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

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 attr;        // the queried name, e.g. "simdjson"
+    std::string version;     // empty when the derivation has no version
+    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