The dbg!() macro should behave like a function call in how it evaluates
its arguments, and captures by Rust closures are not fully compatible
with path-specific borrowing. Specifically:
let borrow = &mut obj.foo;
dbg!("{}", obj.bar);
would fail because the closure inside dbg!() would borrow all of obj
instead of just obj.foo.
Fix this by using the format_args!() macro to evaluate the dbg!()
arguments and produce an fmt::Arguments object which can be safely
passed to the thread-local closure for printing.
The arguments are still evaluated inside an if { .. } which
constant-folds away in release builds.
81ff9bac72