Issue #3511 - Rationalize temporary lifetimes.

Major changes:

- Define temporary scopes in a syntax-based way that basically defaults
  to the innermost statement or conditional block, except for in
  a `let` initializer, where we default to the innermost block. Rules
  are documented in the code, but not in the manual (yet).
  See new test run-pass/cleanup-value-scopes.rs for examples.
- Refactors Datum to better define cleanup roles.
- Refactor cleanup scopes to not be tied to basic blocks, permitting
  us to have a very large number of scopes (one per AST node).
- Introduce nascent documentation in trans/doc.rs covering datums and
  cleanup in a more comprehensive way.

1 files changed, 11 insertions, 4 deletions

diff --git a/src/libsyntax/ext/format.rs b/src/libsyntax/ext/format.rs
index a4b8dd78403..3ad5857395d 100644
--- a/src/libsyntax/ext/format.rs
+++ b/src/libsyntax/ext/format.rs
@@ -634,17 +634,24 @@ impl<'a> Context<'a> {
                                      self.ecx.expr_ident(e.span, lname)));
         }
 
+        // Now create a vector containing all the arguments
+        let slicename = self.ecx.ident_of("__args_vec");
+        {
+            let args = names.move_iter().map(|a| a.unwrap());
+            let mut args = locals.move_iter().chain(args);
+            let args = self.ecx.expr_vec_slice(self.fmtsp, args.collect());
+            lets.push(self.ecx.stmt_let(self.fmtsp, false, slicename, args));
+        }
+
         // Now create the fmt::Arguments struct with all our locals we created.
-        let args = names.move_iter().map(|a| a.unwrap());
-        let mut args = locals.move_iter().chain(args);
         let fmt = self.ecx.expr_ident(self.fmtsp, static_name);
-        let args = self.ecx.expr_vec_slice(self.fmtsp, args.collect());
+        let args_slice = self.ecx.expr_ident(self.fmtsp, slicename);
         let result = self.ecx.expr_call_global(self.fmtsp, ~[
                 self.ecx.ident_of("std"),
                 self.ecx.ident_of("fmt"),
                 self.ecx.ident_of("Arguments"),
                 self.ecx.ident_of("new"),
-            ], ~[fmt, args]);
+            ], ~[fmt, args_slice]);
 
         // We did all the work of making sure that the arguments
         // structure is safe, so we can safely have an unsafe block.