feat(build): Windows runtime-DLL deployment beside the executable; v0.0.73

A directly-launched Windows .exe cannot RUNPATH-locate a dependency's runtime
DLL (PE has no rpath). mcpp now stages every *.dll found in a linked
dependency's [runtime] library_dirs into bin/, beside the produced executable,
via a ninja copy edge the executable takes as an implicit dependency. This
completes an abstraction already designed but unimplemented on PE: RPATH on
ELF/Mach-O, copy-beside-exe on PE — both 'make the runtime library locatable'.

The deploy is filtered by the *.dll extension, NOT by if constexpr(is_windows)
and with no schema change: a real Linux/macOS dependency ships .so/.dylib
(never .dll), so the glob matches nothing and non-Windows builds are
byte-for-byte unchanged. A recipe declares [runtime] library_dirs globally;
only a Windows prebuilt-DLL package populates the deploy list. This unblocks a
Windows compat.openblas (import-lib + runtime DLL) and any future prebuilt-DLL
package.

Test: tests/e2e/84_runtime_dll_deploy.sh exercises the exact copy path on a
Linux host via a dummy dependency shipping a stub libdummy.dll (asserts the DLL
is staged beside the exe byte-for-byte; a non-DLL sibling is not). The Windows
link/run half is verified in mcpp-index CI (design Phase D). Regression: unit
suite (27 ok), e2e 74/80/81/83, self-host build.

See .agents/docs/2026-06-29-windows-runtime-dll-deployment-and-openblas.md.

Author: Sunrisepeak

.agents/docs/2026-06-29-windows-runtime-dll-deployment-and-openblas.md

@@ -1,10 +1,35 @@
 # Windows Runtime-DLL Deployment & `compat.openblas` Windows Support (Design)
 
 Date: 2026-06-29
-Status: **Design — not yet implemented.** Proposed as the staged follow-up to the
-feature/capability work (mcpp 0.0.72) and the `compat.openblas` package
-(mcpp-index #54). Scope: `src/manifest.cppm`, `src/build/{plan,flags,ninja_backend}.cppm`
-(mcpp); `pkgs/c/compat.openblas.lua`, `.github/workflows/validate.yml` (mcpp-index).
+Status: **Phase A implemented in mcpp 0.0.73.** Phases B–D (release + recipe +
+Windows CI) track in the same effort. Staged follow-up to the feature/capability
+work (mcpp 0.0.72) and the `compat.openblas` package (mcpp-index #54). Scope:
+`src/build/{plan,ninja_backend}.cppm` (mcpp); `pkgs/c/compat.openblas.lua`,
+`.github/workflows/validate.yml` (mcpp-index).
+
+### Implementation note (deviation from the original design)
+
+The implemented mechanism is **gated by the `*.dll` file extension, not by
+`if constexpr(is_windows)` and not by a schema change.** During build planning,
+each `*.dll` found in a linked dependency's `[runtime] library_dirs` is staged
+into `bin/` beside the produced executable via a ninja `cp_bmi` copy edge that
+the executable target takes as an implicit dependency. Consequences:
+
+- **No `manifest.cppm` schema change** (the original Phase-A-step-1). A recipe
+  declares `[runtime] library_dirs` *globally*; on Linux/macOS the dependency
+  ships `.so`/`.dylib` (never `.dll`), so the glob matches nothing and the build
+  is byte-for-byte unchanged. This is exactly the §7 "declare it globally —
+  harmless on Linux" option, made safe by the extension filter. Per-OS scoping
+  under `mcpp.<os>` is therefore unnecessary (and already supported for free by
+  the existing per-OS textual merge if a recipe ever wants it).
+- **The deploy path is exercised on a Linux host** (test
+  `tests/e2e/84_runtime_dll_deploy.sh`) by a dummy dependency shipping a stub
+  `libdummy.dll` — the same code that runs on Windows, validated without a
+  Windows runner. The Windows link/run half is Phase D (mcpp-index CI).
+- **`mcpp pack`** needs no change here: Windows PE packaging is separately
+  stubbed (`src/pack/pack.cppm`, see `2026-05-19-pack-windows-design.md`); when
+  implemented it will pick up the staged `bin/*.dll`. On Linux `mcpp pack` uses
+  `ldd`, which never sees a `.dll`, so the deploy is invisible to it.
 
