Allow overlapping implementations for marker traits

0000-allow-overlapping-impls-on-marker-traits.md

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+- Feature Name: Allow overlapping impls for marker traits
+- Start Date: 2015-09-02
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Preventing overlapping implementations of a trait makes complete sense in the
+context of determining method dispatch. There must not be ambiguity in what code
+will actually be run for a given type. However, for marker traits, there are no
+associated methods for which to indicate ambiguity. There is no harm in a type
+being marked as `Sync` for multiple reasons.
+
+# Motivation
+
+This is purely to improve the ergonomics of adding/implementing marker traits.
+While specialization will certainly make all cases not covered today possible,
+removing the restriction entirely will improve the ergonomics in several edge
+cases.
+
+# Detailed design
+
+For the purpose of this RFC, the definition of a marker trait is a trait with no
+associated items. The design here is quite straightforward. The following code
+fails to compile today:
+
+```rust
+trait Marker<A> {}
+
+struct GenericThing<A, B> {
+    a: A,
+    b: B,
+}
+
+impl<A, B> Marker<GenericThing<A, B>> for A {}
+impl<A, B> Marker<GenericThing<A, B>> for B {}
+```
+
+The two impls are considered overlapping, as there is no way to prove currently
+that `A` and `B` are not the same type. However, in the case of marker traits,
+there is no actual reason that they couldn't be overlapping, as no code could
+actually change based on the `impl`.
+
+For a concrete use case, consider some setup like the following:
+
+```rust
+trait QuerySource {
+    fn select<T, C: Selectable<T, Self>>(&self, columns: C) -> SelectSource<C, Self> {
+        ...
+    }
+}
+
+trait Column<T> {}
+trait Table: QuerySource {}
+trait Selectable<T, QS: QuerySource>: Column<T> {}
+
+impl<T: Table, C: Column<T>> Selectable<T, T> for C {}
+```
+
+However, when the following becomes introduced:
+
+```rust
+struct JoinSource<Left, Right> {
+    left: Left,
+    right: Right,
+}
+
+impl<Left, Right> QuerySource for JoinSource<Left, Right> where
+    Left: Table + JoinTo<Right>,
+    Right: Table,
+{
+    ...
+}
+```
+
+It becomes impossible to satisfy the requirements of `select`. The following
+impl is disallowed today:
+
+```rust
+impl<Left, Right, C> Selectable<Left, JoinSource<Left, Right>> for C where
+    Left: Table + JoinTo<Right>,
+    Right: Table,
+    C: Column<Left>,
+{}
+
+impl<Left, Right, C> Selectable<Right, JoinSource<Left, Right>> for C where
+    Left: Table + JoinTo<Right>,
+    Right: Table,
+    C: Column<Right>,
+{}
+```
+
+Since `Left` and `Right` might be the same type, this causes an overlap.
+However, there's also no reason to forbid the overlap. There is no way to work
+around this today. Even if you write an impl that is more specific about the
+tables, that would be considered a non-crate local blanket implementation. The
+only way to write it today is to specify each column individually.
+
+# Drawbacks
+
+With this change, adding any methods to an existing marker trait, even
+defaulted, would be a breaking change. Once specialization lands, this could
+probably be considered an acceptable breakage.
+
+# Alternatives
+
+Once specialization lands, there does not appear to be a case that is impossible
+to write, albeit with some additional boilerplate, as you'll have to manually
+specify the empty impl for any overlap that might occur.
+
+# Unresolved questions
+
+None at this time.