[M5+] nix-cmake-scan walks the dev output for multi-output packages

CHANGELOG.md

@@ -91,6 +91,18 @@ All notable changes to cargoxx will be documented in this file.
   6 cases against fixtures derived from a real fmt 10.2.1 response;
   `tests/devbox_resolve_live.cpp` (gated by `CARGOXX_NETWORK_TESTS=1`)
   hits the live API.
+- Fix: `nix_cmake_scan` walked only the package's default output. For
+  multi-output Nix packages — `boost`, `openssl`, `libllvm`,
+  `libclang`, glib, … — CMake configs live in the `dev` output
+  (`<store>/lib/cmake/llvm/LLVMConfig.cmake`), not under the default
+  output, so `cargoxx add libllvm` (or any other multi-output dep)
+  failed with "Conan/vcpkg/nix-scan all empty". `NixpkgsInfo` now
+  carries an optional `dev_path`; the `nix eval` apply expression
+  reports `if p ? dev then p.dev.outPath else ""`. `discover` scans
+  the dev output preferentially and falls back to `out`. Live
+  verified: `cargoxx add libllvm` now reaches the verify-link step
+  (the linker-time libstdc++/libc++ ABI mismatch is a separate
+  documented limit and applies to abseil-cpp too).
 - Fix: pinned-dep `buildInputs` line used the curated linkdb's
   `nixpkgs_attr` (e.g. `fmt_10`) regardless of the pinned rev, so
   `cargoxx add fmt@12.1.0` emitted `pkgs_nixpkgs_fmt_12_1_0.fmt_10`

src/resolver/discover.cpp

@@ -89,9 +89,25 @@ auto discover(const std::string& name, const std::string& version_spec,
     if (auto v = vcpkg_probe(name); v) {
         candidates.push_back({"vcpkg", recipe_from_vcpkg(*v, name, "vcpkg")});
     }
-    if (auto n = nix_cmake_scan(info->out_path, name); n) {
+    // Multi-output nix packages keep CMake configs in the `dev` output;
+    // scan it first when available, fall back to the default `out`.
+    auto try_scan = [&](const fs::path& root)
+        -> std::optional<NixCmakeCandidate> {
+        if (root.empty()) {
+            return std::nullopt;
+        }
+        if (auto r = nix_cmake_scan(root, name); r) {
+            return std::move(*r);
+        }
+        return std::nullopt;
+    };
+    auto scan_hit = try_scan(info->dev_path);
+    if (!scan_hit) {
+        scan_hit = try_scan(info->out_path);
+    }
+    if (scan_hit) {
         candidates.push_back(
-            {"nix-probe", recipe_from_nix_scan(*n, name, "nix-probe")});
+            {"nix-probe", recipe_from_nix_scan(*scan_hit, name, "nix-probe")});
     }
 
     if (candidates.empty()) {

src/resolver/nixpkgs_probe.cpp

@@ -15,8 +15,14 @@ namespace {
 // `outPath` is a magic attribute name that triggers nix's derivation
 // coercion in --json mode (the attrset gets serialized as a bare path
 // string). Renaming the field to `path` keeps it a proper JSON object.
+//
+// `dev_path` is captured for multi-output packages (boost, openssl,
+// llvm, …) where CMake configs live under the `dev` output rather
+// than `out`. `p ? dev` is false for single-output packages, leaving
+// the field as the empty string.
 constexpr std::string_view APPLY_FN =
-    "p: { version = p.version or \"\"; path = p.outPath; }";
+    "p: { version = p.version or \"\"; path = p.outPath; "
+    "dev_path = if p ? dev then p.dev.outPath else \"\"; }";
 
 auto make_error(util::ErrorCode code, std::string msg) -> util::Error {
     return util::Error{code, std::move(msg), "", std::nullopt, std::nullopt};
@@ -48,7 +54,9 @@ auto parse_nix_eval_json(std::string_view attr, std::string_view json)
             "nix eval JSON is not an object"));
     }
 
-    NixpkgsInfo info{.attr = std::string{attr}, .version = {}, .out_path = {}};
+    NixpkgsInfo info{
+        .attr = std::string{attr}, .version = {}, .out_path = {}, .dev_path = {},
+    };
 
     if (j.contains("path") && j["path"].is_string()) {
         info.out_path = j["path"].get<std::string>();
@@ -62,6 +70,10 @@ auto parse_nix_eval_json(std::string_view attr, std::string_view json)
         info.version = j["version"].get<std::string>();
     }
 
+    if (j.contains("dev_path") && j["dev_path"].is_string()) {
+        info.dev_path = j["dev_path"].get<std::string>();
+    }
+
     return info;
 }
 

src/resolver/resolver.cppm

@@ -10,11 +10,17 @@ export namespace cargoxx::resolver {
 
 // What `nix eval nixpkgs#<pkg>` reports for a package: a confirmation that
 // the attribute exists, a best-effort version string, and the realized
-// nix-store path so later probes can scan its installed CMake configs.
+// nix-store path(s) so later probes can scan its installed CMake configs.
+//
+// Multi-output packages (boost, openssl, llvm, ...) expose CMake configs
+// in their `dev` output, not in the default `out`. When the package has
+// a separate dev output its store path is captured here so callers can
+// scan it preferentially.
 struct NixpkgsInfo {
     std::string attr;        // the queried name, e.g. "simdjson"
     std::string version;     // empty when the derivation has no version
-    std::string out_path;   // absolute /nix/store/... path
+    std::string out_path;    // default output's absolute /nix/store/... path
+    std::string dev_path;    // dev output's path; empty when no dev output
 };
 
 // Pure parser exposed for unit testing. Accepts the raw JSON returned by

tests/nixpkgs_probe_parse.cpp

@@ -53,3 +53,29 @@ TEST_CASE("parse_nix_eval_json fails on a non-object root",
     REQUIRE_FALSE(r.has_value());
     REQUIRE(r.error().code == ErrorCode::ResolutionNetworkError);
 }
+
+TEST_CASE("parse_nix_eval_json captures dev_path when present",
+          "[resolver][nixpkgs]") {
+    constexpr std::string_view input = R"({
+        "version": "21.1.8",
+        "path": "/nix/store/abc-llvm-21.1.8",
+        "dev_path": "/nix/store/def-llvm-21.1.8-dev"
+    })";
+    auto r = parse_nix_eval_json("libllvm", input);
+    REQUIRE(r.has_value());
+    REQUIRE(r->out_path == "/nix/store/abc-llvm-21.1.8");
+    REQUIRE(r->dev_path == "/nix/store/def-llvm-21.1.8-dev");
+}
+
+TEST_CASE("parse_nix_eval_json leaves dev_path empty for single-output packages",
+          "[resolver][nixpkgs]") {
+    constexpr std::string_view input = R"({
+        "version": "2.12.2",
+        "path": "/nix/store/xyz-hello-2.12.2",
+        "dev_path": ""
+    })";
+    auto r = parse_nix_eval_json("hello", input);
+    REQUIRE(r.has_value());
+    REQUIRE(r->out_path == "/nix/store/xyz-hello-2.12.2");
+    REQUIRE(r->dev_path.empty());
+}