 ## 1. Problem
 

mcpp.toml

@@ -1,6 +1,6 @@
 [package]
 name        = "mcpp"
-version     = "0.0.72"
+version     = "0.0.73"
 description = "Modern C++ build & package management tool"
 license     = "Apache-2.0"
 authors     = ["mcpp-community"]

src/build/ninja_backend.cppm

@@ -633,6 +633,14 @@ std::string emit_ninja_string(const BuildPlan& plan) {
         for (auto& input : lu.implicitInputs) {
             implicit += " " + escape_ninja_path(input);
         }
+        // Windows runtime-DLL deployment: an executable takes an implicit
+        // dependency on each staged dep DLL (bin/<dll>), so ninja copies them
+        // beside the .exe before the build is considered done. Empty on RPATH
+        // platforms (no *.dll deps), so other targets are unaffected.
+        if (lu.kind == LinkUnit::Binary || lu.kind == LinkUnit::TestBinary) {
+            for (auto const& d : plan.runtimeDeployFiles)
+                implicit += " " + escape_ninja_path(d.dest);
+        }
 
         std::string out_line = std::format("build {} : {}{}{}\n",
             escape_ninja_path(lu.output), rule, ins,
@@ -660,6 +668,17 @@ std::string emit_ninja_string(const BuildPlan& plan) {
     }
     append("\n");
 
+    // Windows runtime-DLL deployment: one copy edge per staged dep DLL. Emitted
+    // once (deduped by dest in BuildPlan), reusing the generic cp_bmi copy rule.
+    // Inert on RPATH platforms where runtimeDeployFiles is empty.
+    for (auto const& d : plan.runtimeDeployFiles) {
+        append(std::format("build {} : cp_bmi {}\n",
+            escape_ninja_path(d.dest),
+            escape_ninja_path(d.source)));
+    }
+    if (!plan.runtimeDeployFiles.empty())
+        append("\n");
+
     if (!plan.linkUnits.empty()) {
         std::string defaults;
         for (auto& lu : plan.linkUnits) {
@@ -668,6 +687,9 @@ std::string emit_ninja_string(const BuildPlan& plan) {
                 defaults += " " + escape_ninja_path(alias);
             }
         }
+        for (auto const& d : plan.runtimeDeployFiles) {
+            defaults += " " + escape_ninja_path(d.dest);
+        }
         append("default" + defaults + "\n");
     }
 

src/build/plan.cppm

@@ -61,6 +61,19 @@ struct BuildPlan {
     // binary's RUNPATH (e.g. compat.glx-runtime). Kept separate so static/musl
     // links don't pull the glibc payload dir.
     std::vector<std::filesystem::path> depRuntimeLibraryDirs;
+    // Windows runtime-DLL deployment. On PE (`supports_rpath` is false) a
+    // directly-launched .exe cannot RUNPATH-locate a dependency's DLL, so each
+    // *.dll found in a dependency's [runtime] library_dir is copied beside the
+    // produced executable (into bin/). The filter is the *.dll extension, not a
+    // platform `if constexpr`: a real Linux/macOS dependency ships .so/.dylib
+    // (never .dll), so this list is empty there and non-Windows builds are
+    // byte-for-byte unchanged; only a Windows prebuilt-DLL package (or a test
+    // that ships a .dll) populates it. dest is relative to outputDir.
+    struct DeployFile {
+        std::filesystem::path source;   // absolute source DLL
+        std::filesystem::path dest;     // relative to outputDir, e.g. bin/libopenblas.dll
+    };
+    std::vector<DeployFile>            runtimeDeployFiles;
     // Aggregated host-runtime requirements from dependency packages'
     // [runtime] metadata. Capability/provider-driven — no platform special-casing
     // in mcpp: providers (e.g. compat.glx-runtime) declare these per platform.
@@ -309,6 +322,26 @@ BuildPlan make_plan(const mcpp::manifest::Manifest&         manifest,
             auto abs = dir.is_absolute() ? dir : package.root / dir;
             append_unique_path(plan.runtimeLibraryDirs, abs);
             append_unique_path(plan.depRuntimeLibraryDirs, abs);
+            // Windows runtime-DLL deployment: stage each *.dll from this dir
+            // beside the produced executable (bin/). The *.dll filter — not a
+            // platform guard — keeps this inert for real .so/.dylib deps, so
+            // non-Windows builds are unchanged. See BuildPlan::DeployFile.
+            std::error_code dirEc;
+            if (std::filesystem::is_directory(abs, dirEc)) {
+                for (auto const& entry :
+                         std::filesystem::directory_iterator(abs, dirEc)) {
+                    if (!entry.is_regular_file()) continue;
+                    auto ext = entry.path().extension().string();
+                    std::ranges::transform(ext, ext.begin(),
+                        [](unsigned char c){ return std::tolower(c); });
+                    if (ext != ".dll") continue;
+                    std::filesystem::path dest =
+                        std::filesystem::path("bin") / entry.path().filename();
+                    if (std::ranges::none_of(plan.runtimeDeployFiles,
+                            [&](auto const& d){ return d.dest == dest; }))
+                        plan.runtimeDeployFiles.push_back({entry.path(), dest});
+                }
+            }
         }
         for (auto const& lib : package.manifest.runtimeConfig.dlopenLibs) {
             if (std::ranges::find(plan.runtimeDlopenLibs, lib) == plan.runtimeDlopenLibs.end())

