auto merge of #12812 : sfackler/rust/attr-arm, r=alexcrichton

This is really only useful for #[cfg()]. For example:

```rust
enum Foo {
    Bar,
    Baz,
    #[cfg(blob)]
    Blob
}

fn match_foos(f: &Foo) {
    match *f {
        Bar => {}
        Baz => {}
        #[cfg(blob)]
        Blob => {}
    }
}
```

This is a kind of weird place to allow attributes, so it should probably
be discussed before merging.

6 files changed, 18 insertions, 2 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 116411c9e05..4a93fed8d03 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -440,6 +440,7 @@ pub enum Decl_ {
 
 #[deriving(Clone, Eq, TotalEq, Encodable, Decodable, Hash)]
 pub struct Arm {
+    pub attrs: Vec<Attribute>,
     pub pats: Vec<@Pat>,
     pub guard: Option<@Expr>,
     pub body: @Expr,
diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index e1174ea6cc4..b0dbd8b635a 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -726,6 +726,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
 
     fn arm(&self, _span: Span, pats: Vec<@ast::Pat> , expr: @ast::Expr) -> ast::Arm {
         ast::Arm {
+            attrs: vec!(),
             pats: pats,
             guard: None,
             body: expr
diff --git a/src/libsyntax/ext/deriving/primitive.rs b/src/libsyntax/ext/deriving/primitive.rs
index 0a7aa591657..9978a2edce9 100644
--- a/src/libsyntax/ext/deriving/primitive.rs
+++ b/src/libsyntax/ext/deriving/primitive.rs
@@ -112,6 +112,7 @@ fn cs_from(name: &str, cx: &mut ExtCtxt, trait_span: Span, substr: &Substructure
 
                         // arm for `_ if $guard => $body`
                         let arm = ast::Arm {
+                            attrs: vec!(),
                             pats: vec!(cx.pat_wild(span)),
                             guard: Some(guard),
                             body: body,
@@ -131,6 +132,7 @@ fn cs_from(name: &str, cx: &mut ExtCtxt, trait_span: Span, substr: &Substructure
 
             // arm for `_ => None`
             let arm = ast::Arm {
+                attrs: vec!(),
                 pats: vec!(cx.pat_wild(trait_span)),
                 guard: None,
                 body: cx.expr_none(trait_span),
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 04b289b9fca..9f05db5f807 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -119,6 +119,7 @@ pub trait Folder {
 
     fn fold_arm(&mut self, a: &Arm) -> Arm {
         Arm {
+            attrs: a.attrs.iter().map(|x| fold_attribute_(*x, self)).collect(),
             pats: a.pats.iter().map(|x| self.fold_pat(*x)).collect(),
             guard: a.guard.map(|x| self.fold_expr(x)),
             body: self.fold_expr(a.body),
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 8808312bed7..f30d756d854 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -2539,6 +2539,7 @@ impl<'a> Parser<'a> {
         self.commit_expr_expecting(discriminant, token::LBRACE);
         let mut arms: Vec<Arm> = Vec::new();
         while self.token != token::RBRACE {
+            let attrs = self.parse_outer_attributes();
             let pats = self.parse_pats();
             let mut guard = None;
             if self.eat_keyword(keywords::If) {
@@ -2557,7 +2558,12 @@ impl<'a> Parser<'a> {
                 self.eat(&token::COMMA);
             }
 
-            arms.push(ast::Arm { pats: pats, guard: guard, body: expr });
+            arms.push(ast::Arm {
+                attrs: attrs,
+                pats: pats,
+                guard: guard,
+                body: expr
+            });
         }
         let hi = self.span.hi;
         self.bump();
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index f768bf22ce0..b0130f127a3 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1286,9 +1286,14 @@ impl<'a> State<'a> {
                 try!(self.bopen());
                 let len = arms.len();
                 for (i, arm) in arms.iter().enumerate() {
-                    try!(space(&mut self.s));
+                    // I have no idea why this check is necessary, but here it
+                    // is :(
+                    if arm.attrs.is_empty() {
+                        try!(space(&mut self.s));
+                    }
                     try!(self.cbox(indent_unit));
                     try!(self.ibox(0u));
+                    try!(self.print_outer_attributes(arm.attrs.as_slice()));
                     let mut first = true;
                     for p in arm.pats.iter() {
                         if first {