How can I use a dynamic format string with the format! macro?

Question

I want to use the format! macro with a String as first argument, but because the macro expects a string literal, I am not able pass anything different to it.

I want to do this to dynamically add strings into the current string for use in a view engine. I'm open for suggestions if there might be a better way to do it.

let test = String::from("Test: {}"); let test2 = String::from("Not working!"); println!(test, test2); 

What I actually want to achieve is the below example, where main.html contains {content}.

use std::io::prelude::*; use std::fs::File; use std::io;  fn main() {     let mut buffer = String::new();     read_from_file_using_try(&mut buffer);      println!(&buffer, content="content"); }  fn read_from_file_using_try(buffer: &mut String) -> Result<(), io::Error> {     let mut file = try!(File::open("main.html"));     try!(file.read_to_string(buffer));     Ok(()) } 

So I want to print the contents of main.html after I formatted it.

Answer 1

Short answer: it cannot be done.

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Long answer: the format! macro (and its derivatives) requires a string literal, that is a string known at compilation-time. In exchange for this requirement, if the arguments provided do not match the format, a compilation error is raised.

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What you are looking for is known as a template engine. A non-exhaustive list of Rust template engines in no particular order:

• Handlebars
• Rustache
• Maud
• Horrorshow
• fomat-macros
• ...

Template engines have different characteristics, and notably differ by the degree of validation occurring at compile-time or run-time and their flexibility (I seem to recall that Maud was very HTML-centric, for example). It's up to you to find the one most fitting for your use case.

Answer 2

Check out the strfmt library, it is the closest I've found to do dynamic string formatting.