${mcpp.bin_dir} in an action a workspace member's build program declares is that member's product directory

The validation project's cross-verification against this pull request stopped
at mcpp pack: its build script stages ${mcpp.bin_dir}/<target>.pdb, and the
placeholder expanded to the plan's bin/, where a member's binaries are not.
e2e 838 A4 states it; docs/30 (both languages) state the expansion.

Author: speak-agent

.agents/docs/2026-09-29-workspace-build-graph-design.md

@@ -560,6 +560,7 @@ read from the member, or made a value of the plan:
 | the shared libraries of a member's closure | only those the member's units link were placed | every graph-built shared library of the closure, with its aliases, as §15 states | e2e 835 L3 |
 | the runtime files of a program shipped through `artifacts` (2026.9.29.3) | its link waited for the plan's deploy set, which a workspace plan does not place; its own runtime files were not in the member's directory | the link waits for no plan-level file; a member's runtime set includes the closures its `artifacts` edges reach | e2e 833 G9 |
 | the link line of a program shipped through `artifacts` (2026.9.29.4) | the plan's line, which pools the dependencies' flags and not a member's, so a library its package's build program states was missing | a link group of its own closure that places nothing (`LinkGroup::linkOnly`) | e2e 838 |
+| `${mcpp.bin_dir}` in a member's action (2026.9.29.4) | the plan's `bin/` | the declaring member's product directory | e2e 838 A4 |
 
 Each criterion fails on 2026.9.29.1 and passes on 2026.9.29.2. The resource
 case also showed a defect of every build: a quoted `#include` in a script was

CHANGELOG.md

@@ -7,8 +7,10 @@
 ## [2026.9.29.4] - 2026-09-29
 
 This release links a program that a workspace member ships through
-`artifacts` with the link line of its own package's closure. The validation
-project's post-release build of 2026.9.29.3 found it.
+`artifacts` with the link line of its own package's closure, and expands
+`${mcpp.bin_dir}` to where the declaring member's binaries land. The validation
+project's post-release build of 2026.9.29.3 found the first, and its
+cross-verification against this release's pull request the second.
 
 ### Fixed
 
@@ -20,6 +22,10 @@ project's post-release build of 2026.9.29.3 found it.
   program now has a link group of its own that holds its closure's line and
   runtime contract and places nothing, since the members that ship the program
   place it.
+- **`${mcpp.bin_dir}` in an action a workspace member's build program declares
+  is that member's product directory.** It was the plan's `bin/`, where a
+  member's binaries are not, so an action that named a file beside the
+  member's program (a `.pdb`, say) read or wrote the wrong place (e2e 838 A4).
 
 ## [2026.9.29.3] - 2026-09-29
 

docs/30-build-mcpp.md

@@ -1003,7 +1003,7 @@ none to rely on), and the only interpolations are a closed set:
 | Variable | Value |
 |---|---|
 | `${mcpp.out_dir}` | the build output directory |
-| `${mcpp.bin_dir}` | where produced binaries land |
+| `${mcpp.bin_dir}` | where produced binaries land: `bin/`, or, for an action a workspace member's build program declares, that member's product directory (2026.9.29.4+) |
 | `${mcpp.compile_db}` | path to `compile_commands.json` (what clang-tidy's `-p` wants) |
 | `${mcpp.target_file:<name>}` | the built file of target `<name>` |
 | `${mcpp.stage_dir}` *(2026.9.11.1+)* | the tree `mcpp pack` staged, absolute. `artifact` role only, and only under `mcpp pack --format <name>` |

docs/zh/30-build-mcpp.md

@@ -848,7 +848,7 @@ mcpp 会写出 `<暂存树>.stage-manifest` —— 一个兄弟文件，永不
 | 变量 | 含义 |
 |---|---|
 | `${mcpp.out_dir}` | 构建输出目录 |
-| `${mcpp.bin_dir}` | 产出的二进制所在目录 |
+| `${mcpp.bin_dir}` | 产出的二进制所在目录:`bin/`;工作区成员的构建程序声明的 action 中为该成员的产物目录(2026.9.29.4+) |
 | `${mcpp.compile_db}` | `compile_commands.json` 的路径（clang-tidy 的 `-p` 要的就是它） |
 | `${mcpp.target_file:<name>}` | target `<name>` 构建出的文件 |
 | `${mcpp.stage_dir}` *(2026.9.11.1+)* | `mcpp pack` 暂存出的那棵树，绝对路径。仅 `artifact` role 可用，且仅在 `mcpp pack --format <name>` 下可用 |

