Auto merge of #143924 - davidtwco:sve-infrastructure, r=workingjubilee

`rustc_scalable_vector(N)`

Supercedes rust-lang/rust#118917.

Initial experimental implementation of rust-lang/rfcs#3838. Introduces a `rustc_scalable_vector(N)` attribute that can be applied to types with a single `[$ty]` field (for `u{16,32,64}`, `i{16,32,64}`, `f{32,64}`, `bool`). `rustc_scalable_vector` types are lowered to scalable vectors in the codegen backend.

As with any unstable feature, there will necessarily be follow-ups as we experiment and find cases that we've not considered or still need some logic to handle, but this aims to be a decent baseline to start from.

See rust-lang/rust#145052 for request for a lang experiment.

Author: bors

rustc_public/src/abi.rs

@@ -225,6 +225,10 @@ pub enum ValueAbi {
         element: Scalar,
         count: u64,
     },
+    ScalableVector {
+        element: Scalar,
+        count: u64,
+    },
     Aggregate {
         /// If true, the size is exact, otherwise it's only a lower bound.
         sized: bool,
@@ -235,7 +239,15 @@ impl ValueAbi {
     /// Returns `true` if the layout corresponds to an unsized type.
     pub fn is_unsized(&self) -> bool {
         match *self {
-            ValueAbi::Scalar(_) | ValueAbi::ScalarPair(..) | ValueAbi::Vector { .. } => false,
+            ValueAbi::Scalar(_)
+            | ValueAbi::ScalarPair(..)
+            | ValueAbi::Vector { .. }
+            // FIXME(rustc_scalable_vector): Scalable vectors are `Sized` while the
+            // `sized_hierarchy` feature is not yet fully implemented. After `sized_hierarchy` is
+            // fully implemented, scalable vectors will remain `Sized`, they just won't be
+            // `const Sized` - whether `is_unsized` continues to return `false` at that point will
+            // need to be revisited and will depend on what `is_unsized` is used for.
+            | ValueAbi::ScalableVector { .. } => false,
             ValueAbi::Aggregate { sized } => !sized,
         }
     }

rustc_public/src/unstable/convert/stable/abi.rs

@@ -256,6 +256,9 @@ impl<'tcx> Stable<'tcx> for rustc_abi::BackendRepr {
             rustc_abi::BackendRepr::SimdVector { element, count } => {
                 ValueAbi::Vector { element: element.stable(tables, cx), count }
             }
+            rustc_abi::BackendRepr::ScalableVector { element, count } => {
+                ValueAbi::ScalableVector { element: element.stable(tables, cx), count }
+            }
             rustc_abi::BackendRepr::Memory { sized } => ValueAbi::Aggregate { sized },
         }
     }