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docs: cxx_runtime was inert on MSVC until 2026.8.16, and upgrading is silent #450

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Sunrisepeak merged 1 commit into from
Aug 17, 2026
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docs: cxx_runtime was inert on MSVC until 2026.8.16, and upgrading is silent #450

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8 changes: 8 additions & 0 deletions docs/05-mcpp-toml.md
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Expand Up @@ -423,6 +423,14 @@ machine that has only the pinned toolset and no Visual Studio at all.
The debug CRT (`vcruntime140d.dll` and friends, under `debug_nonredist\`) is
never staged: it may not be redistributed.

> **Upgrading from 2026.8.15 or earlier?** This key used to be **inert** on the
> MSVC ABI — it reported `not implemented for the MSVC runtime yet` and every
> value fell back to `/MD`. Since 2026.8.16 it is honoured, so a manifest that
> carries `cxx_runtime = "self-contained"` from that era **changes CRT model on
> upgrade**, from `/MD` to `/MT`. It is not a stricter version of the same
> model, and the switch is silent because the value was always valid. If your
> project set it while the key did nothing, decide which one you actually want.

Combining it with `/MT` is a contradiction rather than a missing feature — a
static CRT leaves no DLL to couple to — so it is reported and resolved to
`self-contained`. `mcpp pack` enforces the other half: a mode that bundles
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7 changes: 7 additions & 0 deletions docs/zh/05-mcpp-toml.md
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Expand Up @@ -375,6 +375,13 @@ Windows 组件(Win10 起),mcpp 从不分发它;而 `vcruntime140.dll` /
调试版 CRT(`debug_nonredist\` 下的 `vcruntime140d.dll` 等)永远不会被放进去:
它不可再分发。

> **从 2026.8.15 或更早版本升上来?** 这条键在 MSVC ABI 上曾经是**空操作** ——
> 它会报 `not implemented for the MSVC runtime yet`,写什么都退回 `/MD`。
> 自 2026.8.16 起它真的生效,于是一份从那个年代带着
> `cxx_runtime = "self-contained"` 的 manifest **会在升级时换掉 CRT 模型**:
> 从 `/MD` 变成 `/MT`。它不是同一个模型的更严格版本,而且因为这个值一直是合法的,
> 切换是**静默**的。如果你的工程是在这条键还不起作用时写下它的,请重新决定你真正要哪一个。

把它和 `/MT` 一起用是**矛盾**而不是缺功能 —— 静态 CRT 根本没有 DLL 可以耦合 ——
所以会被报出来并落到 `self-contained`。另一半由 `mcpp pack` 兜底:什么都不打包的
模式(`--mode system`、`--mode static`)兑现不了 `toolchain-coupled`,会直接拒绝。
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