src/build/prepare/plan.cpp

@@ -932,6 +932,11 @@ static std::expected<void, std::string> step13_build_graph_actions(PrepareState&
         // NAMES ids and a collision there costs a duplicate line, not a wrong
         // edge.
         bool thisActionUsesStageDir = false;
+        // Where the declaring package's binaries land: `bin/`, or in a
+        // workspace plan the product directory of the member that declared
+        // the action (§15 of the 2026-09-29 workspace design). Set per
+        // package by `collect`.
+        std::filesystem::path binDir = ctx.plan.outputDir / "bin";
         const bool stagePass = !state.overrides.pack_stage_dir.empty();
         auto substitute = [&](std::string s, const char* actionId,
                               mcpp::manifest::BuildAction::Role role) {
@@ -940,7 +945,7 @@ static std::expected<void, std::string> step13_build_graph_actions(PrepareState&
                     s.replace(p, what.size(), with);
             };
             rep("${mcpp.out_dir}",    ctx.plan.outputDir.string());
-            rep("${mcpp.bin_dir}",    (ctx.plan.outputDir / "bin").string());
+            rep("${mcpp.bin_dir}",    binDir.string());
             rep("${mcpp.compile_db}", ctx.plan.compileDbPath.string());
             // The engine's own executable, absolute (2026.9.13.1+). An action
             // whose command is an argv with no shell has no portable way to
@@ -1024,6 +1029,9 @@ static std::expected<void, std::string> step13_build_graph_actions(PrepareState&
             // program has no idea which package the engine loaded it for.
             // mcpp#534's ordering edge is scoped to this name.
             auto owner = mcpp::build::qualified_package_name(mm);
+            binDir = ctx.plan.outputDir / "bin";
+            for (auto const& g : ctx.plan.linkGroups)
+                if (!g.linkOnly && g.member == owner) binDir = ctx.plan.outputDir / g.productDir;
             for (auto a : mm.buildConfig.actions) {
                 thisActionUsesStageDir = false;
                 const auto sub = [&](std::string v) {

tests/e2e/838_a_program_shipped_through_artifacts_links_its_own_closure.sh

@@ -11,7 +11,11 @@
 #
 #   A1  `mcpp build --workspace` links the program shipped through `artifacts`;
 #   A2  the program in the shipping member's product directory runs;
-#   A3  the program as its own member's product runs too.
+#   A3  the program as its own member's product runs too;
+#   A4  `${mcpp.bin_dir}` in an action the helper's build program declares is
+#       the helper's product directory, where its binaries land (2026.9.29.3
+#       expanded it to the plan's `bin/`, so a file named after the program
+#       was looked for where the program is not).
 set -e
 
 TMP=$(mktemp -d)
@@ -57,6 +61,15 @@ import mcpp;
 int main() {
     mcpp::link_search("vendor/lib");
     mcpp::link_lib("vend");
+    mcpp::action a;
+    a.id = "copy-helper";
+    a.role = mcpp::roles::artifact;
+    a.arg("${mcpp.self}").arg("stage").arg("--verify").arg("content")
+     .arg("--output").arg("${mcpp.bin_dir}/helper.copy")
+     .arg("${mcpp.target_file:helper}")
+     .input("${mcpp.target_file:helper}")
+     .output("${mcpp.bin_dir}/helper.copy")
+     .submit();
     return 0;
 }
 EOF
@@ -87,4 +100,8 @@ own=$(find target -path '*/bin/helper/helper' -type f | head -1)
 [ -n "$own" ] || fail "A3: bin/helper/helper is missing" b.log
 "$own" || fail "A3: bin/helper/helper did not run"
 
+# A4
+[ -f "$(dirname "$own")/helper.copy" ] \
+    || { find target -name 'helper.copy'; fail "A4: \${mcpp.bin_dir} is not the helper's product directory" b.log; }
+
 echo "PASS: 838_a_program_shipped_through_artifacts_links_its_own_closure"