Rollup merge of #136985 - zachs18:backend-repr-remove-uninhabited, r=workingjubilee

Do not ignore uninhabited types for function-call ABI purposes. (Remove BackendRepr::Uninhabited)

Accepted MCP: https://github.com/rust-lang/compiler-team/issues/832

Fixes #135802

Do not consider the inhabitedness of a type for function call ABI purposes.

* Remove the [`rustc_abi::BackendRepr::Uninhabited`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_abi/enum.BackendRepr.html) variant
  * Instead calculate the `BackendRepr` of uninhabited types "normally" (as though they were not uninhabited "at the top level", but still considering inhabitedness of variants to determine enum layout, etc)
* Add an `uninhabited: bool` field to [`rustc_abi::LayoutData`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_abi/struct.LayoutData.html) so inhabitedness of a `LayoutData` can still be queried when necessary (e.g. when determining if an enum variant needs a tag value allocated to it).

This should not affect type layouts (size/align/field offset); this should only affect function call ABI, and only of uninhabited types.

cc ``@RalfJung``

compiler/rustc_target/src/callconv/loongarch.rs

@@ -80,7 +80,7 @@ where
                 }
             }
         },
-        BackendRepr::Vector { .. } | BackendRepr::Uninhabited => return Err(CannotUseFpConv),
+        BackendRepr::Vector { .. } => return Err(CannotUseFpConv),
         BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => match arg_layout.fields {
             FieldsShape::Primitive => {
                 unreachable!("aggregates can't have `FieldsShape::Primitive`")

compiler/rustc_target/src/callconv/mod.rs

@@ -38,7 +38,7 @@ mod xtensa;
 pub enum PassMode {
     /// Ignore the argument.
     ///
-    /// The argument is either uninhabited or a ZST.
+    /// The argument is a ZST.
     Ignore,
     /// Pass the argument directly.
     ///
@@ -350,7 +350,6 @@ impl<'a, Ty> ArgAbi<'a, Ty> {
         scalar_attrs: impl Fn(&TyAndLayout<'a, Ty>, Scalar, Size) -> ArgAttributes,
     ) -> Self {
         let mode = match layout.backend_repr {
-            BackendRepr::Uninhabited => PassMode::Ignore,
             BackendRepr::Scalar(scalar) => {
                 PassMode::Direct(scalar_attrs(&layout, scalar, Size::ZERO))
             }

compiler/rustc_target/src/callconv/riscv.rs

@@ -86,7 +86,7 @@ where
                 }
             }
         },
-        BackendRepr::Vector { .. } | BackendRepr::Uninhabited => return Err(CannotUseFpConv),
+        BackendRepr::Vector { .. } => return Err(CannotUseFpConv),
         BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => match arg_layout.fields {
             FieldsShape::Primitive => {
                 unreachable!("aggregates can't have `FieldsShape::Primitive`")

compiler/rustc_target/src/callconv/x86.rs

@@ -108,9 +108,7 @@ where
                 Ty: TyAbiInterface<'a, C> + Copy,
             {
                 match layout.backend_repr {
-                    BackendRepr::Uninhabited
-                    | BackendRepr::Scalar(_)
-                    | BackendRepr::ScalarPair(..) => false,
+                    BackendRepr::Scalar(_) | BackendRepr::ScalarPair(..) => false,
                     BackendRepr::Vector { .. } => true,
                     BackendRepr::Memory { .. } => {
                         for i in 0..layout.fields.count() {

compiler/rustc_target/src/callconv/x86_64.rs

@@ -51,8 +51,6 @@ where
         }
 
         let mut c = match layout.backend_repr {
-            BackendRepr::Uninhabited => return Ok(()),
-
             BackendRepr::Scalar(scalar) => match scalar.primitive() {
                 Primitive::Int(..) | Primitive::Pointer(_) => Class::Int,
                 Primitive::Float(_) => Class::Sse,

compiler/rustc_target/src/callconv/x86_win64.rs

@@ -8,7 +8,7 @@ use crate::spec::{HasTargetSpec, RustcAbi};
 pub(crate) fn compute_abi_info<Ty>(cx: &impl HasTargetSpec, fn_abi: &mut FnAbi<'_, Ty>) {
     let fixup = |a: &mut ArgAbi<'_, Ty>, is_ret: bool| {
         match a.layout.backend_repr {
-            BackendRepr::Uninhabited | BackendRepr::Memory { sized: false } => {}
+            BackendRepr::Memory { sized: false } => {}
             BackendRepr::ScalarPair(..) | BackendRepr::Memory { sized: true } => {
                 match a.layout.size.bits() {
                     8 => a.cast_to(Reg::i8()),