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Jonathan Brouwer
dbf9492774
Introduce a `#[diagnostic::on_unknown]` attribute This PR introduces a `#[diagnostic::on_unknown]` attribute that allows crate authors to customize the error messages emitted by unresolved imports. The main usecase for this is using this attribute as part of a proc macro that expects a certain external module structure to exist or certain dependencies to be there. For me personally the motivating use-case are several derives in diesel, that expect to refer to a `tabe` module. That is done either implicitly (via the name of the type with the derive) or explicitly by the user. This attribute would allow us to improve the error message in both cases: * For the implicit case we could explicity call out our assumptions (turning the name into lower case, adding an `s` in the end) + point to the explicit variant as alternative * For the explicit variant we would add additional notes to tell the user why this is happening and what they should look for to fix the problem (be more explicit about certain diesel specific assumptions of the module structure) I assume that similar use-cases exist for other proc-macros as well, therefore I decided to put in the work implementing this new attribute. I would also assume that this is likely not useful for std-lib internal usage. related rust-lang/rust#152900 and rust-lang/rust#128674