RFC: pattern expansion in macros

Author: Keegan McAllister

active/0000-pattern-macros.md

@@ -0,0 +1,54 @@
+- Start Date: 2014-05-21
+- RFC PR #:
+- Rust Issue #:
+
+# Summary
+
+Allow macro expansion in patterns, i.e.
+
+~~~ .rs
+match x {
+    my_macro!() => 1,
+    _ => 2,
+}
+~~~
+
+# Motivation
+
+This is consistent with allowing macros in expressions etc.  It's also a year-old [open issue](https://github.com/mozilla/rust/issues/6830).
+
+I have [implemented](https://github.com/mozilla/rust/pull/14298) this feature already (with some bugs) and I'm [using it](https://github.com/kmcallister/html5/blob/937684f107090741c8e87135efc6e5476489857b/src/tree_builder/mod.rs#L111-L117) to [condense](https://github.com/kmcallister/html5/blob/937684f107090741c8e87135efc6e5476489857b/src/tree_builder/mod.rs#L261-L269) some ubiquitous patterns in the [HTML parser](https://github.com/kmcallister/html5) I'm writing.  This makes the code more concise and easier to cross-reference with the spec.
+
+# Drawbacks / alternatives
+
+A macro invocation in this position:
+
+~~~ .rs
+match x {
+    my_macro!()
+~~~
+
+could potentially expand to any of three different syntactic elements:
+
+* A pattern, i.e. `Foo(x)`
+* The left side of a `match` arm, i.e. `Foo(x) | Bar(x) if x > 5`
+* An entire `match` arm, i.e. `Foo(x) | Bar(x) if x > 5 => 1`
+
+This RFC proposes only the first of these, but the others would be more useful in some cases.  Supporting multiple of the above would be significantly more complex.
+
+Another alternative is to use a macro for the entire `match` expression, e.g.
+
+~~~ .rs
+my_match!(x {
+    my_new_syntax => 1,
+    _ => 2,
+})
+~~~
+
+This doesn't involve any language changes, but requires writing a complicated procedural macro.  (My sustained attempts to do things like this with MBE macros have all failed.)  Perhaps I could alleviate some of the pain with a library for writing `match`-like macros, or better use of the existing parser in `libsyntax`.
+
+The `my_match!` approach is also not very composable.
+
+# Unresolved questions
+
+I need to fix the ICE described in [the pull request](https://github.com/mozilla/rust/pull/14298).  I don't expect this to have any design impact, but I can't be sure.