Though this might be better than using format!() directly, the hope would be to remove all unnecessary allocations along the parsing path. Instead of using format!() to create a string to compare, perform the comparison directly. That is, instead of:

"ab" == format!("{}{}", "a", "b");

Do something closer to:

"ab"[..1] == "a" && "ab"[1..] == "b"

But safely, of course, considering all bounds and possible string lengths.

We could imagine a generalized function:

fn string_is_parts(string: &str, parts: &[&str]) -> bool;

string_is_parts("ab", &["a", "b"]);
string_is_parts(maybe_boundary, &[constants::CRLF, constants::BOUNDARY_EXT, boundary]);