refactor: cleanup from @joerick

README.md

@@ -102,7 +102,7 @@ Options
 |---|--------|-------------|
 | **Build selection** | [`CIBW_PLATFORM`](https://cibuildwheel.readthedocs.io/en/stable/options/#platform)  | Override the auto-detected target platform |
 |   | [`CIBW_BUILD`](https://cibuildwheel.readthedocs.io/en/stable/options/#build-skip)  <br> [`CIBW_SKIP`](https://cibuildwheel.readthedocs.io/en/stable/options/#build-skip)  | Choose the Python versions to build |
-|   | [`CIBW_ARCHS`](https://cibuildwheel.readthedocs.io/en/stable/options/#archs)  | Control available architectures |
+|   | [`CIBW_ARCHS`](https://cibuildwheel.readthedocs.io/en/stable/options/#archs)  | Change the architectures built on your machine by default |
 |   | [`CIBW_PROJECT_REQUIRES_PYTHON`](https://cibuildwheel.readthedocs.io/en/stable/options/#requires-python)  | Manually set the Python compatibility of your project |
 | **Build customization** | [`CIBW_ENVIRONMENT`](https://cibuildwheel.readthedocs.io/en/stable/options/#environment)  | Set environment variables needed during the build |
 |   | [`CIBW_BEFORE_ALL`](https://cibuildwheel.readthedocs.io/en/stable/options/#before-all)  | Execute a shell command on the build system before any wheels are built. |

cibuildwheel/architecture.py

@@ -82,24 +82,16 @@ def all_archs(platform: PlatformName) -> 'Set[Architecture]':
             assert_never(platform)
 
     @staticmethod
-    def bitiness_archs(platform: PlatformName, bitiness: Literal["64", "32"]) -> 'Set[Architecture]':
-        native_architecture = Architecture(platform_module.machine())
-
-        if native_architecture in {Architecture.x86_64, Architecture.aarch64, Architecture.ppc64le, Architecture.s390x, Architecture.AMD64}:
-            if bitiness == "64":
-                return {native_architecture}
-            else:
-                if native_architecture == Architecture.x86_64 and platform != "macos":
-                    return {Architecture.i686}
-                elif native_architecture == Architecture.AMD64:
-                    return {Architecture.x86}
-                else:
-                    return set()
-        elif native_architecture in {Architecture.i686, Architecture.x86}:
-            return {native_architecture} if bitiness == "32" else set()
+    def bitiness_archs(platform: PlatformName, bitiness: Literal['64', '32']) -> 'Set[Architecture]':
+        archs_32 = {Architecture.i686, Architecture.x86}
+        auto_archs = Architecture.auto_archs(platform)
+
+        if bitiness == '64':
+            return auto_archs - archs_32
+        elif bitiness == '32':
+            return auto_archs & archs_32
         else:
-            # assert_never doesn't work here, oddly, maybe due to set checking above
-            raise RuntimeError("Cannot be reached")
+            assert_never(bitiness)
 
 
 def allowed_architectures_check(

docs/options.md

@@ -172,21 +172,21 @@ Options:
 - macOS: `x86_64` `arm64` `universal2`
 - Windows: `AMD64` `x86`
 - `auto`: The default archs for your machine - see the table below.
-    - `auto64`: Just the 64-bit auto archs (will result in an empty set on a 32-bit runner)
-    - `auto32`: Just the 32-bit auto archs (will result in an empty set on macOS)
+    - `auto64`: Just the 64-bit auto archs
+    - `auto32`: Just the 32-bit auto archs
 - `native`: the native arch of the build machine - Matches [`platform.machine()`](https://docs.python.org/3/library/platform.html#platform.machine).
 - `all` : expands to all the architectures supported on this OS. You may want
   to use [CIBW_BUILD](#build-skip) with this option to target specific
   architectures via build selectors.
 
 Default: `auto`
 
-| Runner | `native` | `auto`
-|---|---|---
-| Linux / Intel | `x86_64` | `x86_64` `i686`
-| Windows / Intel | `AMD64` | `AMD64` `x86`
-| macOS / Intel | `x86_64` | `x86_64`
-| macOS / Apple Silicon | `arm64` | `arm64` `universal2`
+| Runner | `native` | `auto` | `auto64` | `auto32` |
+|---|---|---|---|---|
+| Linux / Intel | `x86_64` | `x86_64` `i686` | `x86_64` | `i686` |
+| Windows / Intel | `AMD64` | `AMD64` `x86` | `AMD64` | `x86` |
+| macOS / Intel | `x86_64` | `x86_64` | `x86_64` |  |
+| macOS / Apple Silicon | `arm64` | `arm64` `universal2` | `arm64` `universal2`|  |
 
 If not listed above, `auto` is the same as `native`.
 
@@ -196,6 +196,8 @@ If not listed above, `auto` is the same as `native`.
 Platform-specific variants also available:<br/>
  `CIBW_ARCHS_MACOS` | `CIBW_ARCHS_WINDOWS` | `CIBW_ARCHS_LINUX`
 
+This option can also be set using the [command-line option](#command-line) `--archs`.
+
 #### Examples
 
 ```yaml
@@ -208,10 +210,6 @@ CIBW_ARCHS_MACOS: "x86_64 universal2 arm64"
 CIBW_ARCHS_LINUX: "auto aarch64"
 ```
 
-Platform-specific variants also available:<br/>
-`CIBW_ARCHS_MACOS` | `CIBW_ARCHS_WINDOWS` | `CIBW_ARCHS_LINUX`
-
-This option can also be set using the [command-line option](#command-line) `--archs`.
 
 ###  `CIBW_PROJECT_REQUIRES_PYTHON` {: #requires-python}
 > Manually set the Python compatibility of your project