Improves handling of statement macros.

Statement macros are now treated somewhat like item macros, in that a
statement macro can now expand into a series of statements, rather than
just a single statement.

This allows statement macros to be nested inside other kinds of macros and
expand properly, where previously the expansion would only work when no
nesting was present.

See: src/test/run-pass/macro-stmt_macro_in_expr_macro.rs
     src/test/run-pass/macro-nested_stmt_macro.rs

This changes the interface of the MacResult trait.  make_stmt has become
make_stmts and now returns a vector, rather than a single item.  Plugin
writers who were implementing MacResult will have breakage, as well as
anyone using MacEager::stmt.

See: src/libsyntax/ext/base.rs

This also causes a minor difference in behavior to the diagnostics
produced by certain malformed macros.

See: src/test/compile-fail/macro-incomplete-parse.rs

src/libsyntax/ext/base.rs

@@ -208,10 +208,11 @@ impl<F> IdentMacroExpander for F
 }
 
 // Use a macro because forwarding to a simple function has type system issues
-macro_rules! make_stmt_default {
+macro_rules! make_stmts_default {
     ($me:expr) => {
         $me.make_expr().map(|e| {
-            P(codemap::respan(e.span, ast::StmtExpr(e, ast::DUMMY_NODE_ID)))
+            SmallVector::one(P(codemap::respan(
+                e.span, ast::StmtExpr(e, ast::DUMMY_NODE_ID))))
         })
     }
 }
@@ -238,12 +239,12 @@ pub trait MacResult {
         None
     }
 
-    /// Create a statement.
+    /// Create zero or more statements.
     ///
     /// By default this attempts to create an expression statement,
     /// returning None if that fails.
-    fn make_stmt(self: Box<Self>) -> Option<P<ast::Stmt>> {
-        make_stmt_default!(self)
+    fn make_stmts(self: Box<Self>) -> Option<SmallVector<P<ast::Stmt>>> {
+        make_stmts_default!(self)
     }
 }
 
@@ -276,7 +277,7 @@ make_MacEager! {
     pat: P<ast::Pat>,
     items: SmallVector<P<ast::Item>>,
     impl_items: SmallVector<P<ast::ImplItem>>,
-    stmt: P<ast::Stmt>,
+    stmts: SmallVector<P<ast::Stmt>>,
 }
 
 impl MacResult for MacEager {
@@ -292,10 +293,10 @@ impl MacResult for MacEager {
         self.impl_items
     }
 
-    fn make_stmt(self: Box<Self>) -> Option<P<ast::Stmt>> {
-        match self.stmt {
-            None => make_stmt_default!(self),
-            s => s,
+    fn make_stmts(self: Box<Self>) -> Option<SmallVector<P<ast::Stmt>>> {
+        match self.stmts.as_ref().map_or(0, |s| s.len()) {
+            0 => make_stmts_default!(self),
+            _ => self.stmts,
         }
     }
 
@@ -384,10 +385,11 @@ impl MacResult for DummyResult {
             Some(SmallVector::zero())
         }
     }
-    fn make_stmt(self: Box<DummyResult>) -> Option<P<ast::Stmt>> {
-        Some(P(codemap::respan(self.span,
-                               ast::StmtExpr(DummyResult::raw_expr(self.span),
-                                             ast::DUMMY_NODE_ID))))
+    fn make_stmts(self: Box<DummyResult>) -> Option<SmallVector<P<ast::Stmt>>> {
+        Some(SmallVector::one(P(
+            codemap::respan(self.span,
+                            ast::StmtExpr(DummyResult::raw_expr(self.span),
+                                          ast::DUMMY_NODE_ID)))))
     }
 }