set_host("localhost") on file:// URL produces non-roundtripping serialization

Summary

Calling set_host(Some("localhost")) on a file:/// URL produces file://localhost/..., but re-parsing that serialization strips localhost per the WHATWG URL spec (which normalizes file://localhost/ to file:///). This means the URL produced by set_host does not roundtrip through Url::parse.

Reproduction

use url::Url;

fn main() {
    let mut url = Url::parse("file:///tmp/test").unwrap();
    url.set_host(Some("localhost")).unwrap();

    let modified = url.as_str().to_string();
    println!("After set_host: {:?}", modified);
    // After set_host: "file://localhost/tmp/test"

    let reparsed = Url::parse(&modified).unwrap();
    println!("Reparsed:       {:?}", reparsed.as_str());
    // Reparsed:       "file:///tmp/test"

    assert_eq!(modified, reparsed.as_str()); // FAILS
}

Analysis

Per the WHATWG URL standard, file://localhost/ is normalized to file:/// during parsing. The set_host method should apply the same normalization — when setting the host to "localhost" on a file:// URL, it should either:

1. Normalize it away (set host to empty/None), or
2. Serialize it without localhost so the output roundtrips

Currently set_host stores localhost literally, producing a serialization that the parser will normalize differently on re-parse.

Found by

This bug was found by fuzzing with the fuzz_url_setters target (see #1100).

Start by tracing set_host and Url::parse for file:// URLs, then inspect the fuzz_url_setters target mentioned in the issue. Reproduce the localhost case and compare setter serialization with parser normalization. Done means the behavior follows the WHATWG rule and the serialized URL roundtrips consistently.