Unseal Spanned again #1524
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Unseal Spanned again #1524
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Spanned was not sealed in syn 1. Currently in syn 2 it is nominally sealed, but since quote::ToTokens isn't sealed, Spanned *can* be implemented for any foreign type, provided one is willing to provide a (possibly broken) implemntation of ToTokens. This means that sealing Spanned has no semver advantage, and neither does it defend syn from any bad behaviour on the part of downstream implementors. See dtolnay#1441
dtolnay
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Oct 30, 2023
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I am not interested in this until possibly after Span::join works on stable. You are correct that ToTokens is the supported way to implement Spanned. That means that syn::Error::new_spanned and other APIs can behave usefully on stable when a direct implementation of Spanned would not.
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Spanned was not sealed in syn 1.
Currently in syn 2 it is nominally sealed, but since quote::ToTokens isn't sealed, Spanned can be implemented for any foreign type, provided one is willing to provide a (possibly broken) implemntation of ToTokens.
This means that sealing Spanned has no semver advantage, and neither does it defend syn from any bad behaviour on the part of downstream implementors.
This is blocking at least one of my downstreams of syn from updating to syn 2.
See #1441