docs(mctx): say why the MINIMAL_INDEX_SOURCE match arms differ - #1134

Comment-only follow-up to #1132. No behaviour change.

The trap

Both arms of the map_err produce Error::Other(anyhow!(...)) and read as interchangeable:

.map_err(|e| match e {
    RemoteError::Config(msg) => Error::Other(anyhow::anyhow!("{msg}")),
    other => Error::Other(anyhow::anyhow!("{other}")),
})?

They are not. rcache::Error's Display is:

impl<BE: Debug> Display for Error<BE> {
    fn fmt(&self, f) -> Result { write!(f, "{:?}", self) }
}

It Debug-formats itself, so the fallback arm renders a Config as Config("MINIMAL_INDEX_SOURCE: unknown index source \"banana\" ...") — variant name and escaped quotes included. That is precisely the string gominimal/inbox#447 was filed about. Destructuring Config and formatting the inner msg is what yields the clean user-facing message.

Collapsing this to a single arm would reintroduce the defect while looking like a simplification.

Why it's worth a comment

I found this reviewing #1132 and my first instinct was to file exactly that simplification. I only caught it by going and reading the Display impl in another crate. The next reader will have the same instinct with less reason to check.

Verification

• cargo check -p mctx --locked — clean
• cargo fmt --all -- --check — clean

🤖 Generated with Claude Code