Are nested structs supported in Rust? - struct

Question

When I try to declare a struct inside of another struct:
struct Test {
struct Foo {}
}
The compiler complains:
error: expected identifier, found keyword `struct`
--> src/lib.rs:2:5
|
2 | struct Foo {}
| ^^^^^^ expected identifier, found keyword
help: you can escape reserved keywords to use them as identifiers
|
2 | r#struct Foo {}
| ^^^^^^^^
error: expected `:`, found `Foo`
--> src/lib.rs:2:12
|
2 | struct Foo {}
| ^^^ expected `:`
I could not find any documentation in either direction; are nested structs even supported in Rust?

Answer 1

No, they are not supported. You should use separate struct declarations and regular fields:
struct Foo {}
struct Test {
foo: Foo,
}

Answer 2

They are not supported by Rust.
But you can write yourself a proc macro that emulates them. I have, it turns
structstruck::strike!{
struct Test {
foo: struct {}
}
}
into
struct Foo {}
struct Test {
foo: Foo,
}
You haven't explicitly said so, but I suspect that your goal for using nested structs is not more easily readable data structure declarations, but namespacing?
You can't actually have a struct named Test and access Foo as Test::Foo, but you could make yourself a proc macro that at least automatically creates a mod test { Foo {} }.

Answer 3

I found the optional answer for your request.
Maybe it can help you:
https://internals.rust-lang.org/t/nested-struct-declaration/13314/4
Structural records 592 would give you the nesting (without the privacy controls or naming).
// From the RFC
struct RectangleTidy {
dimensions: {
width: u64,
height: u64,
},
color: {
red: u8,
green: u8,
blue: u8,
},
}
Below is my old answer, please ignore.
Build using the Stable version: 1.65.0.
Here's a example for you.
rust playground
codes:
#[derive(Debug)]
struct Inner {
i: i32,
}
#[derive(Debug)]
struct Outer {
o: i32,
inner: Inner,
}
pub fn test() {
// add your code here
let obj = Outer {
o: 10,
inner: Inner { i: 9 },
};
assert!(10i32 == obj.o);
assert!(9i32 == obj.inner.i);
println!("{}", obj.o);
println!("{}", obj.inner.i);
println!("{:?}", obj);
}
fn main() {
test();
}