Commits on Jul 25, 2022

1. 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 `$TRAIT can't be derived on this ...`.
     This is more accurate, because it's just $TRAIT and *this* packed
     struct that are a problem, not *all* derived traits on *all* packed
     structs.
   - Removed the E0133 error code, because it's incorrect, being a code
     relating to `unsafe`.
   - 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.

   

   nnethercote committed Jul 25, 2022
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