Provide a way to render lax-parsed SPDX expressions as valid SPDX

[Amy Keibler]

Is your feature request related to a problem? Please describe.

Thank you for creating such a useful library. We're using it in the CycloneDX SBOM Rust implementation. One rough edge we've come across is that we want to be able to accept mostly-correct SPDX identifiers (e.g. the "MIT/Apache-2.0" convention that's present in several crates), but we want to ensure that we're only producing valid SPDX expressions in our output, to maximize ecosystem tooling compatibility.

Describe the solution you'd like

It would be useful for the Expression type that is generated via parse_mode with ParseMode::LAX to be able to output as a valid SPDX identifier. Currently, it stores the original string and just outputs that when to_string is called. Changing the output to_string would be a breaking change, so maybe there could be an additional function named render or something similar?

Describe alternatives you've considered

Currently, we have a temporary solution that replaces / with OR, but a proper solution would be more robust.

Additional context

If this change is welcome, I might have time to implement this myself and submit a PR over the next few weeks.

[Jake Shadle]

I think a less invasive change would probably be to instead have a transform function exposed that can take a string and ParseMode, and just fix the string based on the settings in the ParseMode, eg. in your case ParseMode { allow_slash_as_or_operator: false, ..Default::default() would mean an all / would be replaced with OR etc, and could either be executed at all times and the resulting string handed to Expression::parse, or else attempted as a fallback if parsing fails. Right now all of the parsing errors are predicated on the original input string, it feels like adding an additional way to output the expression would make the API more cumbersome.