Add an intrinsic for ptr::metadata

compiler/rustc_middle/src/mir/syntax.rs

@@ -1434,6 +1434,13 @@ pub enum UnOp {
     Not,
     /// The `-` operator for negation
     Neg,
+    /// Get the metadata `M` from a `*const/mut impl Pointee<Metadata = M>`.
+    ///
+    /// For example, this will give a `()` from `*const i32`, a `usize` from
+    /// `*mut [u8]`, or a pointer to a vtable from a `*const dyn Foo`.
+    ///
+    /// Allowed only in [`MirPhase::Runtime`]; earlier it's an intrinsic.
+    PtrMetadata,
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, PartialOrd, Ord, Eq, Hash)]

compiler/rustc_middle/src/mir/tcx.rs

@@ -180,7 +180,10 @@ impl<'tcx> Rvalue<'tcx> {
                 let rhs_ty = rhs.ty(local_decls, tcx);
                 op.ty(tcx, lhs_ty, rhs_ty)
             }
-            Rvalue::UnaryOp(UnOp::Not | UnOp::Neg, ref operand) => operand.ty(local_decls, tcx),
+            Rvalue::UnaryOp(op, ref operand) => {
+                let arg_ty = operand.ty(local_decls, tcx);
+                op.ty(tcx, arg_ty)
+            }
             Rvalue::Discriminant(ref place) => place.ty(local_decls, tcx).ty.discriminant_ty(tcx),
             Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf | NullOp::OffsetOf(..), _) => {
                 tcx.types.usize
@@ -282,6 +285,27 @@ impl<'tcx> BinOp {
     }
 }
 
+impl<'tcx> UnOp {
+    pub fn ty(&self, tcx: TyCtxt<'tcx>, arg_ty: Ty<'tcx>) -> Ty<'tcx> {
+        match self {
+            UnOp::Not | UnOp::Neg => arg_ty,
+            UnOp::PtrMetadata => {
+                let pointee_ty = arg_ty
+                    .builtin_deref(true)
+                    .unwrap_or_else(|| bug!("PtrMetadata of non-dereferenceable ty {arg_ty:?}"));
+                if pointee_ty.is_trivially_sized(tcx) {
+                    tcx.types.unit
+                } else {
+                    let Some(metadata_def_id) = tcx.lang_items().metadata_type() else {
+                        bug!("No metadata_type lang item while looking at {arg_ty:?}")
+                    };
+                    Ty::new_projection(tcx, metadata_def_id, [pointee_ty])
+                }
+            }
+        }
+    }
+}
+
 impl BorrowKind {
     pub fn to_mutbl_lossy(self) -> hir::Mutability {
         match self {

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1579,8 +1579,10 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
                 let formatted_op = match op {
                     UnOp::Not => "!",
                     UnOp::Neg => "-",
+                    UnOp::PtrMetadata => "PtrMetadata",
                 };
                 let parenthesized = match ct.kind() {
+                    _ if op == UnOp::PtrMetadata => true,
                     ty::ConstKind::Expr(Expr::UnOp(c_op, ..)) => c_op != op,
                     ty::ConstKind::Expr(_) => true,
                     _ => false,