Initial batch file hack

Again, this is a squashed commit with around three hours of attempts
to write something working. See comments in the file for more details.
I'm too lazy to document this stuff properly.

index.js

@@ -1,6 +1,100 @@
 const core = require('@actions/core')
+const execFile = require('util').promisify(require('child_process').execFile)
+const fs = require('fs').promises
+const os = require('os')
+const path = require('path')
+const process = require('process')
+
+const MSVS_2017 = '%ProgramFiles(x86)%\\Microsoft Visual Studio\\2017\\Enterprise\\VC\\Auxiliary\\Build'
+const MSVS_2019 = '%ProgramFiles(x86)%\\Microsoft Visual Studio\\2019\\Enterprise\\VC\\Auxiliary\\Build'
+
+const InterestingVariables = [
+    'INCLUDE',
+    'LIB',
+    'LIBPATH',
+    'Path',
+    'Platform',
+    /^VCTools/,
+    /^VSCMD_/,
+    /^WindowsSDK/i,
+]
 
 async function main() {
+    if (process.platform != 'win32') {
+        core.info('This is not a Windows virtual environment, bye!')
+        return
     }
 
-main().catch(() => core.setFailed('could not setup Developer Command Prompt'))
+    const arch    = core.getInput('arch')
+    const sdk     = core.getInput('sdk')
+    const toolset = core.getInput('toolset')
+    const uwp     = core.getInput('uwp')
+    const spectre = core.getInput('spectre')
+
+    // Due to the way Microsoft Visual C++ is configured, we have to resort to the following hack:
+    // write a helper batch file which calls the configuration batch file and then outputs the
+    // configured environment variables, which we then pass to GitHub Actions.
+
+    const helper = path.join(os.homedir(), 'msvc-dev-cmd.bat')
+
+    var args = [arch]
+    if (uwp == 'true') {
+        args.push('uwp')
+    }
+    if (sdk) {
+        args.push(sdk)
+    }
+    if (toolset) {
+        args.push(`-vcvars_ver=${toolset}`)
+    }
+    if (spectre == 'true') {
+        args.push('-vcvars_spectre_libs=spectre')
+    }
+    core.debug(`Arguments: ${args.join(' ')}`)
+
+    core.debug(`Writing helper file: ${helper}`)
+    await fs.writeFile(helper, `
+        @IF EXIST "${MSVS_2017}\\vcvarsall.bat" GOTO :2017
+        @IF EXIST "${MSVS_2019}\\vcvarsall.bat" GOTO :2019
+        @ECHO "Microsoft Visual Studio not found"
+        @EXIT 1
+        :2017
+        @CALL "${MSVS_2017}\\vcvarsall.bat" ${args.join(' ')}
+        @GOTO ENV
+        :2019
+        @CALL "${MSVS_2019}\\vcvarsall.bat" ${args.join(' ')}
+        @GOTO ENV
+        :ENV
+        @IF ERRORLEVEL 1 EXIT
+        @SET
+    `)
+
+    var environment
+    try {
+        core.debug('Executing helper')
+        const { stdout } = await execFile('cmd.exe', ['/q', '/c', helper])
+        environment = stdout.split('\r\n')
+    }
+    catch (error) {
+        core.debug(`Helper failed: ${error.message}`)
+        throw error
+    }
+    finally {
+        core.debug('Removing helper')
+        await fs.unlink(helper)
+    }
+
+    for (string of environment) {
+        const [name, value] = string.split('=')
+        for (pattern of InterestingVariables) {
+            if (name.match(pattern)) {
+                core.exportVariable(name, value)
+                break
+            }
+        }
+    }
+
+    core.info(`Configured Developer Command Prompt`)
+}
+
+main().catch(() => core.setFailed('Could not setup Developer Command Prompt'))