add new attribute rustc_insignificant_dtor and a query to check if a type has a significant drop

compiler/rustc_middle/src/query/mod.rs

@@ -1035,6 +1035,10 @@ rustc_queries! {
     query needs_drop_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` needs drop", env.value }
     }
+    /// Query backing `TyS::has_significant_drop_raw`.
+    query has_significant_drop_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
+        desc { "computing whether `{}` has a significant drop", env.value }
+    }
 
     /// Query backing `TyS::is_structural_eq_shallow`.
     ///
@@ -1055,6 +1059,17 @@ rustc_queries! {
         cache_on_disk_if { true }
     }
 
+    /// A list of types where the ADT requires drop if and only if any of those types
+    /// has significant drop. A type marked with the attribute `rustc_insignificant_dtor`
+    /// is considered to not be significant. A drop is significant if it is implemented
+    /// by the user or does anything that will have any observable behavior (other than
+    /// freeing up memory). If the ADT is known to have a significant destructor then
+    /// `Err(AlwaysRequiresDrop)` is returned.
+    query adt_significant_drop_tys(def_id: DefId) -> Result<&'tcx ty::List<Ty<'tcx>>, AlwaysRequiresDrop> {
+        desc { |tcx| "computing when `{}` has a significant destructor", tcx.def_path_str(def_id) }
+        cache_on_disk_if { false }
+    }
+
     query layout_raw(
         env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>
     ) -> Result<&'tcx rustc_target::abi::Layout, ty::layout::LayoutError<'tcx>> {

compiler/rustc_middle/src/ty/util.rs

@@ -791,6 +791,39 @@ impl<'tcx> ty::TyS<'tcx> {
         }
     }
 
+    /// Checks if `ty` has has a significant drop.
+    ///
+    /// Note that this method can return false even if `ty` has a destructor
+    /// attached; even if that is the case then the adt has been marked with
+    /// the attribute `rustc_insignificant_dtor`.
+    ///
+    /// Note that this method is used to check for change in drop order for
+    /// 2229 drop reorder migration analysis.
+    #[inline]
+    pub fn has_significant_drop(
+        &'tcx self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> bool {
+        // Avoid querying in simple cases.
+        match needs_drop_components(self, &tcx.data_layout) {
+            Err(AlwaysRequiresDrop) => true,
+            Ok(components) => {
+                let query_ty = match *components {
+                    [] => return false,
+                    // If we've got a single component, call the query with that
+                    // to increase the chance that we hit the query cache.
+                    [component_ty] => component_ty,
+                    _ => self,
+                };
+                // This doesn't depend on regions, so try to minimize distinct
+                // query keys used.
+                let erased = tcx.normalize_erasing_regions(param_env, query_ty);
+                tcx.has_significant_drop_raw(param_env.and(erased))
+            }
+        }
+    }
+
     /// Returns `true` if equality for this type is both reflexive and structural.
     ///
     /// Reflexive equality for a type is indicated by an `Eq` impl for that type.