Tuple struct's private field can be accessed outside the defining module

[akonradi]

I tried this code:

mod my {
    pub struct Foo(&'static str);
}

impl AsRef<str> for my::Foo {
    fn as_ref(&self) -> &str {
        let Self(s) = self;
        s
    }
}

I expected to see this happen: compilation should fail because the trait impl is defined outside of the module for the type but refers to its private field.

Instead, this happened: this compiled successfully

Meta

This reproduces on the playground for stable Rust, version 1.69.0: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=20f12a0f682d8dd6530d75f1b5e7a8dd

[KittyBorgX]

Adding a main function and attempting to call the do_evil (playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1e96d7ddeb6f23ccfa9b99c5b27c9c77) raises the compilation error:

[taiki-e]

This can be reproduced with cross-crate (e.i., any other crate can access the private field). This means that all encapsulations that take advantage of the field being private can be bypassed. (For example, if we change Vec to a tuple struct, the same thing as set_len could be done in safe code by abusing this bug.)

// crate a
#[derive(Default)]
pub struct A(u32);

// crate b
pub trait B {
    fn f(&self);
}
impl B for b::A {
    fn f(&self) {
        let Self(a) = self;
        println!("{}", a);
    }
}
fn main() {
    let a = b::A::default();
    a.f();
}

@rustbot label +I-unsound

Also, this can be reproduced in all versions where self_struct_ctor is stable (e.i., 1.32+). https://godbolt.org/z/dWbKP3voc

---

@KittyBorgX It's a matter of how you initialize Foo. If you provide a constructor for Foo, you won't get a compile error: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3e9ec99b1aa66a2f767d61bf71cab9f2

[y21]

Just for completeness, turning this into a segfault without external crates/just std:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=cd936b8cd757d76d84b7307c794bbef1

[Jules-Bertholet]

@rustbot label A-visibility

[lcnr]

this has been unsound ever since #56365 in version 1.32

https://rust.godbolt.org/z/hPzMTY435

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high