Old notes on KnownLayout

[joshlf]

These are some of my old notes on how to implement KnownLayout. They're no longer relevant, but I'm writing them down for posterity.

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Replace KnownLayout::LAYOUT with associated types which represent:

• Base size
• Alignment
• Trailing slice element type

When deriving KnownLayout for Foo { a: A, b: B, c: C }, synthesize the following type:

FooSynthesized {
    a: A,
    b: B,
    c_sized: C::Base,  // For T: Sized, T::Base = T, so for Foo: Sized, FooSynthesized has the same layout as Foo
    _align: [C::TrailingSliceElement; 0]
}

For T: Sized, T::Base = T and T::TrailingSliceElement = (). Note that () has alignment 1, which means that, for Foo: Sized, Foo: Synthesized has the same layout as Foo (c_sized is just C, and _align adds no padding after c_sized).

We still need:

• Some way to distinguish between sized and unsized types (since you can have an unsized type whose trailing slice element is ())
• Is there any way to teach the type system that T: Sized is special? We don’t have a blanket impl to avoid impl conflicts. What are the advantages of being able to teach this to type system? At the time of writing, I'm not remembering why that was important, but maybe it has to do with this?