examples/10-graphics: 依赖不能被 layer 条件化,反向腿改成比较像素

这个示例最初把 Vulkan loader、运行时适配器与软件设备三条都门控在
`cfg(accelerator = "vulkan")` 上。**这条做不到,而且它失败的方式是安静的一半**:
`accelerator` 是从依赖图里解出来的,所以一个由它选出的依赖会决定它自己正在问的那个
答案 —— mcpp 忽略这个谓词并给出警告,而同一个谓词下的 `[build]` 源照常生效。于是包
被丢掉、包含它的源被留下:

    src/vulkan/render.cpp:20:10: fatal error: vulkan/vulkan.h: No such file or directory

谓词改成按平台写(只有 Linux 有的两项用 `cfg(linux)`),包无条件,accelerator 只选
`[build]` 源 —— 既有的 examples/09-heterogeneous/vulkan 一直是这个形状。示例 README
把这条写成正文,因为它是使用者会撞上的规则。

反向腿加强:实测两条腿的中心像素逐字节相同(`(124, 70, 62, 255)`),所以 CI 从
「CPU 腿跑起来了」改成「两条腿报出同一个像素、不同的设备名」—— 这才是这个示例声称
的那个契约。

本地实测:lavapipe 腿 `device: llvmpipe (LLVM 22.1.8, 256 bits)`,CPU 腿
`device: cpu rasteriser`,两者 `centre pixel: (124, 70, 62, 255)`。

Author: speak-agent

.agents/docs/2026-09-07-heterogeneous-cross-platform-ecosystem.md

@@ -329,6 +329,23 @@ install/config 形状,而那套形状是按 Linux 写的:
   Metal 的编译器只在 Xcode 内、不可再分发,所以它不是一个打包问题;ROCm 的运行时可
   再分发,但它需要的是一个 `rules-hip` 的 AMD 平台实现,而不是一个包。
 
+### 10.4b 第四处「读出来的写法是错的」:依赖不能被 layer 条件化
+
+图形示例最初把 Vulkan loader、运行时适配器与软件设备三条都门控在
+`cfg(accelerator = "vulkan")` 上,理由是「`--no-accel` 一个字节都不装」。**这条做不
+到,而且它失败的方式是安静的一半**:`accelerator` 是**从依赖图里解出来的**,所以一个
+由它选出的依赖会决定它自己正在问的那个答案 —— mcpp 因此忽略这个谓词并给出警告,而
+**同一个谓词下的 `[build]` 源照常生效**。结果是包被丢掉、包含它的源被留下:
+
+    src/vulkan/render.cpp:20:10: fatal error: vulkan/vulkan.h: No such file or directory
+
+判据是构建本身。谓词现在按平台写(`cfg(linux)` 用于只有 Linux 有的两项),包是无条件
+的,accelerator 只选 `[build]` 源 —— 也就是既有的 `examples/09-heterogeneous/vulkan`
+一直在用的形状。示例 README 把这一条写成了正文,因为它是一条使用者会撞上的规则。
+
+顺带一条读数:两条腿的中心像素**逐字节相同**(`(124, 70, 62, 255)`),所以 CI 的反向
+腿从「CPU 腿跑起来了」加强成「两条腿报出同一个像素、不同的设备名」。
+
 ### 10.5 一条留下的不一致,以及它什么时候消失
 
 `mcpp:plugins` 0.2.5 里 `xim:shaderc` 在 macOS 与 Windows 上是**精确版本**,而

.github/workflows/ci-linux.yml

@@ -312,11 +312,23 @@ jobs:
             || { echo "the run did not reach the lavapipe device"; exit 1; }
           echo "$out" | grep -qE 'centre pixel: \([0-9]+, [0-9]+, [0-9]+, 255\)' \
             || { echo "no centre pixel was reported"; exit 1; }
-          # The reverse leg: the same assertions hold with no accelerator, which
-          # is what makes the pixel test a contract rather than a device quirk.
+          # THE REVERSE LEG, AND IT COMPARES THE PIXELS RATHER THAN JUST
+          # RUNNING. The claim this example makes is that the image is a
+          # contract two independent rasterisers satisfy, not a property of one
+          # device -- so the criterion is that the two legs report the SAME
+          # centre pixel while reporting different devices. Measured: both give
+          # (124, 70, 62, 255), byte for byte.
+          gpu_px=$(echo "$out" | grep -m1 '^centre pixel:')
           "$MCPP" build --no-accel
           "$MCPP" run --no-accel | tee cpu.log
           grep -q 'cpu rasteriser' cpu.log || { echo "the CPU leg did not run"; exit 1; }
+          cpu_px=$(grep -m1 '^centre pixel:' cpu.log)
+          [ -n "$gpu_px" ] && [ "$gpu_px" = "$cpu_px" ] || {
+            echo "the two legs disagree about the image:"
+            echo "  device: $gpu_px"
+            echo "  cpu:    $cpu_px"
+            exit 1; }
+          echo "ok: both legs agree on $cpu_px"
 
