Use default literals instead of Default::default() in const initialization (#66)

lucic71 · web-flow · commit a8625eb34447 · 2026-05-10T18:02:18.000+01:00
`Default::default()` is not a const function, hence it cannot be used to
initialize global objects such as `pub static mut static_outer: Outer =
Outer::default()`.

To overcome this limitation, emit the default literal for the object,
for example:

```rs
pub static mut static_outer: Outer = Outer {
    p1: std::ptr::null_mut(),
    p2: std::ptr::null(),
    arr: [std::ptr::null_mut(); 3],
    cp: std::ptr::null(),
    pp: std::ptr::null_mut(),
    inner: Inner {
        v: 0_i32,
        name: std::ptr::null(),
    },
    x: 0_i32,
    fn_: None,
};
```
diff --git a/cpp2rust/converter/converter.cpp b/cpp2rust/converter/converter.cpp
@@ -445,6 +445,8 @@ void Converter::ConvertVarDecl(clang::VarDecl *decl) {
     return;
   }
   auto qual_type = decl->getType();
+  PushConstInitializer static_init(*this, decl->isFileVarDecl() ||
+                                              decl->isStaticLocal());
   if (decl->hasInit()) {
     StrCat(token::kAssign);
     ConvertVarInit(qual_type, decl->getInit());
@@ -3043,6 +3045,18 @@ std::string Converter::GetDefaultAsStringFallback(clang::QualType qual_type) {
     return std::format("0.0_{}", ToString(qual_type));
   }
 
+  if (auto record = qual_type->getAsRecordDecl();
+      record && in_const_initializer_) {
+    if (auto cxx = clang::dyn_cast<clang::CXXRecordDecl>(record)) {
+      ENSURE(GetUserDefinedDefaultConstructor(cxx) == nullptr &&
+             "Default initializing globals using default constructor is not "
+             "supported");
+    }
+    Buffer buf(*this);
+    EmitDefaultStructLiteral(record);
+    return std::move(buf).str();
+  }
+
   return std::format("<{}>::default()", ToString(qual_type));
 }
 
@@ -3458,13 +3472,15 @@ void Converter::AddDefaultTrait(const clang::RecordDecl *decl) {
     }
   }
 
-  StrCat(struct_name);
-  {
-    PushBrace struct_brace(*this);
-    for (auto *field : decl->fields()) {
-      StrCat(GetNamedDeclAsString(field), token::kColon,
-             GetDefaultAsString(field->getType()), token::kComma);
-    }
+  EmitDefaultStructLiteral(decl);
+}
+
+void Converter::EmitDefaultStructLiteral(const clang::RecordDecl *decl) {
+  StrCat(GetRecordName(decl));
+  PushBrace brace(*this);
+  for (auto *field : decl->fields()) {
+    StrCat(GetNamedDeclAsString(field), token::kColon,
+           GetDefaultAsString(field->getType()), token::kComma);
   }
 }
 
diff --git a/cpp2rust/converter/converter.h b/cpp2rust/converter/converter.h
@@ -464,6 +464,8 @@ class Converter : public clang::RecursiveASTVisitor<Converter> {
 
   virtual void AddDefaultTraitForUnion(const clang::RecordDecl *decl);
 
+  void EmitDefaultStructLiteral(const clang::RecordDecl *decl);
+
   virtual void AddByteReprTrait(const clang::RecordDecl *decl);
 
   virtual void
@@ -517,6 +519,7 @@ class Converter : public clang::RecursiveASTVisitor<Converter> {
   clang::ASTContext &ctx_;
   clang::FunctionDecl *curr_function_ = nullptr;
   bool in_function_formals_ = false;
+  bool in_const_initializer_ = false;
   std::optional<bool> autoref_mut_;
 
   struct PushExplicitAutoref {
@@ -528,6 +531,23 @@ class Converter : public clang::RecursiveASTVisitor<Converter> {
     }
     ~PushExplicitAutoref() { c.autoref_mut_ = prev; }
   };
+
+  struct PushConstInitializer {
+    Converter &c;
+    bool prev;
+    bool enabled;
+    PushConstInitializer(Converter &c, bool enabled)
+        : c(c), prev(c.in_const_initializer_), enabled(enabled) {
+      if (enabled) {
+        c.in_const_initializer_ = true;
+      }
+    }
+    ~PushConstInitializer() {
+      if (enabled) {
+        c.in_const_initializer_ = prev;
+      }
+    }
+  };
   std::stack<clang::Expr *> curr_for_inc_;
   std::stack<clang::QualType> curr_init_type_;
 
