im-select-rs

A tiny Windows input method helper written in Rust. It switches, toggles and queries the input method between English (United States, locale 1033) and Chinese (Simplified) - Microsoft Pinyin (locale 2052).

It is designed to be called from a terminal editor (Neovim, etc.) running inside WSL, where a Linux input method framework is not available and the actual IME lives on the Windows side.

Features

• Toggle between English and Microsoft Pinyin.
• Switch directly to a specific locale.
• Query the current input method state as a single token, for display in a statusline (e.g. lualine).
• Distinguishes Microsoft Pinyin's Chinese mode from its English (latin pass-through) mode by reading the IME conversion status.

Usage

im-select-rs.exe             Toggle between English (1033) and Chinese (2052)
im-select-rs.exe 1033        Switch to English (United States)
im-select-rs.exe 2052        Switch to Chinese (Simplified) - Microsoft Pinyin
im-select-rs.exe 简中, zh    Switch Microsoft Pinyin to Chinese mode
im-select-rs.exe 简英, py_en Switch Microsoft Pinyin to English mode
im-select-rs.exe -s, --status   Print current input method state token
im-select-rs.exe -r, --raw      Print raw locale/IME values for debugging
im-select-rs.exe -h, --help     Show this help

Status tokens (--status)

Prints exactly one token on stdout, intended for machine consumption:

Token	Meaning	Label
eng	English keyboard layout / IME closed	ENG
zh	Microsoft Pinyin, Chinese (native) mode	简中
py_en	Microsoft Pinyin, English (latin pass-through) mode	简英

Example:

> im-select-rs.exe --status
zh

Raw output (--raw)

Prints the underlying values for verification / tuning on your own machine:

> im-select-rs.exe --raw
locale=2052 open=true conv=0x1 status=zh

• locale - the active input language LANGID low word (1033 / 2052).
• open - whether the IME is open.
• conv - the fdwConversion bitmask; bit 0x0001 (IME_CMODE_NATIVE) is what separates Chinese mode from English mode.

If the Chinese / English distinction does not behave as expected for your IME, run --raw in each state and check whether the conv bit toggles.

How it works

• The active input language is read from the foreground window's thread via GetKeyboardLayout. The low word of the HKL is the LANGID.
• Switching is done with LoadKeyboardLayoutW + ActivateKeyboardLayout, followed by broadcasting WM_INPUTLANGCHANGEREQUEST.
• 简中 / zh and 简英 / py_en first switch to Microsoft Pinyin, then send WM_IME_CONTROL (IMC_SETOPENSTATUS / IMC_SETCONVERSIONMODE) to request the Chinese or English sub-mode.
• The IME Chinese/English sub-mode is queried by sending WM_IME_CONTROL (IMC_GETOPENSTATUS / IMC_GETCONVERSIONMODE) to the foreground window's default IME window (ImmGetDefaultIMEWnd) using SendMessageTimeoutW. This is the standard cross-process way to read IME state, since the IME input context is thread-local and cannot be read directly from another process.

Building

Requires the Rust toolchain with the x86_64-pc-windows-msvc target and the MSVC build tools.

cargo build --release

The resulting binary is target/release/im-select-rs.exe.

Requirements

• Windows 10 / 11.
• The input languages 1033 (English (United States)) and 2052 (Chinese (Simplified) - Microsoft Pinyin) installed.
• "Let me use a different input method for each app window" should be enabled (Settings > Time & Language > Typing > Advanced keyboard settings) so that the foreground window reflects what the user is actually typing.
• For the zh / py_en distinction to work, the foreground window must be a classic Win32 app (see Troubleshooting).

Troubleshooting

If --raw reports open=false conv=0x0 while Microsoft Pinyin is the active layout, the foreground window is a modern TSF application (Windows Terminal, modern PowerShell, UWP/Edge-style apps). Such windows do not expose the legacy IMM fdwConversion bits to other processes, so the Chinese/English sub-mode of Microsoft Pinyin cannot be read cross-process from a separate .exe (TSF compartments are thread-local; there is no global API for it).

This is a platform limitation, not a bug in the tool. Enabling "Use previous version of Microsoft Pinyin" only helps when the foreground is a classic Win32 window (e.g. GVim); it does not help inside Windows Terminal.

When the sub-mode is unreadable, --status falls back to zh for an active Microsoft Pinyin (its primary mode), rather than always reporting py_en. To get a reliable on/off signal that always works, switch layouts with Win+Space (1033 = eng, 2052 = zh) instead of toggling Pinyin's internal sub-mode.

License

MIT