fix(ci): force MSVC host toolchain on Windows to prevent dlltool errors

The self-hosted Windows runner has a persistent rustup settings file
(C:\Windows\system32\config\systemprofile\.rustup\settings.toml) with
default_host_triple set to x86_64-pc-windows-gnu. The setup-rust-toolchain
action reinstalls rustup which reads this file, causing bare toolchain
names like nightly-2026-02-09 to resolve to the GNU variant. The GNU
toolchain requires dlltool.exe which is not present on MSVC-only runners.

Fix with three layers of defense:
- Use fully qualified RUSTUP_TOOLCHAIN on Windows (nightly-2026-02-09-x86_64-pc-windows-msvc)
  in taskfiles/ci.yml to eliminate host triple ambiguity
- Add rustup set default-host x86_64-pc-windows-msvc before toolchain
  install in both the composite action and the Taskfile
- Pass PINNED_RUST explicitly from the composite action to task ci:setup
  on Windows, bypassing unreliable grep/awk resolution in Task's sh:
- Add host verification to the setup-rust-windows status check
- Document the constraint in AGENTS.md and docs/builds.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pythoninthegrass

.github/actions/setup-tauri-build/action.yml

@@ -41,6 +41,14 @@ runs:
         $gitPath = (Get-ItemProperty 'HKLM:\SOFTWARE\GitForWindows' -ErrorAction SilentlyContinue).InstallPath
         if ($gitPath) { echo "$gitPath\usr\bin" >> $env:GITHUB_PATH }
 
+    - name: Set rustup default host to MSVC (Windows)
+      if: runner.os == 'Windows'
+      shell: pwsh
+      run: |
+        if (Get-Command rustup -ErrorAction SilentlyContinue) {
+          rustup set default-host x86_64-pc-windows-msvc
+        }
+
     - name: Install Task
       uses: go-task/setup-task@v1
 
@@ -129,7 +137,7 @@ runs:
     - name: Setup CI environment (Windows)
       if: runner.os == 'Windows'
       shell: pwsh
-      run: task ci:setup
+      run: task ci:setup PINNED_RUST=${{ steps.rust-version-windows.outputs.toolchain }}
 
     - name: Windows Defender exclusions
       if: runner.os == 'Windows'

AGENTS.md

@@ -84,6 +84,16 @@ Three directories are involved — do NOT cross-link them:
 
 `cargo.yml` adds `~/.cargo/bin` to PATH so rustup tools are always findable. The `_setup-cargo-home` task symlinks project-local `CARGO_HOME/bin/` to `~/.cargo/bin/` — never to the mise install dir, which has its own internal symlink structure.
 
+### Windows CI Toolchain Constraint
+
+On Windows CI runners, `RUSTUP_TOOLCHAIN` MUST use the fully qualified name with host triple suffix (e.g. `nightly-2026-02-09-x86_64-pc-windows-msvc`). Bare channel names like `nightly-2026-02-09` resolve using the runner's default host triple, which can be stale GNU — causing `dlltool.exe` not found errors. See [Build Configuration — Windows Toolchain Pinning](docs/builds.md#windows-toolchain-pinning) for details.
+
+When modifying `taskfiles/ci.yml` or `.github/actions/setup-tauri-build/action.yml`:
+
+- Never set `RUSTUP_TOOLCHAIN` to a bare toolchain name on Windows
+- Always run `rustup set default-host x86_64-pc-windows-msvc` before toolchain installation
+- Pass `PINNED_RUST` explicitly from the composite action rather than relying on shell extraction in Task
+
 ## Queue and Shuffle Behavior
 
 The queue store (`app/frontend/js/stores/queue.js`) maintains tracks in **play order** — the `items` array always reflects the order tracks will be played.

docs/builds.md

@@ -522,7 +522,7 @@ Runs on `ubuntu-latest`:
 
 Runs on a self-hosted `[self-hosted, Windows, X64]` runner:
 