diff --git a/tests/unit/default_in_statics.cpp b/tests/unit/default_in_statics.cpp
@@ -0,0 +1,82 @@
+#include <cassert>
+#include <cstddef>
+
+typedef int (*FnPtr)(int);
+
+struct Inner {
+  int v;
+  const char *name;
+};
+
+struct Outer {
+  int *p1;
+  const int *p2;
+  int *arr[3];
+  const char *cp;
+  int **pp;
+  Inner inner;
+  int x;
+  FnPtr fn;
+};
+
+struct Foo {
+  const char *s1;
+  const char *s2;
+  FnPtr fn1;
+  FnPtr fn2;
+  int n;
+};
+
+static FnPtr static_fn;
+static Outer static_outer;
+static Inner static_inner_array[2];
+
+static Foo static_foo = {"hello", 0, 0, 0, 42};
+
+static Foo static_foo_array[2] = {
+    {"first", 0, 0, 0, 1},
+    {"second", 0, 0, 0, 2},
+};
+
+void check_local_static() {
+  static Outer local_outer;
+  static FnPtr local_fn;
+  static int *local_p;
+  assert(local_outer.p1 == nullptr);
+  assert(local_outer.fn == nullptr);
+  assert(local_fn == nullptr);
+  assert(local_p == nullptr);
+}
+
+int main() {
+  assert(static_fn == nullptr);
+
+  assert(static_outer.p1 == nullptr);
+  assert(static_outer.p2 == nullptr);
+  assert(static_outer.cp == nullptr);
+  assert(static_outer.pp == nullptr);
+  assert(static_outer.fn == nullptr);
+  for (int i = 0; i < 3; ++i) {
+    assert(static_outer.arr[i] == nullptr);
+  }
+  assert(static_outer.inner.name == nullptr);
+
+  for (int i = 0; i < 2; ++i) {
+    assert(static_inner_array[i].name == nullptr);
+  }
+
+  assert(static_foo.s2 == nullptr);
+  assert(static_foo.fn1 == nullptr);
+  assert(static_foo.fn2 == nullptr);
+  assert(static_foo.n == 42);
+
+  for (int i = 0; i < 2; ++i) {
+    assert(static_foo_array[i].s2 == nullptr);
+    assert(static_foo_array[i].fn1 == nullptr);
+    assert(static_foo_array[i].fn2 == nullptr);
+  }
+
+  check_local_static();
+
+  return 0;
+}
diff --git a/tests/unit/out/refcount/default_in_statics.rs b/tests/unit/out/refcount/default_in_statics.rs
@@ -0,0 +1,215 @@
+extern crate libcc2rs;
+use libcc2rs::*;
+use std::cell::RefCell;
+use std::collections::BTreeMap;
+use std::io::prelude::*;
+use std::io::{Read, Seek, Write};
+use std::os::fd::AsFd;
+use std::rc::{Rc, Weak};
+#[derive(Default)]
+pub struct Inner {
+    pub v: Value<i32>,
+    pub name: Value<Ptr<u8>>,
+}
+impl Clone for Inner {
+    fn clone(&self) -> Self {
+        let mut this = Self {
+            v: Rc::new(RefCell::new((*self.v.borrow()))),
+            name: Rc::new(RefCell::new((*self.name.borrow()).clone())),
+        };
+        this
+    }
+}
+impl ByteRepr for Inner {}
+#[derive()]
+pub struct Outer {
+    pub p1: Value<Ptr<i32>>,
+    pub p2: Value<Ptr<i32>>,
+    pub arr: Value<Box<[Ptr<i32>]>>,
+    pub cp: Value<Ptr<u8>>,
+    pub pp: Value<Ptr<Ptr<i32>>>,
+    pub inner: Value<Inner>,
+    pub x: Value<i32>,
+    pub fn_: Value<FnPtr<fn(i32) -> i32>>,
+}
+impl Clone for Outer {
+    fn clone(&self) -> Self {
+        let mut this = Self {
+            p1: Rc::new(RefCell::new((*self.p1.borrow()).clone())),
+            p2: Rc::new(RefCell::new((*self.p2.borrow()).clone())),
+            arr: Rc::new(RefCell::new((*self.arr.borrow()).clone())),
+            cp: Rc::new(RefCell::new((*self.cp.borrow()).clone())),
+            pp: Rc::new(RefCell::new((*self.pp.borrow()).clone())),
+            inner: Rc::new(RefCell::new((*self.inner.borrow()).clone())),
