build error //build/toolchain/win: GN error "Assignment had no effect" for rustc_wrapper_inputs on windows 11

[Emre TEKE]

Hi everyone,

I’m hitting a GN blocking error on Windows while trying to generate build files for Chromium. This appears to be a regression in the toolchain logic, specifically affecting builds where the system toolchain is used.

Environment & Steps Taken:

* Source: Freshly rebased to the latest origin/main and updated (git rebase-update).
* Sync: Executed `gclient sync`.
* Toolchain: Visual Studio 2026 (installed at `C:\Program Files\Microsoft Visual Studio\18\Community`).
* Configuration: `set DEPOT_TOOLS_WIN_TOOLCHAIN=0`.

The Error:

When running `gn gen out\Default`, GN fails with the following message:

```text
ERROR at //build/toolchain/win/toolchain.gni:394:18: Assignment had no effect.
        inputs = rustc_wrapper_inputs
                 ^-------------------
You set the variable "inputs" here and it was unused before it went out of scope.
```

Context:
The error is triggered during the `msvc_toolchain("x86")` invocation. It seems that while `rustc_wrapper_inputs` is being assigned to `inputs` within the template, the variable is never consumed by any subsequent tool or action in this specific scope, causing GN’s strictness check to fail.

Since I am using the local toolchain (`DEPOT_TOOLS_WIN_TOOLCHAIN=0`), it seems the logic for Rust integration in `toolchain.gni` doesn't properly account for cases where these inputs might be idle.

Is this a known issue with the recent Rust-related changes in the build system, or is there a missing GN arg I should be aware of to bypass this unused variable check?

Args used:
`is_component_build = true is_debug = true blink_symbol_level = 2 v8_symbol_level = 0`

Best regards,

Emre.

[Nico Weber]

Hi,

I happened to hit this locally too. In my case, it was caused by my accidentally using an old GN binary. If you manually run `gn gen out/Default`, does that work?

If not, what's the output of `grep -A4 'rule gn' out/Default/build.ninja`?

Nico

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