-1. Sets up the Tauri build environment (Chocolatey installs cmake and rustup; `RUSTUP_TOOLCHAIN=nightly-2026-02-09` is exported to `GITHUB_ENV` and `~/.cargo/bin` is prepended to `GITHUB_PATH` to ensure the nightly toolchain takes precedence)
+1. Sets up the Tauri build environment (Chocolatey installs cmake and rustup; `~/.cargo/bin` is prepended to `GITHUB_PATH`; `RUSTUP_TOOLCHAIN` is set to the fully qualified `nightly-2026-02-09-x86_64-pc-windows-msvc` to ensure the MSVC-hosted toolchain is used — see [Windows Toolchain Pinning](#windows-toolchain-pinning) below)
 2. Installs Windows SDK for `signtool.exe`
 3. Generates a self-signed `CodeSigningCert` and exports to PFX
 4. Builds the frontend explicitly (`npm run build` in `app/frontend/`)
@@ -531,6 +531,22 @@ Runs on a self-hosted `[self-hosted, Windows, X64]` runner:
 7. Attaches the signed `.exe` to the same draft GitHub Release
 8. Cleans up the certificate and config override (runs in `always()` step)
 
+#### Windows Toolchain Pinning
+
+The self-hosted Windows runner can accumulate stale rustup state across runs. In particular, a system-level settings file (`C:\Windows\system32\config\systemprofile\.rustup\settings.toml`) may set `default_host_triple` to `x86_64-pc-windows-gnu`. When `RUSTUP_TOOLCHAIN` is set to a bare channel name like `nightly-2026-02-09`, rustup resolves it using the default host triple — producing the GNU-hosted toolchain, which requires `dlltool.exe` (not present on MSVC-only runners) and fails with:
+
+```text
+error: error calling dlltool 'dlltool.exe': program not found
+```
+
+To prevent this, the CI configuration uses three layers of defense:
+
+1. **Fully qualified `RUSTUP_TOOLCHAIN`** (`taskfiles/ci.yml`): On Windows, the env var is set to `nightly-2026-02-09-x86_64-pc-windows-msvc` (with the host triple suffix), eliminating any ambiguity in toolchain resolution.
+
+2. **`rustup set default-host x86_64-pc-windows-msvc`** (`.github/actions/setup-tauri-build/action.yml` and `taskfiles/ci.yml`): Run before toolchain installation to override any stale GNU default.
+
+3. **Explicit `PINNED_RUST` passthrough** (`.github/actions/setup-tauri-build/action.yml`): The composite action extracts the Rust version from `.tool-versions` using PowerShell and passes it to `task ci:setup PINNED_RUST=...`, bypassing the Taskfile's `sh: grep | awk` which may fail when Git Bash utilities aren't in PATH.
+
 ## Linux System Dependencies
 
 Tauri on Ubuntu/Debian requires these packages:

taskfiles/ci.yml

@@ -21,7 +21,7 @@ vars:
 env:
   PATH: "{{.CARGO_BIN}}:{{.MISE_SHIMS}}:{{.PATH}}"
   RUSTC_WRAPPER: ""
-  RUSTUP_TOOLCHAIN: "{{.PINNED_RUST}}"
+  RUSTUP_TOOLCHAIN: '{{if eq OS "windows"}}{{.PINNED_RUST}}-x86_64-pc-windows-msvc{{else}}{{.PINNED_RUST}}{{end}}'
 
 tasks:
   setup:
@@ -50,12 +50,14 @@ tasks:
     desc: "Configure rustup nightly toolchain (Windows)"
     platforms: [windows]
     cmds:
+      - rustup set default-host x86_64-pc-windows-msvc
       - rustup toolchain install {{.PINNED_RUST}} --profile minimal --no-self-update --target x86_64-pc-windows-msvc
       - rustup default {{.PINNED_RUST}}
       - rustc --version
       - cargo --version
     status:
       - rustup run {{.PINNED_RUST}} rustc --version
+      - 'rustup run {{.PINNED_RUST}} rustc --version --verbose | findstr "host: x86_64-pc-windows-msvc"'
       - rustup target list --installed --toolchain {{.PINNED_RUST}} | findstr x86_64-pc-windows-msvc
 
   setup-cargo-tools: