`#[track_caller]` in miniextendr

The #[miniextendr] macro automatically adds #[track_caller] to Rust functions.

🔗What it actually does

#[track_caller] changes what std::panic::Location::caller() reports inside a function: instead of the exact line where caller() (or a panicking .unwrap() / .expect() / assert!) sits, the location resolves to the function’s call site. When several #[track_caller] functions call each other, the location keeps walking up until it exits the attributed chain.

For a #[miniextendr] function, the caller is the macro-generated wrapper, whose call-site span resolves to the #[miniextendr] attribute line of your function. The observable behavior (pinned by the track_caller_is_active() / track_caller_chain_location() fixtures in rpkg/src/rust/r_wrapper_attrs.rs):

#[miniextendr]              // <- panic locations report THIS line
pub fn process_data(x: i32) {
    let result: Option<i32> = None;
    result.unwrap();        // NOT this line
}

• Without the automatic attribute, .unwrap() (which is itself #[track_caller] in std) would report the exact .unwrap() line.
• With it, the location walks past the function body to the wrapper’s call site — the #[miniextendr] line.

So the automatic attribute trades line-level precision inside the function for a stable function-level location: a panic always identifies which exported function failed, in your source file, never a location inside generated code or the standard library.

🔗Propagation through call chains

Marking helpers #[track_caller] extends the walk — their reported location also resolves to the #[miniextendr] line, not to the helper call site:

#[track_caller]
fn helper() {
    let x: Option<i32> = None;
    x.unwrap();             // reports the #[miniextendr] line below
}

#[miniextendr]              // <- reported location
pub fn my_function() {
    helper();
}

Without #[track_caller] on helper(), the panic reports the .unwrap() line inside helper() — standard Rust behavior.

🔗Where the location surfaces

The panic message transported to R (the rust_panic condition) contains only the payload text, not the location. The location appears on stderr via the panic hook, and in Location::caller()-based code like the fixtures above.

🔗Skipped cases

The macro does NOT add #[track_caller] when:

1. The function already has #[track_caller]
2. The function has an explicit ABI (e.g., extern "C-unwind" — the attribute is not supported there)