[feat] cargo should explain doctest binary path location

[loynoir]

Problem

cargo does not explain doctest binary path location

Thus, need to hard code doctest binary path location.

https://doc.rust-lang.org/rustc/instrument-coverage.html#including-doc-tests

$ llvm-cov report \
    $( \
      for file in \
        $( \
          RUSTFLAGS="-C instrument-coverage" \
          RUSTDOCFLAGS="-C instrument-coverage -Z unstable-options --persist-doctests target/debug/doctestbins" \
            cargo test --no-run --message-format=json \
              | jq -r "select(.profile.test == true) | .filenames[]" \
              | grep -v dSYM - \
        ) \
        target/debug/doctestbins/*/rust_out; \
      do \
        [[ -x $file ]] && printf "%s %s " -object $file; \
      done \
    ) \
  --instr-profile=json5format.profdata --summary-only # and/or other options

Proposed Solution

cargo should explain doctest binary path location

$ "${cmd[@]}" --no-run --message-format=json | jq -r ".filenames[]" | grep doctestbins
target/debug/doctestbins/XXX/rust_out
target/debug/doctestbins/XXX/rust_out
target/debug/doctestbins/XXX/rust_out

Notes

channel = "nightly-2025-04-14"

[epage]

Currently, cargo test delegates compiling, running, and reporting of doctests to rustdoc. For this to be fixed in the short term, that would be on rustdoc.

Long term, I'd like to make this less special and run like any other test which would resolve this. Keeping this open for now for that.