A plugin hands a dependency on with reexport = true on its [dependencies] edge (#797)

CHANGELOG.md

@@ -29,7 +29,8 @@
   directory as `engineModules`.
 - Read a unit that imports `mcpp.core` as a build-plugin unit (SPEC-007 §10). A
   plugin's `[dependencies]` are compiled with it for the build program, so a
-  plugin may build on plugins and libraries; `mcpp build` and `mcpp test` in a
+  plugin may build on plugins and libraries, and one it marks `reexport = true`
+  is importable by its consumers' build programs; `mcpp build` and `mcpp test` in a
   plugin plan it in the build-program configuration. A plugin reached through
   a target's `[dependencies]`, a program target that imports `mcpp`, `--target`
   for another machine on a plugin, and `mcpp pack` of a plugin are refused,

docs/31-authoring-a-rule-package.md

@@ -178,9 +178,11 @@ the build-program configuration only. Three consequences follow for its author.
 - **What the rule's library imports is in its `[dependencies]`.** A rule built
   on another rule (`mcpp:plugins`' `plugins-core`, say) or on an ordinary
   library (a TOML parser) names it there. When a consumer's build program uses
-  the rule, those dependencies are compiled for the build program with it. The
-  rule's `[build-dependencies]` serve the rule package's own `build.mcpp`, as
-  any package's do.
+  the rule, those dependencies are compiled for the build program with it. A
+  dependency the consumers' build programs import themselves is handed on with
+  `reexport = true` on its entry; without it the import is refused, naming the
+  key. The rule's `[build-dependencies]` serve the rule package's own
+  `build.mcpp`, as any package's do.
 - **The package builds and tests as the root of a build.** `mcpp build` in the
   package compiles its library for this machine, with the engine interface, and
   `mcpp test` runs its tests under `tests/` the same way: a test may import the

docs/zh/31-authoring-a-rule-package.md

@@ -163,8 +163,9 @@ bool compile(options opt = {});
 
 - **规则的库所导入的东西写在它的 `[dependencies]` 中。** 基于另一条规则（例如
   `mcpp:plugins` 的 `plugins-core`）或普通库（例如 TOML 解析器）的规则在那里写出它。消费方的
-  构建程序使用这条规则时，这些依赖与它一起为构建程序编译。规则的 `[build-dependencies]`
-  服务规则包自己的 `build.mcpp`，与任何包相同。
+  构建程序使用这条规则时，这些依赖与它一起为构建程序编译。消费方的构建程序自己也导入的依赖，
+  在其条目上写 `reexport = true` 交出；不写时该导入被拒绝，诊断写出这个键。规则的
+  `[build-dependencies]` 服务规则包自己的 `build.mcpp`，与任何包相同。
 - **规则包可以作为构建的根构建与测试。** 在包中执行 `mcpp build` 会为本机编译它的库并带上
   引擎接口，`mcpp test` 以同样的方式运行 `tests/` 下的测试：测试可以导入规则的模块与 `mcpp`，
   在没有消费方工程的情况下检查规则的逻辑。

src/build/prepare/host_graph.cpp

@@ -307,7 +307,16 @@ std::expected<void, std::string> check_program_imports(PrepareState& state, std:
     }
     const auto wants = mcpp::pm::imported_module_names(text);
     if (wants.empty()) return {};
-    const auto libs = program_libraries(state, i);
+    auto libs = program_libraries(state, i);
+    // What a library hands on: a dependency on a `reexport = true` edge of its
+    // own `[dependencies]` is importable by the programs that use the library,
+    // as a `[build-dependencies]` entry it re-exports is (provisions).
+    for (std::size_t k = 0; k < libs.size(); ++k)
+        for (auto const& e : state.dependencyEdges)
+            if (e.consumerPackageIndex == libs[k] && !e.buildTable && e.reexport
+                && e.dependencyPackageIndex < state.packages.size()
+                && std::ranges::find(libs, e.dependencyPackageIndex) == libs.end())
+                libs.push_back(e.dependencyPackageIndex);
     std::set<std::string> available{"std", "std.compat", "mcpp", "mcpp.core"};
     std::map<std::string, std::size_t> providerOf;
     for (auto d : libs)

tests/e2e/910_a_build_plugin_is_a_package_of_the_build_program.sh

@@ -11,7 +11,10 @@
 #   D  P1: a plugin reached through [dependencies] of a target is refused,
 #      naming the chain and [build-dependencies];
 #   E  P4: a program target importing mcpp is refused;
-#   F  P5: `mcpp pack` refuses a plugin.
+#   F  P5: `mcpp pack` refuses a plugin;
+#   G  a library a plugin depends on is importable by the plugin's consumers'
+#      build programs when the plugin hands it on (`reexport = true` on its
+#      [dependencies] edge), and refused, naming `reexport`, when it does not.
 set -e
 TMP=$(mktemp -d)
 trap 'rm -rf "$TMP"' EXIT
@@ -95,4 +98,12 @@ grep -q "exists only in a build" e.log || fail "E: the refusal does not say why"
 cd "$TMP/rule"
 "$MCPP" pack > f.log 2>&1 && fail "F: mcpp pack accepted a plugin" f.log
 grep -q "is a build plugin" f.log || fail "F: the refusal does not say why" f.log
+# G
+cd "$TMP/app"
+printf 'import acme.rule;\nimport acme.util;\nint main() { if (util_base() == 40) acme::rule::go(); }\n' > build.mcpp
+"$MCPP" build > g1.log 2>&1 && fail "G: a library the plugin does not hand on was importable" g1.log
+grep -q "reexport = true" g1.log || fail "G: the refusal does not name reexport" g1.log
+sed -i 's|util = { path = "../util" }|util = { path = "../util", reexport = true }|' "$TMP/rule/mcpp.toml"
+"$MCPP" build > g2.log 2>&1 || fail "G: a library the plugin hands on was refused" g2.log
+[ "$("$MCPP" run 2>/dev/null | grep '^V=' | tail -1)" = "V=42" ] || fail "G: value" g2.log
 echo "PASS: 910_a_build_plugin_is_a_package_of_the_build_program"