Rollup merge of #124251 - scottmcm:unop-ptr-metadata, r=oli-obk

Add an intrinsic for `ptr::metadata`

The follow-up to #123840, so we can remove `PtrComponents` and `PtrRepr` from libcore entirely (well, after a bootstrap update).

As discussed in <https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/.60ptr_metadata.60.20in.20MIR/near/435637808>, this introduces `UnOp::PtrMetadata` taking a raw pointer and returning the associated metadata value.

By no longer going through a `union`, this should also help future PRs better optimize pointer operations.

r? ``@oli-obk``

Author: jieyouxu

src/base.rs

@@ -616,22 +616,34 @@ fn codegen_stmt<'tcx>(
                 Rvalue::UnaryOp(un_op, ref operand) => {
                     let operand = codegen_operand(fx, operand);
                     let layout = operand.layout();
-                    let val = operand.load_scalar(fx);
                     let res = match un_op {
-                        UnOp::Not => match layout.ty.kind() {
-                            ty::Bool => {
-                                let res = fx.bcx.ins().icmp_imm(IntCC::Equal, val, 0);
-                                CValue::by_val(res, layout)
+                        UnOp::Not => {
+                            let val = operand.load_scalar(fx);
+                            match layout.ty.kind() {
+                                ty::Bool => {
+                                    let res = fx.bcx.ins().icmp_imm(IntCC::Equal, val, 0);
+                                    CValue::by_val(res, layout)
+                                }
+                                ty::Uint(_) | ty::Int(_) => {
+                                    CValue::by_val(fx.bcx.ins().bnot(val), layout)
+                                }
+                                _ => unreachable!("un op Not for {:?}", layout.ty),
                             }
-                            ty::Uint(_) | ty::Int(_) => {
-                                CValue::by_val(fx.bcx.ins().bnot(val), layout)
+                        }
+                        UnOp::Neg => {
+                            let val = operand.load_scalar(fx);
+                            match layout.ty.kind() {
+                                ty::Int(_) => CValue::by_val(fx.bcx.ins().ineg(val), layout),
+                                ty::Float(_) => CValue::by_val(fx.bcx.ins().fneg(val), layout),
+                                _ => unreachable!("un op Neg for {:?}", layout.ty),
                             }
-                            _ => unreachable!("un op Not for {:?}", layout.ty),
-                        },
-                        UnOp::Neg => match layout.ty.kind() {
-                            ty::Int(_) => CValue::by_val(fx.bcx.ins().ineg(val), layout),
-                            ty::Float(_) => CValue::by_val(fx.bcx.ins().fneg(val), layout),
-                            _ => unreachable!("un op Neg for {:?}", layout.ty),
+                        }
+                        UnOp::PtrMetadata => match layout.abi {
+                            Abi::Scalar(_) => CValue::zst(dest_layout),
+                            Abi::ScalarPair(_, _) => {
+                                CValue::by_val(operand.load_scalar_pair(fx).1, dest_layout)
+                            }
+                            _ => bug!("Unexpected `PtrToMetadata` operand: {operand:?}"),
                         },
                     };
                     lval.write_cvalue(fx, res);

src/constant.rs

@@ -100,7 +100,7 @@ pub(crate) fn codegen_const_value<'tcx>(
     assert!(layout.is_sized(), "unsized const value");
 
     if layout.is_zst() {
-        return CValue::by_ref(crate::Pointer::dangling(layout.align.pref), layout);
+        return CValue::zst(layout);
     }
 
     match const_val {

src/value_and_place.rs

@@ -95,6 +95,14 @@ impl<'tcx> CValue<'tcx> {
         CValue(CValueInner::ByValPair(value, extra), layout)
     }
 
+    /// Create an instance of a ZST
+    ///
+    /// The is represented by a dangling pointer of suitable alignment.
+    pub(crate) fn zst(layout: TyAndLayout<'tcx>) -> CValue<'tcx> {
+        assert!(layout.is_zst());
+        CValue::by_ref(crate::Pointer::dangling(layout.align.pref), layout)
+    }
+
     pub(crate) fn layout(&self) -> TyAndLayout<'tcx> {
         self.1
     }