fix(std-module): 一个字符串承载了两个事实,于是第二条命令收到十九个用不上的 flag

    clang++: warning: argument unused during compilation: '-nostdinc++'
    clang++: warning: argument unused during compilation: '-isystem …'
      (共十九条,每个 include 目录一条)

stdModuleFlags 同时携带「给哪台机器」和「头文件在哪」。构建这个模块有两步,只有
第一步两样都要:第二步编译的是 BMI,而 BMI 已经包含头文件贡献的一切。

把前半单独记为 stdModuleTargetFlags,第二步只用它。

⭐ 核验方式是产物而不是「编过了」:同一个 codegen 步骤,用拆分后的 flag 和用完整
flag 各跑一次,std.o **逐字节相同**(sha256 a6d837241e7f02b2,736 字节),而后者
产生十九条警告。⇒ 被丢掉的 flag 确实无用。

⚠️ 这些警告在每个平台上都存在,而在每个平台上都看不见:非 Windows 的命令以 2>&1
结尾,mcpp 又丢弃成功命令的输出 —— Windows 那条没有重定向,所以它是第一次被看见的
地方。⇒ 噪声本身不是缺陷,「十九条正确而无意义的警告排在任何有意义的警告之前」
才是。

Author: speak-agent

src/build/prepare.cppm

@@ -5659,6 +5659,10 @@ prepare_build(bool print_fingerprint,
             for (auto& f : mcpp::toolchain::graph_runtime_compile_flags(*tc))
                 flags += " " + f;
         }
+        // Everything up to here says which machine the module is for; what
+        // follows says where its headers are. The codegen step needs only the
+        // first — see Toolchain::stdModuleTargetFlags.
+        tc->stdModuleTargetFlags = flags;
         for (auto& f : pkg.manifest.stdModuleFlags) {
             // A flag naming a path is relative to the package that named it,
             // for the same reason the module source is.

src/toolchain/clang.cppm

@@ -204,6 +204,10 @@ std::vector<std::string> std_module_build_commands(const Toolchain& tc,
     // built without it is built for whatever machine is doing the building.
     if (!tc.stdModuleFlags.empty()) sysrootFlag = {};
     const std::string& extraFlags = tc.stdModuleFlags;
+    // ⚠️ The codegen step compiles a BMI, which already carries what the
+    // headers contributed; only the machine has to be restated. See
+    // Toolchain::stdModuleTargetFlags.
+    const std::string& codegenFlags = tc.stdModuleTargetFlags;
 #if defined(_WIN32)
     // Windows: use absolute paths, raw binary path as first token
     // (cmd.exe strips leading quotes), shq for args with spaces.
@@ -252,7 +256,7 @@ std::vector<std::string> std_module_build_commands(const Toolchain& tc,
             tc.binaryPath.string(),
             cppStandardFlag,
             sysrootFlag,
-            extraFlags,
+            codegenFlags,
             mcpp::xlings::shq(absBmi),
             mcpp::xlings::shq((cacheDir / "std.o").string()))
     };
@@ -277,7 +281,7 @@ std::vector<std::string> std_module_build_commands(const Toolchain& tc,
             mcpp::xlings::shq(tc.binaryPath.string()),
             cppStandardFlag,
             sysrootFlag,
-            extraFlags,
+            codegenFlags,
             mcpp::xlings::shq(relBmi))
     };
 #endif
@@ -338,6 +342,9 @@ std::vector<std::string> std_compat_build_commands(const Toolchain& tc,
     // this target. Reading that message, the mixture is invisible.
     if (!tc.stdModuleFlags.empty()) sysrootFlag = {};
     const std::string& extraFlags = tc.stdModuleFlags;
+    // Same split as the `std` builder above: the second command compiles a BMI
+    // and needs the machine restated, not the include paths.
+    const std::string& codegenFlags = tc.stdModuleTargetFlags;
     // std.compat depends on std, so we need -fmodule-file=std=<std.pcm>
     // Note: the path after = must NOT be shell-quoted separately; the
     // entire -fmodule-file flag is a single token to the compiler.
@@ -384,7 +391,7 @@ std::vector<std::string> std_compat_build_commands(const Toolchain& tc,
                     mcpp::xlings::shq(tc.binaryPath.string()),
                     cppStandardFlag,
                     sysrootFlag,
-                    extraFlags,
+                    codegenFlags,
                     absStdBmi,
                     mcpp::xlings::shq(absBmi),
                     mcpp::xlings::shq(absObj))

src/toolchain/model.cppm

@@ -93,6 +93,28 @@ struct Toolchain {
     // They reach the cache key without anything further being done: the key is
     // derived from the build COMMANDS, and these are part of them.
     std::string                         stdModuleFlags;
+    // ⭐⭐ THE PART OF THE ABOVE THAT SAYS WHICH MACHINE, SEPARATED FROM THE
+    // PART THAT SAYS WHERE THE HEADERS ARE.
+    //
+    // `stdModuleFlags` is one string carrying two different facts: the target
+    // and its ABI-affecting options, and the include paths the module's SOURCE
+    // needs. Building the module has two steps, and only the first needs both —
+    // the second compiles a BMI, which already contains everything the headers
+    // contributed.
+    //
+    // ⚠️ Passing the whole string to the second step is not wrong, it is noisy,
+    // and the noise is the kind that hides things:
+    //
+    //     clang++: warning: argument unused during compilation: '-nostdinc++'
+    //     clang++: warning: argument unused during compilation: '-isystem …'
+    //       (× 17, once per include directory)
+    //
+    // Seventeen warnings that are correct and mean nothing, in front of any
+    // warning that would mean something. ⚠️ They were present on every platform
+    // and visible on none: the non-Windows command ends in `2>&1` and mcpp
+    // discards a successful command's output, so the Windows leg — which has no
+    // redirection — is where they first appeared.
+    std::string                         stdModuleTargetFlags;
     // A package in the graph supplies a C++ runtime built FOR THIS TARGET.
     // Read by the freestanding flag table, which otherwise forces exceptions
     // and run-time type information off for every unit — right when nothing can