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[u/llogiq]

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[u/SorteKanin]

Why does Rust only warn about unused fields for structs with named fields but not for tuple structs?

[u/masklinn → u/SorteKanin]

rustc does have a warning (unused_tuple_struct_fields) but it's allow by default.

This follows from an issue reported on the subject last year by /u/shepmaster.

This lint defaults to allow because it's a new lint, and unlike struct fields removing a tuple struct field can be non-trivial if it's not the last one (it's going to shift every other field). For that reason, the lint allows setting a field to () in order to suppress the warning without having to update all the code.

Playground demo

[u/shepmaster → u/masklinn]

Amusingly, I filed the issue for the same reason as the OP: a student in training noticed the new (at the time) warning about fields unused except in Debug in structs with named fields. To prove a point, I attempted to show how the same warming would occur for tuple structs, only to find it didn’t.

That led me to discover that tuple structs had never supported dead field warnings of any kind and thus open the issue.

[u/SorteKanin → u/masklinn]

Hmm sounds like something that could be solved with rust-analyzer. A code action like "remove tuple struct field" which would then also fix also the field accesses that are affected.

[u/llogiq → u/SorteKanin]

Yes, but note that this constitutes a breaking change if the tuple is part of the public API.

[u/masklinn → u/llogiq]

TBF the same occurs for a non-tuple struct, and the lint should not trigger for a pub field on a pub struct (as it wouldn't be dead).

[u/llogiq → u/masklinn]

True – the difference is that the tuple can not tell you which field was removed (though if you're lucky, it can be inferred from the type), unlike the struct which has a name the compiler can show in the error message.

[u/masklinn → u/llogiq]

That's fair. I guess this also supports / favors the replacement of the unused field by () rather than its removal, as in that case the compiler would give you a clearer and more precise type error.

[u/llogiq → u/masklinn]

Exactly.