Warn about unused pub fields in non-pub structs

[FabianWolff]

This pull request fixes #85255. The current implementation of dead code analysis is too prudent because it marks all pub fields of structs as live, even though they cannot be accessed from outside of the current crate if the struct itself only has restricted or private visibility.

I have changed this behavior to take the containing struct's visibility into account when looking at field visibility and liveness. This also makes dead code warnings more consistent; consider the example given in #85255:

struct Foo {
    a: i32,
    pub b: i32,
}

struct Bar;

impl Bar {
    fn a(&self) -> i32 { 5 }
    pub fn b(&self) -> i32 { 6 }
}


fn main() {
    let _ = Foo { a: 1, b: 2 };
    let _ = Bar;
}

Current nightly already warns about Bar::b(), even though it is pub (but Bar is not). It should therefore also warn about Foo::b, which it does with the changes in this PR.

[rust-highfive]

r? @varkor

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[varkor]

Thanks!

@bors r+

[bors]

📌 Commit 46d55d6 has been approved by varkor