The first thing the new refusal caught was this repository's own fixtures

Three e2e fixtures built a `build.mcpp` that read `mcpp::device_sources()`,
printed it, and compiled nothing. They were asserting on the env contract, which
is a real thing to assert on -- and they were modelling a project whose device
files compile to nothing, which is exactly the defect the new refusal exists to
catch. So the refusal caught them, and it was right to.

606, 609 and 613 now declare a `check` action per device source, which is the
edge a real rule package declares, and go on asserting on the variable exactly
as before. The action's input has to be ABSOLUTE: `device_sources()` is
package-root-relative and an action does not run in the package root, which the
first attempt got wrong and `cp: cannot stat 'shaders/s.tesc'` said plainly.

EXAMPLE 05 PINS ITS TOOLCHAIN, and this is a different failure that surfaced in
the same run. `mcpp pack` names its output directory after the ABI tag, and the
consumer beside it names that directory literally -- so the pair only agrees on
a machine whose DEFAULT toolchain is the one that produced the tag. Measured:
a runner whose sandbox had been left with an LLVM default produced
`mathkit-0.1.0-x86_64-linux-gnu-clang22-libcxx22-c++23` while the consumer
named the `gcc16-libstdcxx16` one, and the example that had passed for months
failed without anything in it changing.

An example that hard-codes an ABI tag has to name the toolchain that produces
it. That is not a workaround for CI: a packed artifact IS specific to the ABI
it was built against, which is the entire subject of that example, so leaving
the toolchain implicit was the inconsistency.

Author: speak-agent

examples/05-lib-distribution/consumer/mcpp.toml

@@ -7,6 +7,25 @@ license     = "Apache-2.0"
 # A packed library is an ordinary package: a path dependency, a git dependency,
 # a downloaded archive and an index entry all reach it the same way. Point this
 # at whatever `mcpp pack mathkit` left under ../producer/target/dist/.
+
+# THE TOOLCHAIN IS PINNED BECAUSE THE CONSUMER NAMES THE TAG.
+#
+# `mcpp pack` writes a directory whose name carries the ABI tag --
+# `mathkit-0.1.0-x86_64-linux-gnu-gcc16-libstdcxx16-c++23` -- and the consumer
+# beside this one names that directory literally, which is what the README
+# tells a reader to do. The tag therefore depends on which toolchain built it,
+# and without this pin the pair only agrees on a machine whose DEFAULT
+# toolchain happens to be gcc 16. Measured: a CI runner whose sandbox had been
+# left with an LLVM default produced
+# `mathkit-0.1.0-x86_64-linux-gnu-clang22-libcxx22-c++23`, and the consumer
+# named a directory that did not exist.
+#
+# An example that hard-codes an ABI tag has to name the toolchain that
+# produces it. That is not a workaround: a packed artifact IS specific to the
+# ABI it was built against, which is the whole subject of this example.
+[toolchain]
+default = "gcc@16.1.0"
+
 [dependencies]
 mathkit = { path = "../producer/target/dist/mathkit-0.1.0-x86_64-linux-gnu-gcc16-libstdcxx16-c++23" }
 

examples/05-lib-distribution/producer/mcpp.toml

@@ -4,6 +4,25 @@ version     = "0.1.0"
 description = "Demo: shipping a prebuilt library with both a header and a module interface"
 license     = "Apache-2.0"
 
+
+# THE TOOLCHAIN IS PINNED BECAUSE THE CONSUMER NAMES THE TAG.
+#
+# `mcpp pack` writes a directory whose name carries the ABI tag --
+# `mathkit-0.1.0-x86_64-linux-gnu-gcc16-libstdcxx16-c++23` -- and the consumer
+# beside this one names that directory literally, which is what the README
+# tells a reader to do. The tag therefore depends on which toolchain built it,
+# and without this pin the pair only agrees on a machine whose DEFAULT
+# toolchain happens to be gcc 16. Measured: a CI runner whose sandbox had been
+# left with an LLVM default produced
+# `mathkit-0.1.0-x86_64-linux-gnu-clang22-libcxx22-c++23`, and the consumer
+# named a directory that did not exist.
+#
+# An example that hard-codes an ABI tag has to name the toolchain that
+# produces it. That is not a workaround: a packed artifact IS specific to the
+# ABI it was built against, which is the whole subject of this example.
+[toolchain]
+default = "gcc@16.1.0"
+
 [build]
 sources      = ["src/*.cppm", "src/*.cpp", "src/*.c"]
 # The public headers. Published whole — see include/mathkit_c.h.

