From 6d8c00b8f2cae1fc9d4afcb8b31f58ac64ec760b Mon Sep 17 00:00:00 2001
From: Keegan McAllister <kmcallister@mozilla.com>
Date: Wed, 21 May 2014 13:09:29 -0700
Subject: [PATCH 1/3] RFC: pattern expansion in macros

---
 active/0000-pattern-macros.md | 54 +++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)
 create mode 100644 active/0000-pattern-macros.md

diff --git a/active/0000-pattern-macros.md b/active/0000-pattern-macros.md
new file mode 100644
index 00000000000..b0ab6409e86
--- /dev/null
+++ b/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.

From 374d6909b96977085121ff9e34bc85a1a467d016 Mon Sep 17 00:00:00 2001
From: Keegan McAllister <kmcallister@mozilla.com>
Date: Wed, 21 May 2014 15:12:52 -0700
Subject: [PATCH 2/3] Fixed an implementation bug

---
 active/0000-pattern-macros.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/active/0000-pattern-macros.md b/active/0000-pattern-macros.md
index b0ab6409e86..88210095c0d 100644
--- a/active/0000-pattern-macros.md
+++ b/active/0000-pattern-macros.md
@@ -17,7 +17,7 @@ match x {
 
 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.
+I have [implemented](https://github.com/mozilla/rust/pull/14298) this feature already 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
 
@@ -51,4 +51,4 @@ 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.
+None, as far as I know.

From 59bdf1bd683e2872305a895d60406417c61f182c Mon Sep 17 00:00:00 2001
From: Keegan McAllister <kmcallister@mozilla.com>
Date: Wed, 21 May 2014 15:21:22 -0700
Subject: [PATCH 3/3] Note rustdoc limitation

---
 active/0000-pattern-macros.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/active/0000-pattern-macros.md b/active/0000-pattern-macros.md
index 88210095c0d..a501a44cc9b 100644
--- a/active/0000-pattern-macros.md
+++ b/active/0000-pattern-macros.md
@@ -49,6 +49,8 @@ This doesn't involve any language changes, but requires writing a complicated pr
 
 The `my_match!` approach is also not very composable.
 
+Another small drawback: `rustdoc` [can't document](https://github.com/kmcallister/rust/blob/af65e3e9824087a472de3fea3c7cb1efcec4550b/src/librustdoc/clean.rs#L1287-L1291) the name of a function argument which is produced by a pattern macro.
+
 # Unresolved questions
 
 None, as far as I know.