       - name: "Vulkan example: build the device half and run it on lavapipe"
         run: |

examples/10-graphics/README.md

@@ -70,8 +70,15 @@ shaders to the build program rather than to the C++ compiler; and the generated
 headers land on the include path, so `src/vulkan/render.cpp` writes
 `#include "triangle_vert.h"`.
 
-`mcpp build --no-accel` installs nothing at all — the Vulkan loader and the
-software device are both gated on the accelerator.
+**A dependency cannot be conditioned on the accelerator, and this project is
+where that shows.** `accelerator` is resolved from the dependency graph, so a
+dependency chosen by it would decide the answer it is asking for. mcpp says so
+and ignores the predicate. An earlier revision of this manifest gated the Vulkan
+loader on `cfg(accelerator = "vulkan")` and the build failed on
+`vulkan/vulkan.h: No such file or directory` — the header's package had been
+dropped while the source that includes it, selected by the same predicate, was
+kept. Packages are therefore unconditional or conditioned on the platform;
+`[build]` sources are what the accelerator selects.
 
 **One shader per stem.** The generated name is the shader's stem and its stage,
 so `ui/text.vert` and `world/text.vert` would both produce `text_vert.h`

examples/10-graphics/offscreen/mcpp.toml

@@ -15,23 +15,33 @@ import_std = true
 [build-dependencies.mcpp]
 plugins = { version = "0.2.5", features = ["rules-spirv"], host-module = true }
 
-# The Khronos loader, and on Linux the adapter that makes the host's own ICDs
-# reachable from a binary running under mcpp's private loader. The adapter is a
-# Linux concern by construction: macOS resolves through dyld and Windows through
-# the PE loader, and neither has a private loader to work around.
-# GATED ON THE ACCELERATOR, so `mcpp build --no-accel` installs nothing at all.
-# The CPU leg includes no Vulkan header and links no loader, and a build that
-# pays for one anyway is the shape `cfg(accelerator = ...)` exists to remove.
-[target.'cfg(accelerator = "vulkan")'.dependencies.compat]
+# The Khronos loader. Unconditional, and the reason is a rule of the engine
+# rather than a preference: A DEPENDENCY CANNOT BE CONDITIONED ON A LAYER.
+# `accelerator` is resolved FROM the dependency graph, so a dependency chosen by
+# it would decide the answer it is asking. mcpp says so and ignores the
+# predicate -- measured here, where an earlier revision of this file gated these
+# three entries on `cfg(accelerator = "vulkan")` and the build failed on
+# `vulkan/vulkan.h: No such file or directory`, because the header's package had
+# been silently dropped while the source that includes it was kept.
+#
+# `[build]` sources under the same predicate DO apply, which is what makes the
+# seam work: the CPU leg below is selected by the accelerator, and only the
+# packages are unconditional.
+[dependencies.compat]
 vulkan = "1.4.357.0"
 
-[target.'cfg(all(accelerator = "vulkan", linux))'.dependencies.compat]
+# The adapter that makes the host's own ICDs reachable from a binary running
+# under mcpp's private loader. An OS predicate, which IS allowed, and a Linux
+# concern by construction: macOS resolves through dyld and Windows through the
+# PE loader, and neither has a private loader to work around.
+[target.'cfg(linux)'.dependencies.compat]
 vulkan-runtime = "2026.09.07"
 
 # A Vulkan device that needs no GPU, so this example runs on a machine that has
 # none. It is a DEVICE and therefore a payload; the drivers a real GPU needs are
-# the host's, and `compat.vulkan` models those as a capability.
-[target.'cfg(all(accelerator = "vulkan", linux))'.xlings.workspace]
+# the host's, and `compat.vulkan` models those as a capability. Published for
+# Linux alone, so the predicate names the platform rather than the accelerator.
+[target.'cfg(linux)'.xlings.workspace]
 "xim:mesa-lavapipe" = "26.2.1"
 
 [build]