[codegen] Always generate Eq/Ord/Hash where possible #533
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In write-fonts, we will generate these traits for all types. In read-fonts, the situation is more nuanced. In the case of tables, derive is not an option, because tables in read-fonts wrap some chunk of bytes, but that chunk may contain trailing data that is not necessarily part of the table in question. More generally, in read-fonts, the semantics of things like 'equality' are ambiguous. If a table contains an offset to subtable, should equality be based on the raw value of the offset, or based recursively on the contents of the child tables? At some point we may wish to revisit this qusetion, but for now I am punting. We *will* derive extra traits in read-fonts, but only for records, and only when they contain only non-offset scalar values.
rsheeter
approved these changes
Jul 19, 2023
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LGTM but with varying comfort:
- Eq makes sense to me
- Hash on write-fonts types I'm ok with; maybe there are some you wouldn't realistically use it but I don't see why it would be harmful
- Ord makes me uncomfortable, I don't see how we can generate a sane Ord for all types
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(per IM I'm fine with this merging despite mild discomfort around Ord) |
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My stance here is entirely pragmatic: we need ord for some of these types, and it's annoying to go and selectively add it, so lets just derive it. I agree that the particular implementation of |
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In write-fonts, we will generate these traits for all types.
In read-fonts, the situation is more nuanced. In the case of tables, derive is not an option, because tables in read-fonts wrap some chunk of bytes, but that chunk may contain trailing data that is not necessarily part of the table in question.
More generally, in read-fonts, the semantics of things like 'equality' are ambiguous. If a table contains an offset to subtable, should equality be based on the raw value of the offset, or based recursively on the contents of the child tables?
At some point we may wish to revisit this qusetion, but for now I am punting. We will derive extra traits in read-fonts, but only for records, and only when they contain only non-offset scalar values.
closes #532