feat: Clang multi-module support via static-deps path

- Restore is_gcc gate on dyndep (Clang lacks -fdeps-format=p1689r5;
  needs clang-scan-deps for P1689, future work)
- Fix static-deps path: use implicit output (|) for BMI files so $out
  only contains the .o file; set $bmi_out for all compilers (GCC
  ignores it, Clang uses it for -fmodule-output)
- Add E2E test 38_llvm_modules.sh: multi-module Clang project with
  export module + cross-module import, verifying pcm.cache/ layout

Author: Sunrisepeak

src/build/ninja_backend.cppm

@@ -126,7 +126,11 @@ bool is_c_source(const std::filesystem::path& src) {
 }  // namespace
 
 std::string emit_ninja_string(const BuildPlan& plan) {
-    bool dyndep = dyndep_mode_enabled();
+    // Clang uses clang-scan-deps (not -fdeps-format=p1689r5) for P1689
+    // scanning, so dyndep is GCC-only for now. Clang falls through to the
+    // static-deps path which uses BmiTraits for correct pcm.cache/ paths.
+    bool dyndep = dyndep_mode_enabled()
+               && mcpp::toolchain::is_gcc(plan.toolchain);
     auto traits = mcpp::toolchain::bmi_traits(plan.toolchain);
     std::string out;
     auto append = [&](std::string s) { out += std::move(s); };
@@ -368,12 +372,18 @@ std::string emit_ninja_string(const BuildPlan& plan) {
 
             std::string out_line = "build " + escape_ninja_path(cu.object);
             if (cu.providesModule) {
-                out_line += " " + bmi_path(*cu.providesModule);
+                // Use implicit output (|) so $out only contains the .o file.
+                // GCC writes BMI implicitly; Clang uses -fmodule-output=$bmi_out.
+                out_line += " | " + bmi_path(*cu.providesModule);
             }
             out_line += std::format(" : {} {}", rule, escape_ninja_path(cu.source));
             if (!implicit.empty())
                 out_line += " |" + implicit;
             out_line += "\n";
+            // Clang needs $bmi_out to emit -fmodule-output=$bmi_out
+            if (cu.providesModule) {
+                out_line += "  bmi_out = " + bmi_path(*cu.providesModule) + "\n";
+            }
             append(std::move(out_line));
         }
         append("\n");

tests/e2e/38_llvm_modules.sh

@@ -0,0 +1,89 @@
+#!/usr/bin/env bash
+# 38_llvm_modules.sh — multi-module project with LLVM/Clang.
+#
+# Tests: module interface (.cppm) with `export module`, cross-module import,
+# dyndep pipeline, BMI path parameterization (pcm.cache/*.pcm), and
+# -fmodule-output / -fprebuilt-module-path flags.
+set -e
+
+LLVM_ROOT="${HOME}/.mcpp/registry/data/xpkgs/xim-x-llvm/20.1.7"
+if [[ ! -x "$LLVM_ROOT/bin/clang++" ]]; then
+    echo "SKIP: xlings llvm@20.1.7 is not installed"
+    exit 0
+fi
+if [[ ! -f "$LLVM_ROOT/share/libc++/v1/std.cppm" ]]; then
+    echo "SKIP: xlings llvm@20.1.7 has no libc++ std.cppm"
+    exit 0
+fi
+
+TMP=$(mktemp -d)
+trap "rm -rf $TMP" EXIT
+export MCPP_HOME="$TMP/mcpp-home"
+source "$(dirname "$0")/_inherit_toolchain.sh"
+
+mkdir -p "$TMP/proj/src"
+cd "$TMP/proj"
+
+cat > mcpp.toml <<'EOF'
+[package]
+name    = "llvm_modules"
+version = "0.1.0"
+
+[toolchain]
+linux = "llvm@20.1.7"
+EOF
+
+# Module interface unit: exports greet()
+cat > src/greet.cppm <<'EOF'
+export module llvm_modules.greet;
+
+import std;
+
+export std::string greet(std::string_view name) {
+    return std::format("Hello, {}!", name);
+}
+EOF
+
+# Main imports the module
+cat > src/main.cpp <<'EOF'
+import std;
+import llvm_modules.greet;
+
+int main() {
+    std::println("{}", greet("clang"));
+    return 0;
+}
+EOF
+
+"$MCPP" build --no-cache > "$TMP/build.log" 2>&1 || {
+    cat "$TMP/build.log"
+    echo "FAIL: llvm multi-module build failed"
+    exit 1
+}
+
+binary=$(find target -type f -path '*/bin/llvm_modules' | head -1)
+[[ -n "$binary" && -x "$binary" ]] || {
+    find target -maxdepth 5 -type f
+    echo "FAIL: llvm_modules binary missing"
+    exit 1
+}
+
+out=$("$binary")
+[[ "$out" == "Hello, clang!" ]] || {
+    echo "FAIL: wrong runtime output: $out"
+    exit 1
+}
+
+# Verify BMI went to pcm.cache/, not gcm.cache/
+pcm_dir=$(find target -type d -name 'pcm.cache' | head -1)
+[[ -n "$pcm_dir" ]] || {
+    echo "FAIL: pcm.cache/ directory not found (Clang should use pcm.cache)"
+    exit 1
+}
+gcm_dir=$(find target -type d -name 'gcm.cache' | head -1)
+[[ -z "$gcm_dir" ]] || {
+    echo "FAIL: gcm.cache/ directory found (Clang should NOT use gcm.cache)"
+    exit 1
+}
+
+echo "OK"