toolset: the chain's PATH is the tools' own directories, not the engine's whole PATH

The engine's tool_env PATH is the cl.exe directory followed by the PATH of the
process that ran mcpp. Under Git Bash that carries Git's msys tools and
runtime, and icu's configure, run by vcpkg with that PATH kept, failed with
'invalid variable name: 0'. The toolset's and the SDK's binary directories are
what a foreign system needs, as the design's measurement M3c kept.

Author: Sunrisepeak

src/toolset.cppm

@@ -69,8 +69,8 @@ struct resolved_tools {
     // `chain` on the MSVC ABI: the environment the tools run with, without
     // PATH (INCLUDE, LIB, LIBPATH, ...).
     std::vector<std::pair<std::string, std::string>> env;
-    // `chain`: the directories that go first on PATH, in order, without
-    // duplicates: the tools' own directories, then the environment's PATH.
+    // `chain`: the tools' own directories, in order, without duplicates: the
+    // PATH a foreign system needs for them.
     std::vector<std::string> path_dirs;
     std::string identity;           // toolset_identity(): "msvc 14.44.35207; sdk 10.0.26100.0", "clang 22.1.8"
     std::string crt;                // msvc_crt_linkage(): "static", "dynamic", or empty off the MSVC ABI
@@ -175,22 +175,16 @@ inline std::expected<resolved_tools, std::string> chain(const choice& c, resolve
             c.cc == compiler::abi_native ? "ABI-native" : "row", std::string(mcpp::target())));
     if (r.cc.empty()) r.cc = r.cxx;
     if (r.msvc_abi) {
-        auto env = parse_env(mcpp::tool_env());
-        const std::string path = env_value(env, "PATH");
-        for (auto const& kv : env) if (!same_key(kv.first, "PATH")) r.env.push_back(kv);
+        // The environment WITHOUT its PATH. The engine's PATH is the cl.exe
+        // directory followed by the whole PATH of the process that ran mcpp --
+        // under Git Bash that holds Git's own msys tools and runtime, and a
+        // make-based port configured against them failed (`configure: error:
+        // invalid variable name: '0'`, icu, the plugins' CI). The tools'
+        // own directories are the PATH a foreign system needs: the toolset's
+        // bin (cl, link, lib) and the SDK's (rc, mt).
+        for (auto const& kv : parse_env(mcpp::tool_env()))
+            if (!same_key(kv.first, "PATH")) r.env.push_back(kv);
         for (auto const* t : { &r.cxx, &r.cc, &r.ld, &r.rc, &r.mt }) add_dir(r.path_dirs, *t);
-        // The environment's PATH carries the SDK's and the toolset's binary
-        // directories; its entries follow the tools' own, in its order.
-        std::string_view rest = path;
-        while (!rest.empty()) {
-            const auto semi = rest.find(';');
-            const std::string entry = forward(rest.substr(0, semi));
-            rest = semi == std::string_view::npos ? std::string_view{} : rest.substr(semi + 1);
-            if (entry.empty()) continue;
-            bool seen = false;
-            for (auto const& d : r.path_dirs) if (same_key(d, entry)) { seen = true; break; }
-            if (!seen) r.path_dirs.push_back(entry);
-        }
     } else {
         for (auto const* t : { &r.cxx, &r.cc }) add_dir(r.path_dirs, *t);
     }