f-strings would be part of Rust's grammar. `format` macro isn't available in `core` crate, but it is in `alloc` crate, so it needs to allocate memory.
f-strings on other hand would be pretty weird. Would `f"5"` return a &'static str or a String?
f-strings on other hand would be pretty weird. Would `f"5"` return a &'static str or a String?
In your example I would return a String and generate a warning, because the f of the string isn't used.
str (and thus &str) is part of the language, it's a built-in primitive type like i64 or bool, but String is just a struct the alloc crate brings into existence and so it may not be available.
Though could this work by the language feature just working like a Macro? Code without alloc using f strings just wouldn't compile then