RFC: Change syntax of subslice matching

active/0000-subslice-syntax-change.md

@@ -0,0 +1,60 @@
+- Start Date: 2014-08-15
+- RFC PR:
+- Rust Issue:
+
+# Summary
+
+Change syntax of subslices matching from `..xs` to `xs..`
+to be more consistent with the rest of the language
+and allow future backwards compatible improvements.
+
+Small example:
+
+```rust
+match slice {
+    [xs.., _] => xs,
+    [] => fail!()
+}
+```
+
+This is basically heavily stripped version of [RFC 101](https://github.com/rust-lang/rfcs/pull/101).
+
+# Motivation
+
+In Rust, symbol after `..` token usually describes number of things,
+as in `[T, ..N]` type or in `[e, ..N]` expression.
+But in following pattern: `[_, ..xs]`, `xs` doesn't describe any number,
+but the whole subslice.
+
+I propose to move dots to the right for several reasons (including one mentioned above):
+
+1. Looks more natural (but that might be subjective).
+2. Consistent with the rest of the language.
+3. C++ uses `args...` in variadic templates.
+4. It allows extending slice pattern matching as described in [RFC 101](https://github.com/rust-lang/rfcs/pull/101).
+
+# Detailed design
+
+Slice matching grammar would change to (assuming trailing commas;
+grammar syntax as in Rust manual):
+
+    slice_pattern : "[" [[pattern | subslice_pattern] ","]* "]" ;
+    subslice_pattern : ["mut"? ident]? ".." ["@" slice_pattern]? ;
+
+To compare, currently it looks like:
+
+    slice_pattern : "[" [[pattern | subslice_pattern] ","]* "]" ;
+    subslice_pattern : ".." ["mut"? ident ["@" slice_pattern]?]? ;
+
+# Drawbacks
+
+Backward incompatible.
+
+# Alternatives
+
+Don't do it at all.
+
+# Unresolved questions
+
+Whether subslice matching combined with `@` should be written as `xs.. @[1, 2]`
+or maybe in another way: `xs @[1, 2]..`.