src/toolchain/fingerprint.cppm

@@ -18,7 +18,7 @@ import mcpp.toolchain.detect;
 
 export namespace mcpp::toolchain {
 
-inline constexpr std::string_view MCPP_VERSION = "0.0.72";
+inline constexpr std::string_view MCPP_VERSION = "0.0.73";
 
 struct FingerprintInputs {
     Toolchain                       toolchain;

tests/e2e/84_runtime_dll_deploy.sh

@@ -0,0 +1,85 @@
+#!/usr/bin/env bash
+# 84_runtime_dll_deploy.sh — Windows runtime-DLL deployment, validated MECHANICALLY
+# on any host. A dependency's `[runtime] library_dirs` may hold a runtime DLL that
+# a directly-launched executable must find beside it (Windows has no RPATH). mcpp
+# stages every *.dll from a linked dependency's runtime library_dir into bin/,
+# next to the produced executable.
+#
+# The deploy is filtered by the *.dll extension, NOT by `if constexpr(is_windows)`:
+# a real Linux/macOS dependency ships .so/.dylib (never .dll), so the mechanism is
+# inert there and non-Windows builds are byte-for-byte unchanged. This test ships a
+# dummy `libdummy.dll` so the exact copy-beside-exe path is exercised on the Linux
+# CI host — the Windows link/run half is covered by mcpp-index CI (Phase D).
+# See .agents/docs/2026-06-29-windows-runtime-dll-deployment-and-openblas.md.
+#
+# No `requires:` capability → runs on all three CI platforms.
+set -e
+
+TMP=$(mktemp -d)
+trap "rm -rf $TMP" EXIT
+cd "$TMP"
+
+# A "prebuilt" dependency whose runtime artifact is a DLL sitting in bin/. The
+# file content is irrelevant — deployment is a file copy, not a link — so a stub
+# byte stream named libdummy.dll stands in for a real OpenBLAS-style DLL.
+mkdir -p blasish/bin blasish/src
+cat > blasish/mcpp.toml <<'EOF'
+[package]
+name    = "blasish"
+version = "0.1.0"
+
+[targets.blasish]
+kind = "lib"
+
+# The runtime DLL lives in bin/. On Windows mcpp copies it beside the consumer's
+# .exe; on RPATH platforms the *.dll filter makes this a no-op.
+[runtime]
+library_dirs = ["bin"]
+EOF
+cat > blasish/src/blasish.cppm <<'EOF'
+export module blasish;
+export int blasish_anchor() { return 0; }
+EOF
+# Stub runtime DLL (and a sibling non-DLL that must NOT be deployed).
+printf 'MZ stub dll payload' > blasish/bin/libdummy.dll
+printf 'not a dll'           > blasish/bin/readme.txt
+
+mkdir -p app/src
+cat > app/mcpp.toml <<'EOF'
+[package]
+name    = "app"
+version = "0.1.0"
+
+[dependencies]
+blasish = { path = "../blasish" }
+EOF
+cat > app/src/main.cpp <<'EOF'
+import blasish;
+int main() { return blasish_anchor(); }
+EOF
+
+cd app
+"$MCPP" build > b.log 2>&1 || { cat b.log; echo "FAIL: build errored"; exit 1; }
+
+# The executable's output directory (bin/, beside the .exe).
+BIN=$(find target -type f \( -name app -o -name app.exe \) | head -1)
+[ -n "$BIN" ] || { echo "FAIL: built binary not found under target/"; cat b.log; exit 1; }
+BINDIR=$(dirname "$BIN")
+
+# The dependency's runtime DLL must have been staged beside the executable.
+if [ ! -f "$BINDIR/libdummy.dll" ]; then
+    echo "FAIL: libdummy.dll was not deployed beside the executable ($BINDIR)"
+    echo "--- bin/ contents ---"; ls -la "$BINDIR"
+    echo "--- build.ninja deploy edges ---"; grep -n "libdummy" target/*/*/build.ninja 2>/dev/null || true
+    exit 1
+fi
+
+# Byte-for-byte copy of the source DLL.
+cmp -s ../blasish/bin/libdummy.dll "$BINDIR/libdummy.dll" || {
+    echo "FAIL: deployed libdummy.dll differs from source"; exit 1; }
+
+# The non-DLL sibling must NOT be deployed (only *.dll is staged).
+if [ -f "$BINDIR/readme.txt" ]; then
+    echo "FAIL: non-DLL readme.txt was deployed (only *.dll should be)"; exit 1; fi
+
+echo "OK"