Dynamic objects support inheritance

[philberty]

I tried this code:

extern "C" {
    fn printf(s: *const i8, ...);
}

struct Foo(i32);
trait Bar {
    fn baz(&self);
}

trait Baz : Bar {
    fn qux(&self);
}

impl Bar for Foo {
    fn baz(&self) {
        unsafe {
            let a = "baz %i\n\0";
            let b = a as *const str;
            let c = b as *const i8;

            printf(c, self.0);
        }
    }
}

impl Baz for Foo {
    fn qux(&self) {
        unsafe {
            let a = "baz %i\n\0";
            let b = a as *const str;
            let c = b as *const i8;

            printf(c, self.0);
        }
    }
}


fn dynamic_dispatch(t: &dyn Baz) {
    t.baz();
    t.qux();
}

pub fn main() {
    let a;
    a = &Foo(123);
    dynamic_dispatch(a);
}

I expected to see this happen: compile without error

Instead, this happened:

<source>:40:7: error: failed to resolve method for 'baz'
   40 |     t.baz();
      |       ^
<source>:40:5: error: failed to type resolve expression
   40 |     t.baz();
      |     ^
rust1: error: bounds not satisfied for dyn [example::Baz] 'example::Bar' is not satisfied

Meta

• What version of Rust GCC were you using, git sha if possible. 83bfbf0

[bjorn3]

You can't get from &dyn Baz to &dyn Bar on stable rustc. What happens here is that the vtable contains all methods of all super traits. On nightly rustc it also contains pointers to vtables of super traits as necessary to allow upcasting trait objects to super traits.

[liamnaddell]

New behavior: ICE!

