Add SPDX license ID to the source files instead of non-descriptive "look at the top-level of this crate" comment

[sdroege]

@GuillaumeGomez @bilelmoussaoui Any opinions?

[GuillaumeGomez]

Do you have a description of why we would want this change and what it brings? I have no idea what SPDX license ID is about. ^^'

[sdroege]

https://spdx.dev/learn/handling-license-info/#why

[GuillaumeGomez]

Sorry but I don't see what improvements it adds (not against it, please do it if you think it's an improvement!). Why do you need to use SPDX license for that and not just "Licensed under X. Take a look at the LICENSE file"?

[sdroege]

It's machine-readable and the same format in multiple projects. As a human you can just run grep on your code and all your dependencies to compile a list of all applicable licenses, a tool can collect these automatically.

For Rust code it's less of a problem thanks to the license field in the Cargo.toml (which uses a SPDX license ID btw), until you start mixing files with different licenses in the same crate. Having that one line in each file removes all the ambiguity.

[bilelmoussaoui]

I agree, always found the copy pasted sentence not great