921cfbe56f
Improve error messages involving `derive` and `packed`. There are two errors involving `derive` and `packed`. ``` `#[derive]` can't be derived on a `#[repr(packed)]` struct with type or const parameters `#[derive]` can't be derived on a `#[repr(packed)]` struct that does not derive Copy ``` The second one overstates things. It is possible to use derive on a repr(packed) struct that doesn't derive Copy in two cases. - If all the fields within the struct meet the required alignment: 1 for `repr(packed)`, or `N` for `repr(packed(N))`. - If `Default` is the only trait derived. This commit improves things in a few ways. - Changes the errors to say `this trait can't be derived on this ...`. This is more accurate, because it's just *this* trait and *this* packed struct that are a problem, not *all* derived traits on *all* packed structs. - Adds more details to the "ERROR" lines in the test case, enough to distinguish between the two error messages. - Adds more cases to the test case that don't cause errors, e.g. `Default` derives. - Uses a wider variety of builtin traits in the test case, for better coverage. r? `@estebank` |
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