I'd prefer something that would maybe be less concise but easier to read and maintain, using the host language instead of a mini script using in-band signaling and its weird syntax.
"Hello {username}!"
"Hello $username!"
"Hello " . username . "!"
"Hello " + username + "!"
"Hello " << username << "!"
Of those, I find the first and second one by far the easiest to read, and the first one is easier to extend (as rust has done, e.g. "USD{total:>6}" for a left-padded number).
It doesn't add additional syntax (and therefore complexity, compiler steps, build time, etc) to the language; the only convenience applied here is varargs for the arguments besides the first.
A lot of languages started with basic (s)printf and added string interpolation later on, but that brings its own headaches. I'm thinking of PHP, where on the one hand you can do "hello $username", but if it's a property in an object you need to add additional syntax already - "hello {$user->name}".
But really I didn't really mention it because printf syntax seems to me like exactly the "mini script using in-band signaling and its weird syntax" that GP complained about.
In contrast, the old-school stdio sprintf (and relatives) will interpret the format string at run time and then read a varargs list to do the interpolation which can lead to run-time errors and buffer-overflow vulnerabilities and so forth.
Meanwhile, can't complain all that much if zero-cost features can be added to Rust to make it easier to market it to scripting folk. I think that can only be a good thing, even if the feature design is far from ideal.