explain why we dont skip some of this work when there are field projections

compiler/rustc_middle/src/mir/statement.rs

@@ -441,6 +441,8 @@ pub fn ty_from<D>(
     where
         D: ?Sized + HasLocalDecls<'tcx>,
     {
+        // If there's a field projection element in `projection`, we *could* skip everything
+        // before that, but on 2026-01-31 a perf experiment showed no benefit from doing so.
         PlaceTy::from_ty(local_decls.local_decls()[local].ty).multi_projection_ty(tcx, projection)
     }