accept naked exprs with commas in pattern arms

pretty printing will use them, but indentation is slightly off
if the expr is long

src/libsyntax/ast_util.rs

@@ -588,6 +588,13 @@ fn view_path_id(p: @view_path) -> node_id {
     }
 }
 
+fn lone_block_expr(blk: blk) -> option<@ast::expr> {
+    if blk.node.view_items.len() != 0 { ret none; }
+    if blk.node.stmts.len() != 0 { ret none; }
+    if blk.node.rules != default_blk { ret none; }
+    ret blk.node.expr;
+}
+
 // Local Variables:
 // mode: rust
 // fill-column: 78;

src/libsyntax/parse/parser.rs

@@ -1511,8 +1511,25 @@ class parser {
             let pats = self.parse_pats();
             let mut guard = none;
             if self.eat_keyword(~"if") { guard = some(self.parse_expr()); }
-            if self.token == token::FAT_ARROW { self.bump(); }
-            let blk = self.parse_block();
+            let blk = if self.token != token::FAT_ARROW {
+                self.parse_block()
+            } else {
+                self.bump();
+                if self.token == token::LBRACE {
+                    self.parse_block()
+                } else {
+                    let expr = self.parse_expr();
+                    if self.token != token::RBRACE {
+                        self.expect(token::COMMA);
+                    }
+                    {node: {view_items: ~[],
+                            stmts: ~[],
+                            expr: some(expr),
+                            id: self.get_id(),
+                            rules: default_blk},
+                     span: expr.span}
+                }
+            };
             vec::push(arms, {pats: pats, guard: guard, body: blk});
         }
         let mut hi = self.span.hi;

src/libsyntax/print/pprust.rs

@@ -6,7 +6,7 @@ import pp::{break_offset, word, printer,
             inconsistent, eof};
 import diagnostic;
 import ast::{required, provided};
-import ast_util::operator_prec;
+import ast_util::{operator_prec, lone_block_expr};
 import dvec::{dvec, extensions};
 import parse::classify::*;
 import util::interner;
@@ -1034,7 +1034,8 @@ fn print_expr(s: ps, &&expr: @ast::expr) {
         print_maybe_parens_discrim(s, expr);
         space(s.s);
         bopen(s);
-        for arms.each |arm| {
+        let len = arms.len();
+        for arms.eachi |i, arm| {
             space(s.s);
             cbox(s, alt_indent_unit);
             ibox(s, 0u);
@@ -1050,8 +1051,19 @@ fn print_expr(s: ps, &&expr: @ast::expr) {
               some(e) { word_space(s, ~"if"); print_expr(s, e); space(s.s); }
               none { }
             }
-            print_possibly_embedded_block(s, arm.body, block_normal,
-                                          alt_indent_unit);
+            word_space(s, ~"=>");
+            alt lone_block_expr(arm.body) {
+              some(expr) => {
+                end(s); // close the ibox for the pattern
+                print_expr(s, expr);
+                if i < len - 1 { word_space(s, ~","); }
+                end(s); // close enclosing cbox
+              }
+              none => {
+                print_possibly_embedded_block(s, arm.body, block_normal,
+                                              alt_indent_unit);
+              }
+            }
         }
         bclose_(s, expr.span, alt_indent_unit);
       }

src/test/pretty/alt-naked-expr-long.rs

@@ -0,0 +1,17 @@
+// pretty-exact
+
+// actually this doesn't quite look how I want it to, but I can't
+// get the prettyprinter to indent the long expr
+
+fn main() {
+    let x = some(3);
+    let y =
+        alt x {
+          some(_) =>
+          "some" + "very" + "very" + "very" + "very" + "very" + "very" +
+              "very" + "very" + "long" + "string",
+
+          none => "none"
+        };
+    assert y == "some(_)";
+}

src/test/pretty/alt-naked-expr-medium.rs

@@ -0,0 +1,10 @@
+// pretty-exact
+
+fn main() {
+    let x = some(3);
+    let _y =
+        alt x {
+          some(_) => ~[~"some(_)", ~"not", ~"SO", ~"long", ~"string"],
+          none => ~[~"none"]
+        };
+}

src/test/pretty/alt-naked-expr.rs

@@ -0,0 +1,7 @@
+// pretty-exact
+
+fn main() {
+    let x = some(3);
+    let y = alt x { some(_) => "some(_)",  none => "none" };
+    assert y == "some(_)";
+}