tests/e2e/606_constrained_source_globs_narrow_to_the_build.sh

@@ -49,9 +49,36 @@ kind = "bin"
 main = "src/main.cpp"
 EOF
     cat > build.mcpp <<'EOF'
+import std;
 import mcpp;
+
+// A `check` action per device source, and the reason it is here rather than a
+// bare read of the variable: mcpp refuses a device source that reached no
+// action (2026.9.6.5). A build program that only LOOKS at
+// `mcpp::device_sources()` models a project whose device files compile to
+// nothing, which is the defect that refusal exists to catch -- so this fixture
+// declares the edge a real rule package would declare, and asserts on the
+// variable as before.
 int main() {
     const char* d = mcpp::device_sources();
+    std::string flat(d);
+    std::size_t n = 0, start = 0;
+    while (start <= flat.size()) {
+        auto nl = flat.find('\n', start);
+        auto one = flat.substr(start, nl == std::string::npos ? flat.size() - start : nl - start);
+        start = nl == std::string::npos ? flat.size() + 1 : nl + 1;
+        if (one.empty()) continue;
+        auto stamp = std::string(mcpp::out_dir()) + "/dev-" + std::to_string(n++) + ".stamp";
+        mcpp::action a;
+        a.id = "seen";
+        a.role = "check";
+        a.description = "account for a device source";
+        auto abs = std::string(mcpp::manifest_dir()) + "/" + one;
+        a.arg("cp").arg(abs.c_str()).arg(stamp.c_str());
+        a.input(abs.c_str());
+        a.output(stamp.c_str());
+        a.submit();
+    }
     mcpp::warning((*d ? d : "(no device sources)"));
     return 0;
 }

tests/e2e/609_shader_sources_are_device_sources.sh

@@ -39,8 +39,33 @@ mkdir -p shaders
 cat > build.mcpp <<'EOF'
 import std;
 import mcpp;
+
+// A `check` action per device source, and the reason it is here rather than a
+// bare read of the variable: mcpp refuses a device source that reached no
+// action (2026.9.6.5). A build program that only LOOKS at
+// `mcpp::device_sources()` models a project whose device files compile to
+// nothing, which is the defect that refusal exists to catch -- so this fixture
+// declares the edge a real rule package would declare, and asserts on the
+// variable as before.
 int main() {
     std::string flat(mcpp::device_sources());
+    std::size_t n = 0, start = 0;
+    while (start <= flat.size()) {
+        auto nl = flat.find('\n', start);
+        auto one = flat.substr(start, nl == std::string::npos ? flat.size() - start : nl - start);
+        start = nl == std::string::npos ? flat.size() + 1 : nl + 1;
+        if (one.empty()) continue;
+        auto stamp = std::string(mcpp::out_dir()) + "/dev-" + std::to_string(n++) + ".stamp";
+        mcpp::action a;
+        a.id = "seen";
+        a.role = "check";
+        a.description = "account for a device source";
+        auto abs = std::string(mcpp::manifest_dir()) + "/" + one;
+        a.arg("cp").arg(abs.c_str()).arg(stamp.c_str());
+        a.input(abs.c_str());
+        a.output(stamp.c_str());
+        a.submit();
+    }
     for (auto& c : flat) if (c == '\n') c = ' ';
     mcpp::warning(("device=[" + flat + "]").c_str());
     return 0;

tests/e2e/613_sycl_sources_are_device_sources.sh

@@ -34,8 +34,33 @@ mkdir -p src/kernels
 cat > build.mcpp <<'EOF2'
 import std;
 import mcpp;
+
+// A `check` action per device source, and the reason it is here rather than a
+// bare read of the variable: mcpp refuses a device source that reached no
+// action (2026.9.6.5). A build program that only LOOKS at
+// `mcpp::device_sources()` models a project whose device files compile to
+// nothing, which is the defect that refusal exists to catch -- so this fixture
+// declares the edge a real rule package would declare, and asserts on the
+// variable as before.
 int main() {
     std::string flat(mcpp::device_sources());
+    std::size_t n = 0, start = 0;
+    while (start <= flat.size()) {
+        auto nl = flat.find('\n', start);
+        auto one = flat.substr(start, nl == std::string::npos ? flat.size() - start : nl - start);
+        start = nl == std::string::npos ? flat.size() + 1 : nl + 1;
+        if (one.empty()) continue;
+        auto abs   = std::string(mcpp::manifest_dir()) + "/" + one;
+        auto stamp = std::string(mcpp::out_dir()) + "/dev-" + std::to_string(n++) + ".stamp";
+        mcpp::action a;
+        a.id = "seen";
+        a.role = "check";
+        a.description = "account for a device source";
+        a.arg("cp").arg(abs.c_str()).arg(stamp.c_str());
+        a.input(abs.c_str());
+        a.output(stamp.c_str());
+        a.submit();
+    }
     for (auto& c : flat) if (c == '\n') c = ' ';
     mcpp::warning(("device=[" + flat + "]").c_str());
     return 0;