Suggest private non-default field in some cases

[estebank]

Description

If all fields private and have a default field value should emit a lint akin to the clippy lint suggesting #[non_exhaustive] to add a private non-default field to prevent construction outside of the current crate.

CC @epage

Version

all

Additional Labels

@rustbot label C-enhancement

[epage]

pub struct Foo {
    bar: usize = 10,
    baz: usize = 10,
}

impl Foo {
    pub fn new(special: Special) -> Self {
    }
}

A user could:

let foo = Foo { .. };

when that might bypass invariants established by Foo::new

If direct construction is not intended, it should be written as

pub struct Foo {
    bar: usize = 10,
    baz: usize = 10,
    __priv: PhantomData<()>,
}

impl Foo {
    pub fn new(special: Special) -> Self {
    }
}

Likely the condition should be limited to:

• all fields have lower visibility than the type itself
• all fields have default initializers

Potential variant:

• any field has lower visibility than the type itself
• any field has a default initializer

Occasionally, I see a type where all fields are private except for one or two pub field but the end-user is not meant to construct it. Depending on the exact scenario, the author may wish to add __priv.

[estebank]

I was musing about the verbosity of Foo { .., __priv: PhantomData } for the API author, and realized that in this case, because the field is used as a marker without any non-present type parameters, it can be defined (and used) as __priv: ().