codegen: Deduplicate derive traits added by add_derives() parse callback

bindgen-tests/tests/parse_callbacks/add_derives_callback/header_add_derives.h

@@ -0,0 +1,3 @@
+struct SimpleStruct {
+  int a;
+};

bindgen-tests/tests/parse_callbacks/add_derives_callback/mod.rs

@@ -0,0 +1,103 @@
+#[cfg(test)]
+mod tests {
+    use bindgen::callbacks::{DeriveInfo, ParseCallbacks};
+    use bindgen::{Bindings, Builder};
+    use std::path::PathBuf;
+
+    #[derive(Debug)]
+    struct AddDerivesCallback(Vec<String>);
+
+    impl ParseCallbacks for AddDerivesCallback {
+        fn add_derives(&self, _info: &DeriveInfo<'_>) -> Vec<String> {
+            self.0.clone()
+        }
+    }
+
+    struct WriteAdapter<'a>(&'a mut Vec<u8>);
+
+    impl std::io::Write for WriteAdapter<'_> {
+        fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+            self.0.extend_from_slice(buf);
+            Ok(buf.len())
+        }
+        fn flush(&mut self) -> std::io::Result<()> {
+            Ok(())
+        }
+    }
+
+    fn write_bindings_to_string(bindings: &Bindings) -> String {
+        let mut output = Vec::<u8>::new();
+        bindings
+            .write(Box::new(WriteAdapter(&mut output)))
+            .expect("Failed to write generated bindings");
+        String::from_utf8(output)
+            .expect("Failed to convert generated bindings to string")
+    }
+
+    /// Tests that adding a derive trait that's already derived automatically
+    /// does not result in a duplicate derive trait (which would not compile).
+    #[test]
+    fn test_add_derives_callback_dedupe() {
+        let crate_dir =
+            PathBuf::from(std::env::var("CARGO_MANIFEST_DIR").unwrap());
+        let header_path = crate_dir.join(
+            "tests/parse_callbacks/add_derives_callback/header_add_derives.h",
+        );
+
+        let builder = Builder::default()
+            .header(header_path.display().to_string())
+            .derive_debug(true)
+            .derive_copy(false)
+            .derive_default(false)
+            .derive_partialeq(false)
+            .derive_eq(false)
+            .derive_partialord(false)
+            .derive_ord(false)
+            .derive_hash(false)
+            .parse_callbacks(Box::new(AddDerivesCallback(vec![
+                "Debug".to_string()
+            ])));
+        let bindings = builder.generate().expect("Failed to generate bindings");
+        let output = write_bindings_to_string(&bindings);
+        let output_without_spaces = output.replace(' ', "");
+        assert!(
+            output_without_spaces.contains("#[derive(Debug)]") &&
+                !output_without_spaces.contains("#[derive(Debug,Debug)]"),
+            "Unexpected bindgen output:\n{}",
+            output.as_str()
+        );
+    }
+
+    /// Tests that adding a derive trait that's not already derived automatically
+    /// adds it to the end of the derive list.
+    #[test]
+    fn test_add_derives_callback() {
+        let crate_dir =
+            PathBuf::from(std::env::var("CARGO_MANIFEST_DIR").unwrap());
+        let header_path = crate_dir.join(
+            "tests/parse_callbacks/add_derives_callback/header_add_derives.h",
+        );
+
+        let builder = Builder::default()
+            .header(header_path.display().to_string())
+            .derive_debug(true)
+            .derive_copy(false)
+            .derive_default(false)
+            .derive_partialeq(false)
+            .derive_eq(false)
+            .derive_partialord(false)
+            .derive_ord(false)
+            .derive_hash(false)
+            .parse_callbacks(Box::new(AddDerivesCallback(vec![
+                "Default".to_string()
+            ])));
+        let bindings = builder.generate().expect("Failed to generate bindings");
+        let output = write_bindings_to_string(&bindings);
+        let output_without_spaces = output.replace(' ', "");
+        assert!(
+            output_without_spaces.contains("#[derive(Debug,Default)]"),
+            "Unexpected bindgen output:\n{}",
+            output.as_str()
+        );
+    }
+}

bindgen-tests/tests/parse_callbacks/mod.rs

@@ -1,3 +1,4 @@
+mod add_derives_callback;
 mod item_discovery_callback;
 
 use bindgen::callbacks::*;

bindgen/codegen/mod.rs

@@ -199,6 +199,25 @@ fn derives_of_item(
     derivable_traits
 }
 
+/// Appends the contents of the `custom_derives` iterator to the `derives` vector,
+/// ignoring duplicates and preserving order.
+fn append_custom_derives<'a, I>(derives: &mut Vec<&'a str>, custom_derives: I)
+where
+    I: Iterator<Item = &'a str>,
+{
+    // Use a HashSet to track already seen elements.
+    let mut seen: HashSet<&'a str> = Default::default();
+    seen.extend(derives.iter().copied());
+
+    // Add the custom derives to the derives vector, ignoring duplicates.
+    for custom_derive in custom_derives {
+        if !seen.contains(custom_derive) {
+            derives.push(custom_derive);
+            seen.insert(custom_derive);
+        }
+    }
+}
+
 impl From<DerivableTraits> for Vec<&'static str> {
     fn from(derivable_traits: DerivableTraits) -> Vec<&'static str> {
         [
@@ -1043,8 +1062,12 @@ impl CodeGenerator for Type {
                                 })
                             });
                         // In most cases this will be a no-op, since custom_derives will be empty.
-                        derives
-                            .extend(custom_derives.iter().map(|s| s.as_str()));
+                        if !custom_derives.is_empty() {
+                            append_custom_derives(
+                                &mut derives,
+                                custom_derives.iter().map(|s| s.as_str()),
+                            );
+                        }
                         attributes.push(attributes::derives(&derives));
 
                         let custom_attributes =
@@ -2475,7 +2498,12 @@ impl CodeGenerator for CompInfo {
             })
         });
         // In most cases this will be a no-op, since custom_derives will be empty.
-        derives.extend(custom_derives.iter().map(|s| s.as_str()));
+        if !custom_derives.is_empty() {
+            append_custom_derives(
+                &mut derives,
+                custom_derives.iter().map(|s| s.as_str()),
+            );
+        }
 
         if !derives.is_empty() {
             attributes.push(attributes::derives(&derives));
@@ -3678,7 +3706,12 @@ impl CodeGenerator for Enum {
                 })
             });
             // In most cases this will be a no-op, since custom_derives will be empty.
-            derives.extend(custom_derives.iter().map(|s| s.as_str()));
+            if !custom_derives.is_empty() {
+                append_custom_derives(
+                    &mut derives,
+                    custom_derives.iter().map(|s| s.as_str()),
+                );
+            }
 
             attrs.extend(
                 item.annotations()