Meaning of multiple licenses

[SimonSapin]

http://doc.crates.io/manifest.html#package-metadata says:

# This is a string description of the license for this package. Currently
# crates.io will validate the license provided against a whitelist of known
# license identifiers from http://spdx.org/licenses/. Multiple licenses can
# be separated with a `/`
license = "..."

But what is the meaning of having multiple licenses?

MIT / Apache-2.0 is often use to express a "dual-license" like Rust itself. My understanding is that the user chooses which one to apply.

Now I’m publishing https://github.com/servo/rust-egl on crates.io. Some files have an Apache-2 license headers, others something that looks like MIT. So different files have different requirements. This is sort of "this license and this license", where Rust is "that one or that one".

Is there a way to express this difference in the license field of Cargo.toml? Should there be?

CC @larsbergstrom

[alexcrichton]

crates.io currently uses https://crates.io/crates/license-exprs which I believe allows clarifying this ambiguity

[SimonSapin]

Unfortunately https://crates.io/crates/license-exprs doesn’t have a documentation or repository link, I don’t even know how to download the source to reverse-engineer what it does.

[alexcrichton]

Sure, but you can google "withoutboats license-exprs" and get https://github.com/withoutboats/license-exprs :)

[SimonSapin]

Cool. ehuss/license-exprs#2.

Perhaps this syntax is worth mentioning on http://doc.crates.io/manifest.html#package-metadata?

[withoutboats]

The syntax is specified in Appendix IV of the SPDX 2.0 standard. My crate has a few bugs: + should be an operator than can be applied to any license according to the standard, not a part of the license string, and it doesn't support parens. Maybe I'll fix these things today.

EDIT: + is properly treated now.

[codyps]

I'd like to note that the / is a crates.io specific addition, applied before trying to parse SPDX license expressions (using license-exprs). As a result, there isn't much clarity on what / means (except in looking at how projects use it, which isn't great. Docs on http://doc.crates.io/manifest.html just say "Multiple licenses can be separated with a /").

It would be a very good idea to try to deprecate the non-standard use of / in the license fields and stick to SPDX license expressions.

[hobofan]

#2039 related/duplicate issue

[sanmai-NL]

@alexcrichton: It seems like we can quickly remove any Cargo documentation suggesting to use the deprecated notation as per @jmesmon's comment. Can we expedite that fix?

[sanmai-NL]

An appropriate link to replace the wording with the deprecated notation would be https://github.com/david-a-wheeler/spdx-tutorial/blob/master/README.md#spdx-license-expressions

[wking]

/ is deprecated and AND / OR recommended (via the SPDX license expression docs) with the just-landed #4898. The only remaining issue here is the lack of (parens ehuss/license-exprs#3).