Rollup merge of #123840 - scottmcm:aggregate-kind-rawptr, r=cjgillot

Add an intrinsic for `ptr::from_raw_parts(_mut)`

Fixes #123174
cc `@CAD97` `@saethlin`
r? `@cjgillot`

As suggested in rust-lang/rust#123190 (comment), this adds a new `AggregateKind::RawPtr` for creating a pointer from its data pointer and its metadata.

That means that `slice::from_raw_parts` and friends no longer need to hard-code pointer layout into `libcore`, and because it no longer does union hacks the MIR is shorter and more amenable to optimizations.

Author: matthiaskrgr

src/base.rs

@@ -813,6 +813,19 @@ fn codegen_stmt<'tcx>(
                     );
                     lval.write_cvalue(fx, val);
                 }
+                Rvalue::Aggregate(ref kind, ref operands)
+                    if matches!(**kind, AggregateKind::RawPtr(..)) =>
+                {
+                    let ty = to_place_and_rval.1.ty(&fx.mir.local_decls, fx.tcx);
+                    let layout = fx.layout_of(fx.monomorphize(ty));
+                    let [data, meta] = &*operands.raw else {
+                        bug!("RawPtr fields: {operands:?}");
+                    };
+                    let data = codegen_operand(fx, data);
+                    let meta = codegen_operand(fx, meta);
+                    let ptr_val = CValue::pointer_from_data_and_meta(data, meta, layout);
+                    lval.write_cvalue(fx, ptr_val);
+                }
                 Rvalue::Aggregate(ref kind, ref operands) => {
                     let (variant_index, variant_dest, active_field_index) = match **kind {
                         mir::AggregateKind::Adt(_, variant_index, _, _, active_field_index) => {

src/value_and_place.rs

@@ -94,6 +94,23 @@ impl<'tcx> CValue<'tcx> {
         CValue(CValueInner::ByValPair(value, extra), layout)
     }
 
+    /// For `AggregateKind::RawPtr`, create a pointer from its parts.
+    ///
+    /// Panics if the `layout` is not a raw pointer.
+    pub(crate) fn pointer_from_data_and_meta(
+        data: CValue<'tcx>,
+        meta: CValue<'tcx>,
+        layout: TyAndLayout<'tcx>,
+    ) -> CValue<'tcx> {
+        assert!(layout.ty.is_unsafe_ptr());
+        let inner = match (data.0, meta.0) {
+            (CValueInner::ByVal(p), CValueInner::ByVal(m)) => CValueInner::ByValPair(p, m),
+            (p @ CValueInner::ByVal(_), CValueInner::ByRef(..)) if meta.1.is_zst() => p,
+            _ => bug!("RawPtr operands {data:?} {meta:?}"),
+        };
+        CValue(inner, layout)
+    }
+
     pub(crate) fn layout(&self) -> TyAndLayout<'tcx> {
         self.1
     }