Reset PROCESSOR_ARCHITECTURE on start

include/vcpkg/base/message-data.inc.h

@@ -2705,6 +2705,10 @@ DECLARE_MESSAGE(UnexpectedToolOutput,
                 (msg::tool_name, msg::path),
                 "The actual command line output will be appended after this message.",
                 "{tool_name} ({path}) produced unexpected output when attempting to determine the version:")
+DECLARE_MESSAGE(UnexpectedWindowsArchitecture,
+                (msg::actual),
+                "{actual} is the CPU kind we observed like ARM or MIPS",
+                "unexpected Windows host architecture: {actual}")
 DECLARE_MESSAGE(UnknownBaselineFileContent,
                 (),
                 "",

include/vcpkg/base/system.h

@@ -36,6 +36,8 @@ namespace vcpkg
     ExpectedL<std::string> get_registry_string(void* base_hkey, StringView subkey, StringView valuename);
 
     ExpectedL<std::uint32_t> get_registry_dword(void* base_hkey, StringView subkey, StringView valuename);
+
+    void reset_processor_architecture_environment_variable();
 #endif
 
     long get_process_id();

locales/messages.json

@@ -1530,6 +1530,8 @@
   "_UnexpectedSwitch.comment": "Switch is a command line switch like --switch An example of {option} is editable.",
   "UnexpectedToolOutput": "{tool_name} ({path}) produced unexpected output when attempting to determine the version:",
   "_UnexpectedToolOutput.comment": "The actual command line output will be appended after this message. An example of {tool_name} is aria2. An example of {path} is /foo/bar.",
+  "UnexpectedWindowsArchitecture": "unexpected Windows host architecture: {actual}",
+  "_UnexpectedWindowsArchitecture.comment": "{actual} is the CPU kind we observed like ARM or MIPS",
   "UnknownBaselineFileContent": "unrecognizable baseline entry; expected 'port:triplet=(fail|skip|pass)'",
   "UnknownBinaryProviderType": "unknown binary provider type: valid providers are 'clear', 'default', 'nuget', 'nugetconfig','nugettimeout', 'interactive', 'x-azblob', 'x-gcs', 'x-aws', 'x-aws-config', 'http', and 'files'",
   "UnknownBooleanSetting": "unknown boolean setting for {option}: \"{value}\". Valid values are '', '1', '0', 'ON', 'OFF', 'TRUE', and 'FALSE'.",

src/vcpkg.cpp

@@ -233,6 +233,8 @@ int main(const int argc, const char* const* const argv)
     SetConsoleOutputCP(CP_UTF8);
 
     initialize_global_job_object();
+
+    reset_processor_architecture_environment_variable();
 #else
     static const char* const utf8_locales[] = {
         "C.UTF-8",

src/vcpkg/base/system.cpp

@@ -593,6 +593,28 @@ namespace vcpkg
 
         return std::move(maybe_k).error();
     }
+
+    void reset_processor_architecture_environment_variable()
+    {
+        // sometimes we get launched with incorrectly set %PROCESSOR_ARCHITECTURE%; this
+        // corrects that as we launch a lot of bits like CMake that expect it to be correctly set:
+        // https://cmake.org/cmake/help/latest/variable/CMAKE_HOST_SYSTEM_PROCESSOR.html#windows-platforms
+        const wchar_t* value;
+        const auto proc = get_host_processor();
+        switch (proc)
+        {
+            case CPUArchitecture::X86: value = L"X86"; break;
+            case CPUArchitecture::X64: value = L"AMD64"; break;
+            case CPUArchitecture::ARM: value = L"ARM"; break;
+            case CPUArchitecture::ARM64: value = L"ARM64"; break;
+            default:
+                Checks::msg_exit_with_error(
+                    VCPKG_LINE_INFO, msgUnexpectedWindowsArchitecture, msg::actual = to_zstring_view(proc));
+                break;
+        }
+
+        Checks::check_exit(VCPKG_LINE_INFO, SetEnvironmentVariableW(L"PROCESSOR_ARCHITECTURE", value) != 0);
+    }
 #endif
 
     static const Optional<Path>& get_program_files()