One flavour I would expect to be valuable that isn't present here is this:
Process the input (as quickly as possible) and never fail, but replace each invalid sequence of bytes with U+FFFD (bytes 0xEF 0xBF 0xBD).
Process the input (as quickly as possible) and never fail, but replace each invalid sequence of bytes with U+FFFD (bytes 0xEF 0xBF 0xBD).
Edit: Although, it looks like Rust's std already does this, except for preallocating an exactly correct size result buffer: https://doc.rust-lang.org/src/alloc/string.rs.html#538
Looks like it just uses the size of the original slice. If the average broken chunk is less than three bytes (maybe quite common?) then it'll have to grow the buffer, at least doubling it.
>> let bytestring = b"foobar\xcc";
>> bytestring.len()
7
>> let cleaned = String::from_utf8_lossy(bytestring).into_owned();
>> cleaned.len()
9
>> cleaned.capacity()
14It isn't a letter, or a digit, or whitespace, or punctuation, or a word separator, or a control character, it is neither uppercase nor lowercase, it doesn't have any canonical equivalents - it's just a codepoint that exists specifically for this purpose.
As a result it's much less likely that if gibberish sneaks into your system somehow and gets turned into U+FFFD this causes something important to break elsewhere.
And when sooner or later a human is shown this text, it's very obvious that U+FFFD isn't what they expected, whether that was E-acute, a Euro currency symbol, a cat emoji or whatever else, and the human will know something went wrong and can decide if they care about that.