Allow #[repr(align(x))] on enums (#57996)
Tracking issue: #57996
Implements an extension of [RFC 1358](https://github.com/rust-lang/rfcs/blob/master/text/1358-repr-align.md) behind a feature flag (`repr_align_enum`). Originally introduced here for structs: #39999.
It seems like only HIR-level changes are required, since enums are already aware of their alignment (due to alignment of their limbs).
cc @bitshifter
Document that `-C opt-level=0` implies `-C debug-assertions`.
I couldn't find it stated anywhere else (https://doc.rust-lang.org/nightly/rustc/codegen-options/index.html#opt-level).
It was a problem before here: https://github.com/rust-lang/rust/issues/39449, it got lost in the migration to the new documentation I assume.
On a sidenote: I think that `-C opt-level=0` having a sideeffect on another flag should be changed. Having compiler flags affecting others doesn't make much sense to me, they are used to fine tune anyway.
In any case, this plays no role in this PR.
Implement the Re-rebalance coherence RFC
This is the first time I touch anything in the compiler so just tell me if I got something wrong.
Big thanks to @sgrif for the pointers where to look for those things.
cc #55437
Implement RFC 2338, "Type alias enum variants"
This PR implements [RFC 2338](https://github.com/rust-lang/rfcs/pull/2338), allowing one to write code like the following.
```rust
#![feature(type_alias_enum_variants)]
enum Foo {
Bar(i32),
Baz { i: i32 },
}
type Alias = Foo;
fn main() {
let t = Alias::Bar(0);
let t = Alias::Baz { i: 0 };
match t {
Alias::Bar(_i) => {}
Alias::Baz { i: _i } => {}
}
}
```
Since `Self` can be considered a type alias in this context, it also enables using `Self::Variant` as both a constructor and pattern.
Fixes issues #56199 and #56611.
N.B., after discussing the syntax for type arguments on enum variants with @petrochenkov and @eddyb (there are also a few comments on the [tracking issue](https://github.com/rust-lang/rust/issues/49683)), the consensus seems to be treat the syntax as follows, which ought to be backwards-compatible.
```rust
Option::<u8>::None; // OK
Option::None::<u8>; // OK, but lint in near future (hard error next edition?)
Alias::<u8>::None; // OK
Alias::None::<u8>; // Error
```
I do not know if this will need an FCP, but let's start one if so.
docs(rustc): make hello() public
Running the example code [here](https://doc.rust-lang.org/rustc/what-is-rustc.html#basic-usage) throws error:
```
error[E0603]: function `hello` is private
--> src/main.rs:4:10
|
4 | foo::hello();
| ^^^^^
```
Making `hello()` public fixes the problem.
Stabilize `linker-flavor` flag.
Part of #55396.
This commit moves the linker-flavor flag from a debugging option to a
codegen option, thus stabilizing it. There are no feature flags
associated with this flag.
r? @nagisa
This commit moves the linker-flavor flag from a debugging option to a
codegen option, thus stabilizing it. There are no feature flags
associated with this flag.
Update the book to fix some edition-related bugs
I'd love to have this included in a point release if one happens, but the changes aren't worth doing a point release on their own IMO. I'd definitely like to see this backported to beta.
The most urgent changes included here are:
- Chapter 19.6, on macros (including the macro changes that just stabilized), [is cut off](https://github.com/rust-lang/book/issues/1668) because of an ill-placed newline in a comment that [pulldown-cmark interprets incorrectly](https://github.com/raphlinus/pulldown-cmark/issues/124).
- [The `Cargo.toml` shown in the Guessing Game example in Chapter 2 (and one in Chapter 14) doesn't have `edition="2018"`](https://github.com/rust-lang/book/pull/1671), which can be confusing depending on whether the readers have it in theirs or not, think they should remove it if they have it, and the rest of the chapter assumes it's there and the code doesn't compile if you don't have it.
- The redirects implemented as part of only shipping the 2018 edition of the book sometimes lead to [having to click through 2 redirect pages](https://github.com/rust-lang/book/pull/1667) when it could be just one.
There are other small corrections included that were made since the last time the book was updated, but those aren't urgent.
cc @steveklabnik
