Make trans treat all statements as if they introduce a new scope

This is for consistency with borrowck. Changing borrowck would
also be an alternative, but I found it easier to change trans :-)

This eliminates an ICE in trans where the scope for a particular
borrow was a statement ID, but the code in trans that does cleanups
wasn't finding the block with that scope. As per rust-lang#3860

src/librustc/middle/trans/base.rs

@@ -1018,8 +1018,16 @@ fn trans_stmt(cx: block, s: ast::stmt) -> block {
     debuginfo::update_source_pos(cx, s.span);
 
     match s.node {
-        ast::stmt_expr(e, _) | ast::stmt_semi(e, _) => {
-            bcx = expr::trans_into(cx, e, expr::Ignore);
+        ast::stmt_expr(e, s_id) | ast::stmt_semi(e, s_id) => {
+            // We introduce a new scope here because even though
+            // a statement of the form e; doesn't bind a new var,
+            // it might introduce a borrow, and borrowck may
+            // consider any statement to introduce a new scope.
+            bcx = with_scope(bcx, Some({id: s_id, span: s.span}),
+                             stmt_to_str(s, cx.tcx().sess.intr()),
+                             |cx| {
+                                 expr::trans_into(cx, e, expr::Ignore)
+                             });
         }
         ast::stmt_decl(d, _) => {
             match d.node {

src/test/run-pass/issue-3860.rs

@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// xfail-test
 struct Foo { x: int }
 
 impl Foo {
@@ -19,11 +18,7 @@ impl Foo {
 
 fn main() {
     let mut x = @mut Foo { x: 3 };
-    x.stuff(); // error: internal compiler error: no enclosing scope with id 49
-    // storing the result removes the error, so replacing the above
-    // with the following, works:
-    // let _y = x.stuff()
-
-    // also making 'stuff()' not return anything fixes it
-    // I guess the "dangling &ptr" cuases issues?
-}
+    // Neither of the next two lines should cause an error
+    let _ = x.stuff(); 
+    x.stuff();
+}