Rollup merge of #114022 - oli-obk:tait_ice_alias_field_projection, r=cjgillot

Perform OpaqueCast field projection on HIR, too.

fixes #105819

This is necessary for closure captures in 2021 edition, as they capture individual fields, not the full mentioned variables. So it may try to capture a field of an opaque (because the hidden type is known to be something with a field).

See https://github.com/rust-lang/rust/pull/99806 for when and why we added OpaqueCast to MIR.

compiler/rustc_middle/src/ty/closure.rs

@@ -174,6 +174,8 @@ impl<'tcx> CapturedPlace<'tcx> {
                 // Ignore derefs for now, as they are likely caused by
                 // autoderefs that don't appear in the original code.
                 HirProjectionKind::Deref => {}
+                // Just change the type to the hidden type, so we can actually project.
+                HirProjectionKind::OpaqueCast => {}
                 proj => bug!("Unexpected projection {:?} in captured place", proj),
             }
             ty = proj.ty;