Feature Backlog

This document lists practical feature candidates for upcoming maintenance cycles. Items are scoped to be incremental and compatible with the current architecture.

🔗Completed

🔗1. Harden ALTREP registration diagnostics ✓

• validate_altrep_class() checks for null handles after R_make_alt*_class().
• Wired into make_class_by_base(), all 7 impl_inferbase_* macros, and explicit-base codegen path.

🔗2. Structured panic telemetry hook ✓

• panic_telemetry module with PanicSource enum and optional hook via AtomicPtr.
• fire() called at all 3 panic→R-error sites (worker, altrep_bridge, unwind_protect).
• Zero overhead when no hook is set (single atomic load).

🔗3. Strict conversion mode (outputs, standalone fns) ✓

• #[miniextendr(strict)] on standalone functions panics instead of silently widening i64/u64/isize/usize to f64.
• strict module with checked_into_sexp_* helpers.
• Codegen wired via return_type_analysis.rs for lossy scalar and Vec return types.

🔗4. Strict conversion mode — impl methods ✓

• #[miniextendr(r6, strict)] (or any class system + strict) extends strict output conversion to impl block methods.
• strict field added to ImplAttrs, ParsedImpl, CWrapperContext, and builder.
• sexp_conversion_expr() helper on CWrapperContext handles bare, Option<T>, and Result<T, E> return types — delegates to strict_conversion_for_type().
• All 6 IntoR return handling branches (3 main thread + 3 worker thread) updated.

🔗5. Strict conversion mode — inputs (TryCoerce) ✓

• #[miniextendr(strict)] now also validates input parameters for lossy types (i64/u64/isize/usize + Vec variants).
• Only INTSXP and REALSXP accepted; RAWSXP and LGLSXP rejected with “strict conversion failed for parameter ‘{name}’” error.
• REALSXP values go through TryCoerce to catch fractional, NaN, and overflow.
• Strict takes priority over coerce for lossy types.
• Wired through both standalone functions (lib.rs) and impl methods (c_wrapper_builder.rs) via RustConversionBuilder::with_strict().

🔗6. #[miniextendr(internal)] and #[miniextendr(noexport)] attributes ✓

• internal injects @keywords internal and suppresses @export.
• noexport suppresses @export only (no @keywords internal).
• Works on standalone functions and all 6 class system impl blocks (env, R6, S3, S4, S7, vctrs) via ClassDocBuilder::with_export_control().

🔗7. String ALTREP Dataptr ✓

• Bridge-layer materialization: Rust Vec<String>/Box<[String]> materialize into native R STRSXP cached in the ALTREP data2 slot.
• Enables saveRDS/readRDS roundtrip and identical() for string ALTREP vectors.
• Uses DATAPTR_RO with cast (DATAPTR is behind nonapi feature gate).

🔗8. Adapter test coverage expansion ✓

• Added 3–5 edge-case functions per adapter for 13 thin adapter modules (sha2, aho-corasick, time, tinyvec, indexmap, ordered-float, toml, bytes, either, url, bitvec, bitflags, tabled).
• Corresponding R test expectations in test-feature-adapters.R.

🔗9. Conversion behavior matrix ✓

• docs/CONVERSION_MATRIX.md: R input type × Rust target type → behavior reference.
• Covers INTSXP, REALSXP, LGLSXP, RAWSXP, STRSXP against i32, f64, u8, bool, String, i64/u64/isize/usize in normal, coerce, and strict modes.

🔗10. Sparse iterator ALTREP guide ✓

• docs/SPARSE_ITERATOR_ALTREP.md: compute-on-access pattern, Iterator::nth() for efficient skipping, comparison with materialization, usage guidance.

🔗11. vctrs documentation expansion ✓

• Expanded docs/VCTRS.md with record type example, list-of pattern, advanced coercion, and troubleshooting section.

🔗12. Fix has_roxygen_tag for multi-word tags ✓

• has_roxygen_tag("keywords internal") was broken — tag_names() only extracts first word after @. Added multi-word branch matching full content after @.
• Added comprehensive unit tests for has_roxygen_tag, tag_names, find_tag_value.

🔗13. Vec<Option<T>> IntoR for extended numeric types ✓

• Smart i32/f64 conversion for Vec<Option<i64/u64/isize/usize>> — checks if all non-None values fit i32 → INTSXP, otherwise REALSXP with NA_REAL for None.
• Simple coercion for Vec<Option<i8/i16/u16/u32/f32>> via widening macro.
• Strict mode: checked_vec_option_{i64,u64,isize,usize}_into_sexp() helpers.
• Proc-macro detection: strict_conversion_for_type() handles Vec<Option<lossy>>.

🔗14. S7 multi-level inheritance tests ✓

• 3-level chain: S7Animal (abstract) → S7Dog → S7GoldenRetriever.
• R tests verify S7::S7_inherits() through full chain, abstract rejection.
• GAPS.md section 3.1 updated: inheritance chains marked as implemented.

🔗15. Rustdoc examples for feature-gated modules ✓

• Added /// # Examples blocks (ignore-marked) to progress.rs, vctrs.rs, connection.rs for key public items.

🔗16. Field access documentation ✓

• GAPS.md section 3.3 marked RESOLVED via #[r_data] + RSidecar sidecar pattern.
• CLASS_SYSTEMS.md: added “Field Access via Sidecar” subsection.
• GAPS.md section 2.3: Vec<Option<T>> updated to “Works (all scalar types)”.

🔗17. Fix String ALTREP NA serialization ✓

• into_sexp_altrep STRSXP branch now uses Vec<Option<String>> instead of Vec<String>, preserving NA_character_ through saveRDS/readRDS roundtrips.
• Added RegisterAltrep + InferBase for Vec<Option<String>>.
• Test suite: 0 FAIL, 2868 PASS (previously 2 FAIL).

🔗Active: Next Up

🔗Parked: Needs Evidence

These items are plausible but lack a demonstrated need or clear design.

• Pooled PROTECT scope — Rf_protect() is already cheap (stack counter increment). No benchmark shows this is a bottleneck. Revisit if profiling reveals protect overhead in tight loops.
• ALTREP region prefetch — R controls get_region invocation, not the ALTREP class. Can’t prefetch from the implementor side as described. Reframe as docs/examples on efficient get_region implementation if needed.
• Microbenchmark gating in CI — valuable but depends on CI infrastructure (not yet set up). Revisit when CI exists.

🔗Dropped

• Wrapper diff mode — git diff already handles this. Wrappers are mechanical .Call() functions; a dedicated diff tool adds complexity for no real gain.

🔗Backlog: Nice to Have