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runtime-fmt 
A crate for string formatting using runtime format strings.
This crate provides much the same facilities as std::fmt, with the
additional allowance for format strings which are not known until runtime.
Possible applications include internationalization, scripting, or other
customization.
Each of the standard formatting macros format_args!, format!,
print!, println!, write!, and writeln! have corresponding rt_
variants. Calls which previously succeeded unconditionally now return
Result, which may indicate a bad format string or arguments.
The syntax for format strings and for macro invocations is equivalent to
that used by std::fmt, including support for positional and named
arguments. This crate shells out to the standard library implementations
for as much as possible to ensure feature parity.