feat(workspace): build-mcpp member (L3 build.mcpp consumer) (#58)

* feat(workspace): add build-mcpp member (L3 build.mcpp consumer, mcpp>=0.0.78)

A workspace member that exercises the native build.mcpp build program: its
build.mcpp generates src/generated.cpp and emits -DBUILT_BY_BUILD_MCPP; main.cpp
asserts both. Dogfoods the L3 feature through the real pipeline.

(held until mcpp 0.0.78 is released + indexed — CI downloads the released binary)

* ci: bump mcpp 0.0.77 -> 0.0.78 for build-mcpp member

Author: Sunrisepeak

.github/workflows/validate.yml

@@ -11,9 +11,9 @@ on:
   workflow_dispatch:
 
 env:
-  # Bumped to 0.0.77: carries L1 platform-conditional deps/flags + [xlings] env,
+  # Bumped to 0.0.78: carries L3 build.mcpp native build program (build-mcpp member),
   # compat.openblas Windows recipe (bin/libopenblas.dll staged beside the .exe).
-  MCPP_VERSION: "0.0.77"
+  MCPP_VERSION: "0.0.78"
 
 jobs:
   lint:
@@ -326,14 +326,14 @@ jobs:
         include:
           - platform: macos
             os: macos-15
-            archive: mcpp-0.0.77-macosx-arm64.tar.gz
-            root: mcpp-0.0.77-macosx-arm64
+            archive: mcpp-0.0.78-macosx-arm64.tar.gz
+            root: mcpp-0.0.78-macosx-arm64
             mcpp: bin/mcpp
             xlings: registry/bin/xlings
           - platform: windows
             os: windows-latest
-            archive: mcpp-0.0.77-windows-x86_64.zip
-            root: mcpp-0.0.77-windows-x86_64
+            archive: mcpp-0.0.78-windows-x86_64.zip
+            root: mcpp-0.0.78-windows-x86_64
             mcpp: bin/mcpp.exe
             xlings: registry/bin/xlings.exe
     steps:

.gitignore

@@ -4,3 +4,5 @@ tests/examples/*/target/
 tests/examples/*/.mcpp/
 tests/examples/*/compile_commands.json
 tests/examples/*/mcpp.lock
+# build.mcpp-generated source (regenerated each build by the build program)
+tests/examples/build-mcpp/src/generated.cpp

mcpp.toml

@@ -5,6 +5,7 @@
 # .agents/docs/2026-06-29-mcpp-native-workspace-ci-design.md.
 [workspace]
 members = [
+    "tests/examples/build-mcpp",
     "tests/examples/cjson",
     "tests/examples/eigen",
     "tests/examples/nlohmann.json",

tests/examples/build-mcpp/build.mcpp

@@ -0,0 +1,15 @@
+// Native build program. mcpp compiles this with the host toolchain and runs it
+// before the main build; its stdout `mcpp:` directives augment the build. Here we
+// generate a source file and define a macro that main.cpp asserts on.
+#include <cstdio>
+#include <fstream>
+
+int main() {
+    std::ofstream("src/generated.cpp")
+        << "int answer() { return 42; }\n";
+
+    std::puts("mcpp:generated=src/generated.cpp");
+    std::puts("mcpp:cxxflag=-DBUILT_BY_BUILD_MCPP=1");
+    std::puts("mcpp:rerun-if-changed=build.mcpp");
+    return 0;
+}

tests/examples/build-mcpp/mcpp.toml

@@ -0,0 +1,11 @@
+# L3 build.mcpp consumer: a project-local native build program (mcpp >= 0.0.78)
+# generates a source and emits a compile define, both consumed by main.cpp. No
+# registry dependency — this member exercises the build.mcpp pipeline itself
+# (Zig build.zig / Cargo build.rs analog, in C++).
+[package]
+name    = "build-mcpp-example"
+version = "0.1.0"
+
+[targets.build-mcpp-example]
+kind = "bin"
+main = "src/main.cpp"

tests/examples/build-mcpp/src/main.cpp

@@ -0,0 +1,9 @@
+// Consumes both outputs of build.mcpp: the -DBUILT_BY_BUILD_MCPP define and the
+// generated answer() function. Returns 0 only if both took effect.
+#ifndef BUILT_BY_BUILD_MCPP
+#error "build.mcpp cxxflag did not reach this translation unit"
+#endif
+
+int answer();  // defined in the generated src/generated.cpp
+
+int main() { return answer() == 42 ? 0 : 1; }