Auto merge of #99806 - oli-obk:unconstrained_opaque_type, r=estebank

Allow patterns to constrain the hidden type of opaque types

fixes #96572

reverts a revert as original PR was a perf regression that was fixed by reverting it: rust-lang/rust#99368 (comment))

TODO:

* check if rust-lang/rust#99685 is avoided

src/base.rs

@@ -850,6 +850,7 @@ pub(crate) fn codegen_place<'tcx>(
             PlaceElem::Deref => {
                 cplace = cplace.place_deref(fx);
             }
+            PlaceElem::OpaqueCast(ty) => cplace = cplace.place_opaque_cast(fx, ty),
             PlaceElem::Field(field, _ty) => {
                 cplace = cplace.place_field(fx, field);
             }

src/value_and_place.rs

@@ -621,6 +621,14 @@ impl<'tcx> CPlace<'tcx> {
         }
     }
 
+    pub(crate) fn place_opaque_cast(
+        self,
+        fx: &mut FunctionCx<'_, '_, 'tcx>,
+        ty: Ty<'tcx>,
+    ) -> CPlace<'tcx> {
+        CPlace { inner: self.inner, layout: fx.layout_of(ty) }
+    }
+
     pub(crate) fn place_field(
         self,
         fx: &mut FunctionCx<'_, '_, 'tcx>,