+            x: Rc::new(RefCell::new((*self.x.borrow()))),
+            fn_: Rc::new(RefCell::new((*self.fn_.borrow()).clone())),
+        };
+        this
+    }
+}
+impl Default for Outer {
+    fn default() -> Self {
+        Outer {
+            p1: Rc::new(RefCell::new(Ptr::<i32>::null())),
+            p2: Rc::new(RefCell::new(Ptr::<i32>::null())),
+            arr: Rc::new(RefCell::new(
+                (0..3)
+                    .map(|_| Ptr::<i32>::null())
+                    .collect::<Box<[Ptr<i32>]>>(),
+            )),
+            cp: Rc::new(RefCell::new(Ptr::<u8>::null())),
+            pp: Rc::new(RefCell::new(Ptr::<Ptr<i32>>::null())),
+            inner: <Value<Inner>>::default(),
+            x: <Value<i32>>::default(),
+            fn_: Rc::new(RefCell::new(FnPtr::null())),
+        }
+    }
+}
+impl ByteRepr for Outer {}
+#[derive()]
+pub struct Foo {
+    pub s1: Value<Ptr<u8>>,
+    pub s2: Value<Ptr<u8>>,
+    pub fn1: Value<FnPtr<fn(i32) -> i32>>,
+    pub fn2: Value<FnPtr<fn(i32) -> i32>>,
+    pub n: Value<i32>,
+}
+impl Clone for Foo {
+    fn clone(&self) -> Self {
+        let mut this = Self {
+            s1: Rc::new(RefCell::new((*self.s1.borrow()).clone())),
+            s2: Rc::new(RefCell::new((*self.s2.borrow()).clone())),
+            fn1: Rc::new(RefCell::new((*self.fn1.borrow()).clone())),
+            fn2: Rc::new(RefCell::new((*self.fn2.borrow()).clone())),
+            n: Rc::new(RefCell::new((*self.n.borrow()))),
+        };
+        this
+    }
+}
+impl Default for Foo {
+    fn default() -> Self {
+        Foo {
+            s1: Rc::new(RefCell::new(Ptr::<u8>::null())),
+            s2: Rc::new(RefCell::new(Ptr::<u8>::null())),
+            fn1: Rc::new(RefCell::new(FnPtr::null())),
+            fn2: Rc::new(RefCell::new(FnPtr::null())),
+            n: <Value<i32>>::default(),
+        }
+    }
+}
+impl ByteRepr for Foo {}
+thread_local!(
+    pub static static_fn: Value<FnPtr<fn(i32) -> i32>> = Rc::new(RefCell::new(FnPtr::null()));
+);
+thread_local!(
+    pub static static_outer: Value<Outer> = Rc::new(RefCell::new(<Outer>::default()));
+);
+thread_local!(
+    pub static static_inner_array: Value<Box<[Inner]>> = Rc::new(RefCell::new(
+        (0..2).map(|_| <Inner>::default()).collect::<Box<[Inner]>>(),
+    ));
+);
+thread_local!(
+    pub static static_foo: Value<Foo> = Rc::new(RefCell::new(Foo {
+        s1: Rc::new(RefCell::new(Ptr::from_string_literal("hello"))),
+        s2: Rc::new(RefCell::new(Default::default())),
+        fn1: Rc::new(RefCell::new(FnPtr::null())),
+        fn2: Rc::new(RefCell::new(FnPtr::null())),
+        n: Rc::new(RefCell::new(42)),
+    }));
+);
+thread_local!(
+    pub static static_foo_array: Value<Box<[Foo]>> = Rc::new(RefCell::new(Box::new([
+        Foo {
+            s1: Rc::new(RefCell::new(Ptr::from_string_literal("first"))),
+            s2: Rc::new(RefCell::new(Default::default())),
+            fn1: Rc::new(RefCell::new(FnPtr::null())),
+            fn2: Rc::new(RefCell::new(FnPtr::null())),
+            n: Rc::new(RefCell::new(1)),
+        },
+        Foo {
+            s1: Rc::new(RefCell::new(Ptr::from_string_literal("second"))),
+            s2: Rc::new(RefCell::new(Default::default())),
+            fn1: Rc::new(RefCell::new(FnPtr::null())),
