Auto merge of #117511 - gurry:117406-err-packed-structs, r=compiler-errors

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Emit explanatory note for move errors in packed struct derives

Derive expansions for packed structs with non-`Copy` fields cause move errors because they prefer copying over borrowing since borrowing the fields of a packed struct can result in unaligned access.

This underlying cause of the errors, however, is not apparent to the user. This PR adds a diagnostic note to make it clear to the user (the new note is on the second last line):

```
tests/ui/derives/deriving-with-repr-packed-move-errors.rs:13:16
   |
12 | #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Default)]
   |          ----- in this derive macro expansion
13 | struct StructA(String);
   |                ^^^^^^ move occurs because `self.0` has type `String`, which does not implement the `Copy` trait
   |
   = note: `#[derive(Debug)]` triggers a move because taking references to the fields of a packed struct is undefined behaviour
   = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
```

Fixes #117406

Partially addresses #110777

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bors 2023-11-07 00:03:53 +00:00

parent b4d912781a 8c7bc3e0a2

commit 44f5b21abc

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