to an extern type is identical to a raw pointer (ie. it is not a "fat pointer").
+It also means that if we store an extern type at the end of a container (such as a struct or tuple) pointers to that container will also be identical to raw pointers (despite the container as a whole being unsized).
+This is useful to support a pattern found in some C APIs where structs are passed around which have arbitrary data appended to the end of them: eg.
+
+```rust
+extern {
+    type OpaqueTail;
+}
+
+#[repr(C)]
+struct FfiStruct {
+    data: u8,
+    more_data: u32,
+    tail: OpaqueTail,
+}
+```
+
+As a DST, `size_of` and `align_of` do not work, but we must also be careful that `size_of_val` and `align_of_val` do not work either, as there is not necessarily a way at run-time to get the size of extern types either.
+For an initial implementation, those methods can just panic, but before this is stabilized there should be some trait bound or similar on them that prevents their use statically.
+The exact mechanism is more the domain of the custom DST RFC, [RFC 1524](https://github.com/rust-lang/rfcs/pull/1524), and so figuring that mechanism out will be delegated to it.
+
+C's "pointer `void`" (not `()`, but the `void` used in `void*` and similar) is currently defined in two official places: [`std::os::raw::c_void`](https://doc.rust-lang.org/stable/std/os/raw/enum.c_void.html) and [`libc::c_void`](https://doc.rust-lang.org/libc/x86_64-unknown-linux-gnu/libc/enum.c_void.html).
+Unifying these is out of scope for this RFC, but this feature should be used in their definition instead of the current tricks.
+Strictly speaking, this is a breaking change, but the `std` docs explicitly say that `void` shouldn't be used without indirection.
+And `libc` can, in the worst-case, make a breaking change.
+
+# How We Teach This
+[how-we-teach-this]: #how-we-teach-this
+
+Really, the question is "how do we teach *without* this".
+As described above, the current tricks for doing this are wrong.
+Furthermore, they are quite advanced touching upon many advanced corners of the language: zero-sized and uninhabited types are phenomena few programmer coming from mainstream languages have encountered.
+From reading around other RFCs, issues, and internal threads, one gets a sense of two issues:
+First, even among the group of Rust programmers enthusiastic enough to participate in these fora, the semantics of foreign types are not widely understood.
+Second, there is annoyance that none of the current tricks, by nature of them all being flawed in different ways, would become standard.
+
+By contrast, `extern type` does exactly what one wants, with an obvious and guessable syntax, without forcing the user to immediately understand all the nuance about why *these* semantics are indeed the right ones.
+As they see various options fail: moves, stack variables, they can discover these semantics incrementally.
+The benefits are such that this would soon displace the current hacks, making code in the wild more readable through consistent use of a pattern.
+
+This should be taught in the foreign function interface chapter of the rust book in place of where it currently tells people to use uninhabited enums (ack!).
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Very slight addition of complexity to the language.
+
+The syntax has the potential to be confused with introducing a type alias, rather than a new nominal type.
+The use of `extern` here is also a bit of a misnomer as the name of the type does not refer to anything external to Rust.
+
+# Alternatives
+[alternatives]: #alternatives
+
+Not do this.
+
+Alternatively, rather than provide a way to create opaque types, we could just offer one distinguished type (`std::mem::OpaqueData` or something like that).
+Then, to create new opaque types, users just declare a struct with a member of type `OpaqueData`.
+This has the advantage of introducing no new syntax, and issues like FFI-compatibility would fall out of existing rules.
+
+Another alternative is to drop the `extern` and allow a declaration to be written `type A;`.
+This removes the (arguably disingenuous) use of the `extern` keyword although it makes the syntax look even more like a type alias.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+- Should we allow generic lifetime and type parameters on extern types?
+  If so, how do they effect the type in terms of variance?
+
+- [In std's source](https://github.com/rust-lang/rust/blob/164619a8cfe6d376d25bd3a6a9a5f2856c8de64d/src/libstd/os/raw.rs#L59-L64), it is mentioned that LLVM expects `i8*` for C's `void*`.
+  We'd need to continue to hack this for the two `c_void`s in std and libc.
+  But perhaps this should be done across-the-board for all extern types?
+  Somebody should check what Clang does.