syntax: Allow any block-like expr to be used as alt arm w/o comma separator

src/libsyntax/ast_util.rs

@@ -588,13 +588,6 @@ 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

@@ -1477,9 +1477,10 @@ class parser {
     // For distingishing between record literals and blocks
     fn looking_at_record_literal() -> bool {
         let lookahead = self.look_ahead(1);
-        self.token_is_keyword(~"mut", lookahead) ||
-            (is_plain_ident(lookahead) &&
-             self.look_ahead(2) == token::COLON)
+        self.token == token::LBRACE &&
+            (self.token_is_keyword(~"mut", lookahead) ||
+             (is_plain_ident(lookahead) &&
+              self.look_ahead(2) == token::COLON))
     }
 
     fn parse_record_literal() -> expr_ {
@@ -1518,26 +1519,30 @@ class parser {
             let pats = self.parse_pats();
             let mut guard = none;
             if self.eat_keyword(~"if") { guard = some(self.parse_expr()); }
-            let blk = if self.token != token::FAT_ARROW {
-                self.parse_block()
+            let expr = if self.token != token::FAT_ARROW {
+                self.parse_block_expr(self.last_span.lo, default_blk)
             } else {
                 self.bump();
-                if self.token == token::LBRACE
-                    && !self.looking_at_record_literal() {
-                    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}
-                }
+                self.parse_expr_res(RESTRICT_STMT_EXPR)
             };
+
+            let require_comma =
+                classify::expr_requires_semi_to_be_stmt(expr)
+                && self.token != token::RBRACE;
+
+            if require_comma {
+                self.expect(token::COMMA);
+            } else {
+                self.eat(token::COMMA);
+            }
+
+            let blk = {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, lone_block_expr};
+import ast_util::{operator_prec};
 import dvec::{dvec, extensions};
 import parse::classify::*;
 import util::interner;
@@ -1052,17 +1052,21 @@ fn print_expr(s: ps, &&expr: @ast::expr) {
               none { }
             }
             word_space(s, ~"=>");
-            alt lone_block_expr(arm.body) {
+            // Extract the expression from the extra block the parser adds
+            assert arm.body.node.view_items.is_empty();
+            assert arm.body.node.stmts.is_empty();
+            assert arm.body.node.rules == ast::default_blk;
+            alt arm.body.node.expr {
               some(expr) => {
                 end(s); // close the ibox for the pattern
                 print_expr(s, expr);
-                if i < len - 1 { word(s.s, ~","); }
+                if expr_requires_semi_to_be_stmt(expr)
+                    && i < len - 1 {
+                    word(s.s, ~",");
+                }
                 end(s); // close enclosing cbox
               }
-              none => {
-                print_possibly_embedded_block(s, arm.body, block_normal,
-                                              alt_indent_unit);
-              }
+              none => fail
             }
         }
         bclose_(s, expr.span, alt_indent_unit);

src/test/compile-fail/alt-arrows-block-then-binop.rs

@@ -0,0 +1,7 @@
+fn main() {
+
+    alt 0 {
+      0 => {
+      } + 5 //~ ERROR unexpected token: `+`
+    }
+}

src/test/run-pass/alt-arrows-blocky-commas.rs

@@ -0,0 +1,56 @@
+// no-reformat
+// Testing the presense or absense of commas separating block-structure
+// alt arm expressions
+
+fn fun(_f: fn()) {
+}
+
+fn it(_f: fn() -> bool) {
+}
+
+fn main() {
+
+    alt 0 {
+      00 => {
+      }
+      01 => if true {
+      } else {
+      }
+      03 => alt 0 {
+        _ => ()
+      }
+      04 => do fun {
+      }
+      05 => for it {
+      }
+      06 => while false {
+      }
+      07 => loop {
+      }
+      08 => unsafe {
+      }
+      09 => unchecked {
+      }
+      10 => {
+      },
+      11 => if true {
+      } else {
+      },
+      13 => alt 0 {
+        _ => ()
+      },
+      14 => do fun {
+      },
+      15 => for it {
+      },
+      16 => while false {
+      },
+      17 => loop {
+      },
+      18 => unsafe {
+      },
+      19 => unchecked {
+      },
+      _ => ()
+    }
+}