GH-143941: Move WASI-related files to Platforms/WASI

.github/CODEOWNERS

@@ -176,6 +176,7 @@ Tools/wasm/config.site-wasm32-emscripten  @freakboy3742 @emmatyping
 Tools/wasm/emscripten                     @freakboy3742 @emmatyping
 
 # WebAssembly (WASI)
+Platforms/WASI                @brettcannon @emmatyping @savannahostrowski
 Tools/wasm/wasi-env           @brettcannon @emmatyping @savannahostrowski
 Tools/wasm/wasi.py            @brettcannon @emmatyping @savannahostrowski
 Tools/wasm/wasi               @brettcannon @emmatyping @savannahostrowski

.github/workflows/reusable-wasi.yml

@@ -47,14 +47,14 @@ jobs:
     - name: "Runner image version"
       run: echo "IMAGE_OS_VERSION=${ImageOS}-${ImageVersion}" >> "$GITHUB_ENV"
     - name: "Configure build Python"
-      run: python3 Tools/wasm/wasi configure-build-python -- --config-cache --with-pydebug
+      run: python3 Platforms/WASI configure-build-python -- --config-cache --with-pydebug
     - name: "Make build Python"
-      run: python3 Tools/wasm/wasi make-build-python
+      run: python3 Platforms/WASI make-build-python
     - name: "Configure host"
       # `--with-pydebug` inferred from configure-build-python
-      run: python3 Tools/wasm/wasi configure-host -- --config-cache
+      run: python3 Platforms/WASI configure-host -- --config-cache
     - name: "Make host"
-      run: python3 Tools/wasm/wasi make-host
+      run: python3 Platforms/WASI make-host
     - name: "Display build info"
       run: make --directory "${CROSS_BUILD_WASI}" pythoninfo
     - name: "Test"

Misc/NEWS.d/next/Build/2026-01-16-14-27-53.gh-issue-143941.TiaE-3.rst

@@ -0,0 +1,2 @@
+Move WASI-related files to Platforms/WASI. Along the way, leave a deprecated
+Tools/wasm/wasi/__main__.py behind for backwards-compatibility.

Platforms/WASI/.ruff.toml

@@ -0,0 +1,25 @@
+extend = "../../.ruff.toml"  # Inherit the project-wide settings
+
+[format]
+preview = true
+docstring-code-format = true
+
+[lint]
+select = [
+    "C4",      # flake8-comprehensions
+    "E",       # pycodestyle
+    "F",       # pyflakes
+    "I",       # isort
+    "ISC",     # flake8-implicit-str-concat
+    "LOG",     # flake8-logging
+    "PGH",     # pygrep-hooks
+    "PT",      # flake8-pytest-style
+    "PYI",     # flake8-pyi
+    "RUF100",  # Ban unused `# noqa` comments
+    "UP",      # pyupgrade
+    "W",       # pycodestyle
+    "YTT",     # flake8-2020
+]
+ignore = [
+    "E501",    # Line too long
+]

Platforms/WASI/README.md

@@ -0,0 +1,59 @@
+# Python WASI (wasm32-wasi) build
+
+**WASI support is [tier 2](https://peps.python.org/pep-0011/#tier-2).**
+
+This directory contains configuration and helpers to facilitate cross
+compilation of CPython to WebAssembly (WASM) using WASI. WASI builds
+use WASM runtimes such as [wasmtime](https://wasmtime.dev/).
+
+**NOTE**: If you are looking for general information about WebAssembly that is
+not directly related to CPython, please see https://github.com/psf/webassembly.
+
+## Build
+
+See [the devguide on how to build and run for WASI](https://devguide.python.org/getting-started/setup-building/#wasi).
+
+## Detecting WASI builds
+
+### Python code
+
+```python
+import os, sys
+
+if sys.platform == "wasi":
+    # Python on WASI
+    ...
+
+if os.name == "posix":
+    # WASM platforms identify as POSIX-like.
+    # Windows does not provide os.uname().
+    machine = os.uname().machine
+    if machine.startswith("wasm"):
+        # WebAssembly (wasm32, wasm64 potentially in the future)
+```
+
+```python
+>>> import os, sys
+>>> os.uname()
+posix.uname_result(
+    sysname='wasi',
+    nodename='(none)',
+    release='0.0.0',
+    version='0.0.0',
+    machine='wasm32'
+)
+>>> os.name
+'posix'
+>>> sys.platform
+'wasi'
+```
+
+### C code
+
+WASI SDK defines several built-in macros. You can dump a full list of built-ins
+with ``/path/to/wasi-sdk/bin/clang -dM -E - < /dev/null``.
+
+* WebAssembly ``__wasm__`` (also ``__wasm``)
+* wasm32 ``__wasm32__`` (also ``__wasm32``)
+* wasm64 ``__wasm64__``
+* WASI ``__wasi__``