`write!(..)` to a String failure documentation

Is it documented anywhere that calling write!(..) to a String cannot error? This is suggested by the format_push_string Clippy lint:

let mut s = String::new();
s += &format!("0x{:X}", 1024);
s.push_str(&format!("0x{:X}", 1024));

Use instead:

use std::fmt::Write as _; // import without risk of name clashing

let mut s = String::new();
let _ = write!(s, "0x{:X}", 1024);

It's probably not stated explicitly, but implied here (emphasis mine):

Additionally, the return value of this function is fmt::Result which is a type alias of Result<(), std::fmt::Error>. Formatting implementations should ensure that they propagate errors from the Formatter (e.g., when calling write!). However, they should never return errors spuriously. That is, a formatting implementation must and may only return an error if the passed-in Formatter returns an error. This is because, contrary to what the function signature might suggest, string formatting is an infallible operation. This function only returns a result because writing to the underlying stream might fail and it must provide a way to propagate the fact that an error has occurred back up the stack.


I wish it was fallible and used try_reserve internally.


This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.