Make accesses to fields of packed structs unsafe #44884

To handle packed structs with destructors (which you'll think are a rare case, but the `#[repr(packed)] struct Packed<T>(T);` pattern is ever-popular, which requires handling packed structs with destructors to avoid monomorphization-time errors), drops of subfields of packed structs should drop a local move of the field instead of the original one. cc rust-lang#27060 - this should deal with that issue after codegen of drop glue is updated. The new errors need to be changed to future-compatibility warnings, but I'll rather do a crater run first with them as errors to assess the impact.

This is required because the old version depended on tendril 0.3.1, which used `repr(packed)` incorrectly - see kuchiki-rs/kuchiki#38

Fix the derive implementation for repr(packed) structs to move the fields out instead of calling functions on references to each subfield. That's it, `#[derive(PartialEq)]` on a packed struct now does: ```Rust fn eq(&self, other: &Self) { let field_0 = self.0; let other_field_0 = other.0; &field_0 == &other_field_0 } ``` Instead of ```Rust fn eq(&self, other: &Self) { let ref field_0 = self.0; let ref other_field_0 = other.0; &*field_0 == &*other_field_0 } ``` Taking (unaligned) references to each subfield is undefined, unsound and is an error with MIR effectck, so it had to be prevented. This causes a borrowck error when a `repr(packed)` struct has a non-Copy field (and therefore is a [breaking-change]), but I don't see a sound way to avoid that error.

@nrc

There's a trailing whitespace problem in one of the tests. @nrc said he'll go fix it quickly, but until then I'll like to land this PR. I suspect this happened because one of the dependencies in the Cargo.lock I updated had changed the format of the XML they emit, but that has to be investigated.

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Make accesses to fields of packed structs unsafe #44884

Make accesses to fields of packed structs unsafe #44884

Commits on Nov 26, 2017