+            fn2: Rc::new(RefCell::new(FnPtr::null())),
+            n: Rc::new(RefCell::new(2)),
+        },
+    ])));
+);
+pub fn check_local_static_0() {
+    thread_local!(
+        static local_outer: Value<Outer> = Rc::new(RefCell::new(<Outer>::default()));
+    );
+    thread_local!(
+        static local_fn: Value<FnPtr<fn(i32) -> i32>> = Rc::new(RefCell::new(FnPtr::null()));
+    );
+    thread_local!(
+        static local_p: Value<Ptr<i32>> = Rc::new(RefCell::new(Ptr::<i32>::null()));
+    );
+    assert!((*(*local_outer.with(Value::clone).borrow()).p1.borrow()).is_null());
+    assert!((*(*local_outer.with(Value::clone).borrow()).fn_.borrow()).is_null());
+    assert!((*local_fn.with(Value::clone).borrow()).is_null());
+    assert!((*local_p.with(Value::clone).borrow()).is_null());
+}
+pub fn main() {
+    std::process::exit(main_0());
+}
+fn main_0() -> i32 {
+    assert!((*static_fn.with(Value::clone).borrow()).is_null());
+    assert!((*(*static_outer.with(Value::clone).borrow()).p1.borrow()).is_null());
+    assert!((*(*static_outer.with(Value::clone).borrow()).p2.borrow()).is_null());
+    assert!((*(*static_outer.with(Value::clone).borrow()).cp.borrow()).is_null());
+    assert!((*(*static_outer.with(Value::clone).borrow()).pp.borrow()).is_null());
+    assert!((*(*static_outer.with(Value::clone).borrow()).fn_.borrow()).is_null());
+    let i: Value<i32> = Rc::new(RefCell::new(0));
+    'loop_: while ((*i.borrow()) < 3) {
+        assert!(((*(*static_outer.with(Value::clone).borrow()).arr.borrow())
+            [(*i.borrow()) as usize])
+            .is_null());
+        (*i.borrow_mut()).prefix_inc();
+    }
+    assert!(
+        (*(*(*static_outer.with(Value::clone).borrow()).inner.borrow())
+            .name
+            .borrow())
+        .is_null()
+    );
+    let i: Value<i32> = Rc::new(RefCell::new(0));
+    'loop_: while ((*i.borrow()) < 2) {
+        assert!(
+            (*(*static_inner_array.with(Value::clone).borrow())[(*i.borrow()) as usize]
+                .name
+                .borrow())
+            .is_null()
+        );
+        (*i.borrow_mut()).prefix_inc();
+    }
+    assert!((*(*static_foo.with(Value::clone).borrow()).s2.borrow()).is_null());
+    assert!((*(*static_foo.with(Value::clone).borrow()).fn1.borrow()).is_null());
+    assert!((*(*static_foo.with(Value::clone).borrow()).fn2.borrow()).is_null());
+    assert!(((*(*static_foo.with(Value::clone).borrow()).n.borrow()) == 42));
+    let i: Value<i32> = Rc::new(RefCell::new(0));
+    'loop_: while ((*i.borrow()) < 2) {
+        assert!(
+            (*(*static_foo_array.with(Value::clone).borrow())[(*i.borrow()) as usize]
+                .s2
+                .borrow())
+            .is_null()
+        );
+        assert!(
+            (*(*static_foo_array.with(Value::clone).borrow())[(*i.borrow()) as usize]
+                .fn1
+                .borrow())
+            .is_null()
+        );
+        assert!(
+            (*(*static_foo_array.with(Value::clone).borrow())[(*i.borrow()) as usize]
+                .fn2
+                .borrow())
+            .is_null()
+        );
+        (*i.borrow_mut()).prefix_inc();
+    }
+    ({ check_local_static_0() });
+    return 0;
+}
diff --git a/tests/unit/out/unsafe/default_in_statics.rs b/tests/unit/out/unsafe/default_in_statics.rs
