Rustdoc displays private internals of associated constants (regression)

[dtolnay]

pub struct Struct {
    private: (),
}

impl Struct {
    pub const CONST: Self = Struct { private: () };
}

Correct:

Incorrect:

I used the following script to bisect:

#!/bin/bash
cargo doc && ! rg private: $CARGO_TARGET_DIR/doc/repro/struct.Struct.html >/dev/null

cargo-bisect-rustc --script ./bisect.sh

It bisects to nightly-2022-04-14 and specifically #95990, of which #95316 seems the most relevant because it involves associated constants in rustdoc. @fmease @notriddle @GuillaumeGomez

[fmease]

#95316 is indeed the cause. More specifically the following line:

rust/src/librustdoc/html/render/mod.rs

Line 715 in 3dea003

write!(w, " = {}", default.value(cx.tcx()).unwrap_or_else(|| default.expr(cx.tcx())));

.

If we “fail” to evaluate (“failure” also includes “the constant is a struct”) the const value of a provided associated const with .value(), we fall back to .expr() which prints the constant in raw uncensored form including structs with their private fields.

I assume in the non-associated const version (shown below) the call to .expr() does not happen and if .value() fails in such a case, neither the value nor the = is being printed.

pub struct Struct { private: () }
pub const CONST: Struct = Struct { private: () };

Note that I haven't verified all those pieces of information yet, I am going to check them very soon.

I am not sure how to proceed @GuillaumeGomez. This is another instance of constant-printing for provided assoc consts being problematic. Previous incident: Performance (#95316 (comment)).

[GuillaumeGomez]

Maybe we could update the .expr() method so that it can only show the public items to fulfill our need?

[fmease]

The ideal solution (non-hotfix) would be to generally improve the printing of (unevaluated) constants (as returned by .expr() -> String. I've already written about it on Zulip. Not sure how easy it is to implement though.

[GuillaumeGomez]

Seems like we want something similar. I don't think it's very difficult but might involve a lot of code. Want to give it a try?

[fmease]

I might as well try. If successful, my PR needs backporting.

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[eggyal]

I'm not convinced that displaying the value of (provided associated) constants in rustdoc is ever worthwhile and indeed it could be harmful. Users should not need to know what value a constant holds, only what it represents; indeed they should only ever be using the constant itself to represent that value when it's required—that is to say, the value assigned is an implementation detail and nobody should be relying upon it.