Delayed resolution

text/0000-delayed-relosution.md

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+- Feature Name: extern_item
+- Start Date: 2015-12-14
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Generalize the delayed resolution of language items to arbitrary items.
+
+# Motivation
+[motivation]: #motivation
+
+Consider an allocator crate that defines a default allocator:
+
+```rust
+type Default = Jemalloc;
+```
+
+At the point in the dependency tree where the allocator crate is defined, the
+default allocator might not yet exist or downstream crates might want to define
+their own allocator. Users of the allocator crate will, however, want to use the
+default allocator as a default type parameter in their allocating types:
+
+```rust
+struct Vec<T, A: Allocator = alloc::Default>
+```
+
+In order to satisfy both constraints, a delayed resolution of items is added:
+
+```rust
+#[extern]
+type Default: Allocator;
+```
+
+# Detailed design
+[design]: #detailed-design
+
+## Declaring extern items
+
+An `#[extern]` attribute is added. This attribute can be applied to
+
+* type definitions,
+* functions,
+* statics, and
+* constants.
+
+For example
+
+```rust
+#[extern]
+type T: Bounds;
+
+#[extern]
+fn f();
+
+#[extern]
+static S: Type;
+
+#[extern]
+const C: Type;
+```
+
+where `Bounds` is a collection of trait and lifetime bounds. Users of `T` can
+only use the properties exposed by the bounds. And where `Type` is an arbitrary
+(possibly extern) type.
+
+## Defining extern items
+
+The `#[extern]` attribute can be applied in the `#[extern = path]` form to
+define an extern item. For example
+
+```rust
+#[extern = "alloc::Default"]
+struct X;
+
+impl Allocator for X { /* ... */ }
+```
+
+The `path` in `#[extern = path]` can be any path to the item including
+re-exports. Any crate that has a path to an extern item can define it.
+
+The extern definitions must satisfy the bounds, etc. that appear in the
+declarations.
+
+When an executable or static library is compiled, each extern item declared in
+any of the involved crates must have exactly one definition in any of the
+involved crates. Otherwise a helpful error message is printed that shows
+duplicate or missing definitions.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+* None known
+
+# Alternatives
+[alternatives]: #alternatives
+
+* Hacks such as `#[allocator]`.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+* Many