crab1: internal compiler error: in compute_address_for_trait_item, at rust/backend/rust-compile.cc:362
0x10afce3 Rust::Compile::HIRCompileBase::compute_address_for_trait_item(Rust::Resolver::TraitItemReference const*, Rust::TyTy::TypeBoundPredicate const*, std::vector<std::pair<Rust::Resolver::TraitReference*, Rust::HIR::ImplBlock*>, std::allocator<std::pair<Rust::Resolver::TraitReference*, Rust::HIR::ImplBlock*> > >&, Rust::TyTy::BaseType const*, Rust::TyTy::BaseType const*, unsigned int)
        ../../gccrs/gcc/rust/backend/rust-compile.cc:362
0x10af543 Rust::Compile::HIRCompileBase::coerce_to_dyn_object(tree_node*, Rust::TyTy::BaseType const*, Rust::TyTy::DynamicObjectType const*, unsigned int)
        ../../gccrs/gcc/rust/backend/rust-compile.cc:219
0x13cc47f Rust::Compile::HIRCompileBase::resolve_unsized_dyn_adjustment(Rust::Resolver::Adjustment&, tree_node*, unsigned int)
        ../../gccrs/gcc/rust/backend/rust-compile-expr.cc:2000
0x13cc007 Rust::Compile::HIRCompileBase::resolve_unsized_adjustment(Rust::Resolver::Adjustment&, tree_node*, unsigned int)
        ../../gccrs/gcc/rust/backend/rust-compile-expr.cc:1949
0x13cbc8f Rust::Compile::HIRCompileBase::resolve_adjustements(std::vector<Rust::Resolver::Adjustment, std::allocator<Rust::Resolver::Adjustment> >&, tree_node*, unsigned int)
        ../../gccrs/gcc/rust/backend/rust-compile-expr.cc:1890
0x10aeb53 Rust::Compile::HIRCompileBase::coercion_site(unsigned int, tree_node*, Rust::TyTy::BaseType*, Rust::TyTy::BaseType*, unsigned int, unsigned int)
        ../../gccrs/gcc/rust/backend/rust-compile.cc:66
0x13c96fb Rust::Compile::CompileExpr::visit(Rust::HIR::CallExpr&)
        ../../gccrs/gcc/rust/backend/rust-compile-expr.cc:1269
0x1195ec3 Rust::HIR::CallExpr::accept_vis(Rust::HIR::HIRExpressionVisitor&)
        ../../gccrs/gcc/rust/hir/tree/rust-hir.cc:4865
0x13c5157 Rust::Compile::CompileExpr::Compile(Rust::HIR::Expr*, Rust::Compile::Context*)
        ../../gccrs/gcc/rust/backend/rust-compile-expr.cc:46
0x13c3c5f Rust::Compile::CompileStmt::visit(Rust::HIR::ExprStmt&)
        ../../gccrs/gcc/rust/backend/rust-compile-stmt.cc:43
0x1195ccb Rust::HIR::ExprStmt::accept_vis(Rust::HIR::HIRStmtVisitor&)
        ../../gccrs/gcc/rust/hir/tree/rust-hir.cc:4811
0x13c3be7 Rust::Compile::CompileStmt::Compile(Rust::HIR::Stmt*, Rust::Compile::Context*)
        ../../gccrs/gcc/rust/backend/rust-compile-stmt.cc:36
0x13ef6ff Rust::Compile::HIRCompileBase::compile_function_body(tree_node*, Rust::HIR::BlockExpr&, Rust::TyTy::BaseType*)
        ../../gccrs/gcc/rust/backend/rust-compile-base.cc:575
0x13f00f7 Rust::Compile::HIRCompileBase::compile_function(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, Rust::HIR::SelfParam&, std::vector<Rust::HIR::FunctionParam, std::allocator<Rust::HIR::FunctionParam> >&, Rust::HIR::FunctionQualifiers const&, Rust::HIR::Visibility&, std::vector<Rust::AST::Attribute, std::allocator<Rust::AST::Attribute> >&, unsigned int, Rust::HIR::BlockExpr*, Rust::Resolver::CanonicalPath const&, Rust::TyTy::FnType*)
        ../../gccrs/gcc/rust/backend/rust-compile-base.cc:751
0x13c2e9f Rust::Compile::CompileItem::visit(Rust::HIR::Function&)
        ../../gccrs/gcc/rust/backend/rust-compile-item.cc:205
0x11964f3 Rust::HIR::Function::accept_vis(Rust::HIR::HIRStmtVisitor&)
        ../../gccrs/gcc/rust/hir/tree/rust-hir.cc:5033
0x10b0cd3 Rust::Compile::CompileItem::compile(Rust::HIR::Item*, Rust::Compile::Context*, Rust::TyTy::BaseType*, bool, unsigned int)
        ../../gccrs/gcc/rust/backend/rust-compile-item.h:38
0x10aea47 Rust::Compile::CompileCrate::go()
        ../../gccrs/gcc/rust/backend/rust-compile.cc:48
0x10ae98b Rust::Compile::CompileCrate::Compile(Rust::HIR::Crate&, Rust::Compile::Context*)
        ../../gccrs/gcc/rust/backend/rust-compile.cc:41
0x1071e53 Rust::Session::compile_crate(char const*)
        ../../gccrs/gcc/rust/rust-session-manager.cc:710
Please submit a full bug report, with preprocessed source (by using -freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

[liamnaddell]

This ICE isn't related to either t.baz() or t.qux() invocations, it's related to passing Foo(123) as a dynamic reference to Baz. Here's a minified reproducer of the same issue:

#![allow(dead_code)]

struct Foo(i32);
trait Bar {
    fn baz(&self);
}

trait Baz : Bar {
    fn qux(&self);
}

impl Bar for Foo {
    fn baz(&self) {
    }
}

impl Baz for Foo {
    fn qux(&self) {
    }
}

pub fn main() {
    let a = Foo(123);
    let _b: &dyn Baz = &a;
}

[liamnaddell]

As a note, I think @bjorn3 's comment is out of date now. I'm able to compile the example @philberty posted without trouble on stable rustc:

liam@gentoo ~/super $ rustc --version
rustc 1.79.0 (129f3b996 2024-06-10)
liam@gentoo ~/super $ cat main3.rs
... same code as philberty provided ... 
liam@gentoo ~/super $ rustc main3.rs
liam@gentoo ~/super $