Rollup merge of #123043 - GoldsteinE:fix/repr-c-dead-branches, r=oli-obk

Disable dead variant removal for `#[repr(C)]` enums.

This prevents removing dead branches from a `#[repr(C)]` enum (they now get discriminants allocated as if they were inhabited).

Implementation notes: ABI of something like

```rust
#[repr(C)]
enum Foo {
    Foo(!),
}
```

is still `Uninhabited`, but its layout is now computed as if all the branches were inhabited.
This seemed to me like a proper way to do it, especially given that ABI sanity check explicitly asserts that type-level uninhabitedness implies ABI uninhabitedness.

This probably needs some sort of FCP (given that it changes `#[repr(C)]` layout, which is a stable guarantee), but I’m not sure how to call for one or which team is the most relevant.

See https://github.com/rust-lang/unsafe-code-guidelines/issues/500.

2 files changed, 3 insertions, 3 deletions

diff --git a/compiler/rustc_abi/src/layout.rs b/compiler/rustc_abi/src/layout.rs
index 9165b4f2df3..197dd7f9c9e 100644
--- a/compiler/rustc_abi/src/layout.rs
+++ b/compiler/rustc_abi/src/layout.rs
@@ -186,7 +186,7 @@ pub trait LayoutCalculator {
         let (present_first, present_second) = {
             let mut present_variants = variants
                 .iter_enumerated()
-                .filter_map(|(i, v)| if absent(v) { None } else { Some(i) });
+                .filter_map(|(i, v)| if !repr.c() && absent(v) { None } else { Some(i) });
             (present_variants.next(), present_variants.next())
         };
         let present_first = match present_first {
@@ -621,7 +621,7 @@ where
     let discr_type = repr.discr_type();
     let bits = Integer::from_attr(dl, discr_type).size().bits();
     for (i, mut val) in discriminants {
-        if variants[i].iter().any(|f| f.abi.is_uninhabited()) {
+        if !repr.c() && variants[i].iter().any(|f| f.abi.is_uninhabited()) {
             continue;
         }
         if discr_type.is_signed() {
diff --git a/compiler/rustc_abi/src/lib.rs b/compiler/rustc_abi/src/lib.rs
index 31c66a56bea..78332d66f03 100644
--- a/compiler/rustc_abi/src/lib.rs
+++ b/compiler/rustc_abi/src/lib.rs
@@ -1429,7 +1429,7 @@ pub enum Variants<FieldIdx: Idx, VariantIdx: Idx> {
     /// Single enum variants, structs/tuples, unions, and all non-ADTs.
     Single { index: VariantIdx },
 
-    /// Enum-likes with more than one inhabited variant: each variant comes with
+    /// Enum-likes with more than one variant: each variant comes with
     /// a *discriminant* (usually the same as the variant index but the user can
     /// assign explicit discriminant values). That discriminant is encoded
     /// as a *tag* on the machine. The layout of each variant is