RFC: Array pattern adjustments

Author: ftxqxd

text/0000-array-pattern-changes.md

@@ -0,0 +1,152 @@
+- Start Date: 2014-12-03
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+Summary
+=======
+
+Change array/slice patterns in the following ways:
+
+- Make them only match on arrays (`[T, ..n]` and `[T]`), not slices;
+- Make subslice matching yield a value of type `[T, ..n]` or `[T]`, not `&[T]`
+  or `&mut [T]`;
+- Allow multiple mutable references to be made to different parts of the same
+  array or slice in array patterns (resolving rust-lang/rust [issue
+  #8636](https://github.com/rust-lang/rust/issues/8636)).
+
+Motivation
+==========
+
+Before DST (and after the removal of `~[T]`), there were only two types based on
+`[T]`: `&[T]` and `&mut [T]`. With DST, we can have many more types based on
+`[T]`, `Box<[T]>` in particular, but theoretically any pointer type around a
+`[T]` could be used. However, array patterns still match on `&[T]`, `&mut [T]`,
+and `[T, ..n]` only, meaning that to match on a `Box<[T]>`, one must first
+convert it to a slice, which disallows moves. This may prove to significantly
+limit the amount of useful code that can be written using array patterns.
+
+Another problem with today’s array patterns is in subslice matching, which
+specifies that the rest of a slice not matched on already in the pattern should
+be put into a variable:
+
+```rust
+let foo = [1i, 2, 3];
+match foo {
+    [head, tail..] => {
+        assert_eq!(head, 1);
+        assert_eq!(tail, &[2, 3]);
+    },
+    _ => {},
+}
+```
+
+This makes sense, but still has a few problems. In particular, `tail` is a
+`&[int]`, even though the compiler can always assert that it will have a length
+of `2`, so there is no way to treat it like a fixed-length array. Also, all
+other bindings in array patterns are by-value, whereas bindings using subslice
+matching are by-reference (even though they don’t use `ref`). This can create
+confusing errors because of the fact that the `..` syntax is the only way of
+taking a reference to something within a pattern without using the `ref`
+keyword.
+
+Finally, the compiler currently complains when one tries to take multiple
+mutable references to different values within the same array in a slice pattern:
+
+```rust
+let foo: &mut [int] = &mut [1, 2, 3];
+match foo {
+    [ref mut a, ref mut b] => ...,
+    ...
+}
+```
+
+This fails to compile, because the compiler thinks that this would allow
+multiple mutable borrows to the same value (which is not the case).
+
+Detailed design
+===============
+
+- Make array patterns match only on arrays (`[T, ..n]` and `[T]`). For example,
+  the following code:
+
+  ```rust
+  let foo: &[u8] = &[1, 2, 3];
+  match foo {
+      [a, b, c] => ...,
+      ...
+  }
+  ```
+
+  Would have to be changed to this:
+
+  ```rust
+  let foo: &[u8] = &[1, 2, 3];
+  match foo {
+      &[a, b, c] => ...,
+      ...
+  }
+  ```
+
+  This change makes slice patterns mirror slice expressions much more closely.
+
+- Make subslice matching in array patterns yield a value of type `[T, ..n]` (if
+  the array is of fixed size) or `[T]` (if not). This means changing most code
+  that looks like this:
+
+  ```rust
+  let foo: &[u8] = &[1, 2, 3];
+  match foo {
+      [a, b, c..] => ...,
+      ...
+  }
+  ```
+
+  To this:
+
+  ```rust
+  let foo: &[u8] = &[1, 2, 3];
+  match foo {
+      &[a, b, ref c..] => ...,
+      ...
+  }
+  ```
+
+  It should be noted that if a fixed-size array is matched on using subslice
+  matching, and `ref` is used, the type of the binding will be `&[T, ..n]`,
+  *not* `&[T]`.
+
+- Improve the compiler’s analysis of multiple mutable references to the same
+  value within array patterns. This would be done by allowing multiple mutable
+  references to different elements of the same array (including bindings from
+  subslice matching):
+
+  ```rust
+  let foo: &mut [u8] = &mut [1, 2, 3, 4];
+  match foo {
+      &[ref mut a, ref mut b, ref c, ref mut d..] => ...,
+      ...
+  }
+  ```
+
+Drawbacks
+=========
+
+- This will break a non-negligible amount of code, requiring people to add `&`s
+  and `ref`s to their code.
+
+- The modifications to subslice matching will require `ref` or `ref mut` to be
+  used in almost all cases. This could be seen as unnecessary.
+
+Alternatives
+============
+
+- Do a subset of this proposal; for example, the modifications to subslice
+  matching in patterns could be removed.
+
+Unresolved questions
+====================
+
+- What are the precise implications to the borrow checker of the change to
+  multiple mutable borrows in the same array pattern? Since it is a
+  backwards-compatible change, it can be implemented after 1.0 if it turns out
+  to be difficult to implement.