3 files changed, 75 insertions, 7 deletions

diff --git a/src/callbacks.rs b/src/callbacks.rs
index 1cd7f37b..e288af43 100644
--- a/src/callbacks.rs
+++ b/src/callbacks.rs
@@ -1,5 +1,7 @@
 //! A public API for more fine-grained customization of bindgen behavior.
 
+pub use crate::ir::analysis::DeriveTrait;
+pub use crate::ir::derive::CanDerive as ImplementsTrait;
 pub use crate::ir::enum_ty::{EnumVariantCustomBehavior, EnumVariantValue};
 pub use crate::ir::int::IntKind;
 use std::fmt;
@@ -76,4 +78,21 @@ pub trait ParseCallbacks: fmt::Debug + UnwindSafe {
 
     /// This will be called on every file inclusion, with the full path of the included file.
     fn include_file(&self, _filename: &str) {}
+
+    /// This will be called to determine whether a particular blocklisted type
+    /// implements a trait or not. This will be used to implement traits on
+    /// other types containing the blocklisted type.
+    ///
+    /// * `None`: use the default behavior
+    /// * `Some(ImplementsTrait::Yes)`: `_name` implements `_derive_trait`
+    /// * `Some(ImplementsTrait::Manually)`: any type including `_name` can't
+    ///   derive `_derive_trait` but can implemented it manually
+    /// * `Some(ImplementsTrait::No)`: `_name` doesn't implement `_derive_trait`
+    fn blocklisted_type_implements_trait(
+        &self,
+        _name: &str,
+        _derive_trait: DeriveTrait,
+    ) -> Option<ImplementsTrait> {
+        None
+    }
 }
diff --git a/src/ir/analysis/derive.rs b/src/ir/analysis/derive.rs
index fa4ce795..be626661 100644
--- a/src/ir/analysis/derive.rs
+++ b/src/ir/analysis/derive.rs
@@ -16,7 +16,7 @@ use crate::ir::ty::{Type, TypeKind};
 use crate::{Entry, HashMap, HashSet};
 
 /// Which trait to consider when doing the `CannotDerive` analysis.
-#[derive(Debug, Copy, Clone)]
+#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub enum DeriveTrait {
     /// The `Copy` trait.
     Copy,
@@ -139,11 +139,24 @@ impl<'ctx> CannotDerive<'ctx> {
 
     fn constrain_type(&mut self, item: &Item, ty: &Type) -> CanDerive {
         if !self.ctx.allowlisted_items().contains(&item.id()) {
-            trace!(
-                "    cannot derive {} for blocklisted type",
-                self.derive_trait
-            );
-            return CanDerive::No;
+            let can_derive = self
+                .ctx
+                .blocklisted_type_implements_trait(item, self.derive_trait);
+            match can_derive {
+                CanDerive::Yes => trace!(
+                    "    blocklisted type explicitly implements {}",
+                    self.derive_trait
+                ),
+                CanDerive::Manually => trace!(
+                    "    blocklisted type requires manual implementation of {}",
+                    self.derive_trait
+                ),
+                CanDerive::No => trace!(
+                    "    cannot derive {} for blocklisted type",
+                    self.derive_trait
+                ),
+            }
+            return can_derive;
         }
 
         if self.derive_trait.not_by_name(self.ctx, &item) {
diff --git a/src/ir/context.rs b/src/ir/context.rs
index a0d4de1e..ccb05e75 100644
--- a/src/ir/context.rs
+++ b/src/ir/context.rs
@@ -28,7 +28,7 @@ use cexpr;
 use clang_sys;
 use proc_macro2::{Ident, Span};
 use std::borrow::Cow;
-use std::cell::Cell;
+use std::cell::{Cell, RefCell};
 use std::collections::HashMap as StdHashMap;
 use std::iter::IntoIterator;
 use std::mem;
@@ -380,6 +380,10 @@ pub struct BindgenContext {
     /// computed after parsing our IR, and before running any of our analyses.
     allowlisted: Option<ItemSet>,
 
+    /// Cache for calls to `ParseCallbacks::blocklisted_type_implements_trait`
+    blocklisted_types_implement_traits:
+        RefCell<HashMap<DeriveTrait, HashMap<ItemId, CanDerive>>>,
+
     /// The set of `ItemId`s that are allowlisted for code generation _and_ that
     /// we should generate accounting for the codegen options.
     ///
@@ -560,6 +564,7 @@ If you encounter an error missing from this list, please file an issue or a PR!"
             options,
             generated_bindgen_complex: Cell::new(false),
             allowlisted: None,
+            blocklisted_types_implement_traits: Default::default(),
             codegen_items: None,
             used_template_parameters: None,
             need_bitfield_allocation: Default::default(),
@@ -2205,6 +2210,37 @@ If you encounter an error missing from this list, please file an issue or a PR!"
         self.allowlisted.as_ref().unwrap()
     }
 
+    /// Check whether a particular blocklisted type implements a trait or not.
+    /// Results may be cached.
+    pub fn blocklisted_type_implements_trait(
+        &self,
+        item: &Item,
+        derive_trait: DeriveTrait,
+    ) -> CanDerive {
+        assert!(self.in_codegen_phase());
+        assert!(self.current_module == self.root_module);
+
+        let cb = match self.options.parse_callbacks {
+            Some(ref cb) => cb,
+            None => return CanDerive::No,
+        };
+
+        *self
+            .blocklisted_types_implement_traits
+            .borrow_mut()
+            .entry(derive_trait)
+            .or_default()
+            .entry(item.id())
+            .or_insert_with(|| {
+                item.expect_type()
+                    .name()
+                    .and_then(|name| {
+                        cb.blocklisted_type_implements_trait(name, derive_trait)
+                    })
+                    .unwrap_or(CanDerive::No)
+            })
+    }
+
     /// Get a reference to the set of items we should generate.
     pub fn codegen_items(&self) -> &ItemSet {
         assert!(self.in_codegen_phase());