Auto merge of #68122 - Centril:stabilize-transparent-enums, r=petrochenkov

Stabilize `#[repr(transparent)]` on `enum`s in Rust 1.42.0

# Stabilization report

The following is the stabilization report for `#![feature(transparent_enums)]`.

Tracking issue: https://github.com/rust-lang/rust/issues/60405
[Version target](https://forge.rust-lang.org/#current-release-versions): 1.42 (2020-01-30 => beta, 2020-03-12 => stable).

## User guide

A `struct` with only a single non-ZST field (let's call it `foo`) can be marked as `#[repr(transparent)]`. Such a `struct` has the same layout and ABI as `foo`. Here, we also extend this ability to `enum`s with only one variant, subject to the same restrictions as for the equivalent `struct`. That is, you can now write:

```rust
#[repr(transparent)]
enum Foo { Bar(u8) }
```

which, in terms of layout and ABI, is equivalent to:

```rust
#[repr(transparent)]
struct Foo(u8);
```

## Motivation

This is not a major feature that will unlock new and important use-cases. The utility of `repr(transparent)` `enum`s is indeed limited. However, there is still some value in it:

1. It provides conceptual simplification of the language in terms of treating univariant `enum`s and `struct`s the same, as both are product types. Indeed, languages like Haskell only have `data` as the only way to construct user-defined ADTs in the language.

2. In rare occasions, it might be that the user started out with a univariant `enum` for whatever reason (e.g. they thought they might extend it later). Now they want to make this `enum` `transparent` without breaking users by turning it into a `struct`. By lifting the restriction here, now they can.

## Technical specification

The reference specifies [`repr(transparent)` on a `struct`](https://doc.rust-lang.org/nightly/reference/type-layout.html#the-transparent-representation) as:

> ### The transparent Representation
>
>  The `transparent` representation can only be used on `struct`s that have:
>  - a single field with non-zero size, and
>  - any number of fields with size 0 and alignment 1 (e.g. `PhantomData<T>`).
>
> Structs with this representation have the same layout and ABI as the single non-zero sized field.
>
> This is different than the `C` representation because a struct with the `C` representation will always have the ABI of a `C` `struct` while, for example, a struct with the `transparent` representation with a primitive field will have the ABI of the primitive field.
>
> Because this representation delegates type layout to another type, it cannot be used with any other representation.

Here, we amend this to include univariant `enum`s as well with the same static restrictions and the same effects on dynamic semantics.

## Tests

All the relevant tests are adjusted in the PR diff but are recounted here:

- `src/test/ui/repr/repr-transparent.rs` checks that `repr(transparent)` on an `enum` must be univariant, rather than having zero or more than one variant. Restrictions on the fields inside the only variants, like for those on `struct`s, are also checked here.

- A number of codegen tests are provided as well:
    - `src/test/codegen/repr-transparent.rs` (the canonical test)
    - `src/test/codegen/repr-transparent-aggregates-1.rs`
    - `src/test/codegen/repr-transparent-aggregates-2.rs`
    - `src/test/codegen/repr-transparent-aggregates-3.rs`

- `src/test/ui/lint/lint-ctypes-enum.rs` tests the interactions with the `improper_ctypes` lint.

## History

- 2019-04-30, RFC https://github.com/rust-lang/rfcs/pull/2645
  Author: @mjbshaw
  Reviewers: The Language Team

  This is the RFC that proposes allowing `#[repr(transparent)]` on `enum`s and `union`.

- 2019-06-11, PR https://github.com/rust-lang/rust/pull/60463
  Author: @mjbshaw
  Reviewers: @varkor and @rkruppe

  The PR implements the RFC aforementioned in full.

- 2019, PR https://github.com/rust-lang/rust/pull/67323
  Author: @Centril
  Reviewers: @davidtwco

  The PR reorganizes the static checks taking advantage of the fact that `struct`s and `union`s are internally represented as ADTs with a single variant.

- This PR stabilizes `transparent_enums`.

## Related / possible future work

The remaining work here is to figure out the semantics of `#[repr(transparent)]` on `union`s and stabilize those. This work continues to be tracked in https://github.com/rust-lang/rust/issues/60405.

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