Add draft of variadic notes

src/design_notes/variadic_generics_design.md

@@ -0,0 +1,146 @@
+
+# Variadic Generics
+
+A draft for Variadic Generics has existed since ~2013, and there have been multiple postponed RFCs surrounding
+the topic. Despite this, the difficulty of designing such a system as well as lack of singular best choice
+has led to no resolution. This document has been written as an attempt to 'sum up' proposals and discussion in
+this space.
+
+# Proposals so far
+
+Following will be an overview of each of the proposals so far, followed by their individual pros and cons
+
+## EddyB's Draft
+
+This is a simple proposal, and the oldest one. It proposes the basic idea that all variadic types can
+be seen as just a tuple with N elements. The syntax is thus built around allowing expansions of tuple
+types into their components, and taking multiple types that will be condensed into a tuple.
+
+For using these types, it is proposed that one destructure the tuple, generally into a single head, and the remaining
+tail. This matches the recursive style used in C++. There is also some desire to
+be able to iterate in both directions (from left or right) rather than fixing
+the choice to a single direction.
+
+### Pros
+
+- Simple. Doesn't add much syntax while allowing the driving use case, the `Fn` traits and similar designs.
+
+- Allows intuitive function call ergonomics
+
+### Cons
+
+- Mentioned by eddyb, a subtuple type could have different padding than its parent.
+  EG: `&(A, B, C, D) != &A, &(B, C, D)`
+
+- The `..` syntax is already used in ranges, so some other syntax would be needed
+    - Note that `...` syntax may be available, as `..=` is now the inclusive
+      range syntax.
+
+### Syntax
+
+```rust
+type Tuple<..T> = T;
+
+(..(true, false), ..(1, "foo")) == (true, false, 1, "foo")
+
+fn bar(..x: (A, B, C)) {}
+```
+
+## Cramertj's Proposal
+
+Proposes both a syntax similar to C++ as well as a Tuple trait that will be implemented by all tuples.
+The trait would contain helpful types and methods for working with variadic tuples:
+- AsRefs type, `(A, B, C) -> (&A, &B, &C)`
+- AsMuts type, `(A, B, C) -> (&mut A, &mut B, &mut C)`
+- elements_as_refs fn, with signature `(&'a self) -> Self::AsRefs<'a>`
+- elements_as_mut fn, with signature `(&'a mut self) -> Self::AsMuts<'a>`
+
+- Not provided, but proposed as future extensions:
+  - Allowing unpacking tuple types in an argument position, preventing the need to call variadic functions like
+  `foo((1, 2.0, "3"))`
+  - Allowing the `...` syntax in generic type position, preventing the need to write traits such as `Fn` like
+  `Fn<(T1, T2, T3)>`
+
+### Pros
+
+- Proposes simple syntax, similar to eddyb proposal
+
+- Includes Tuple trait, which is helpful for working with variadic arguments
+
+### Cons
+
+- None yet.
+
+### Syntax
+
+```rust
+trait MyTrait: Tuple {}
+
+impl MyTrait for () {}
+
+impl<Head, Tail> MyTrait for (Head, ...Tail) where Tail: Tuple {}
+
+fn foo<T: MyTrait>(args: T) {}
+```
+
+## Fredpointzero's Proposal
+
+Takes a similar path to previous proposals in terms of syntax, but adds on a lot of ergonomics syntax.
+This additional syntax is proposed to allow easier usage of the variadic types, allowing the user to generate
+more imperative loops over the variadic type. A lot of the debate on this proposal surrounded the proposed for
+loop syntax, as many found it hard to read/comprehend at a glance compared to the rest of the proposed syntax.
+
+### Pros
+
+- Ergonomics, this proposal allows more linear usage of variadic types, which the other proposals lack.
+
+- Power, this proposal allows almost full control over the bounds on variadics, and provides many example
+  implementations.
+
+### Cons
+
+- Complexity, much more new syntax is needed than in the other proposals
+
+- No way to terminate recursive functions currently, they just won't compile
+
+### Syntax
+
+```
+struct Foo<(..T)>
+where
+    ..(T: Debug)
+{
+    items: (..T)
+}
+
+fn append<(..L), (..R)>(l: (..L), r: (..R)) -> (..L, ..R) {
+    todo!()
+}
+
+fn foo<(..T)>(args: (..T)) {
+    let (head, tail @ ..) = args;
+
+    (for arg <ARG> @in args <T> {
+        Vec::<Arg>::new();
+    })
+}
+```
+
+## Common notes
+
+All the proposals start with a similar syntax, using two or three dots to represent packing/unpacking types.
+They all act on tuples, extending language syntax to allow tuples of varrying instead of constant arity, and building
+all new functionality on top of that idea.
+
+The overlap with range is a common topic of discussion, but all full proposals seem to still use that syntax.
+Overall, the basic syntax seems to be agreed on, but usability and 'extra' ergonomic functionality still requires
+a lot of work.
+
+## Tuple layout
+
+Another important consideration for these proposals, particularly those resting
+on tuples, is the layout of the tuple. We do not currently guarantee any
+particular ordering of the fields in a tuple, which can limit our ability to
+"subset" the tuple under a reference. See [this
+comment](https://github.com/rust-lang/lang-team/pull/76#issuecomment-857206830)
+for some discussion.