Draft initial implementation

src/main.ts

@@ -1,9 +1,14 @@
 import * as core from '@actions/core';
+import * as rustup from './rustup';
+import * as os from 'os';
 
 async function run() {
   try {
-    const myInput = core.getInput('myInput');
-    core.debug(`Hello ${myInput}`);
+    const version = core.getInput('rust-version');
+
+    if(version && os.platform() != 'win32') {
+      await rustup.install(version);
+    }
   } catch (error) {
     core.setFailed(error.message);
   }

src/rustup.ts

@@ -0,0 +1,79 @@
+import * as core from '@actions/core';
+import * as exec from '@actions/exec';
+import * as io from '@actions/io';
+import * as toolCache from '@actions/tool-cache';
+import * as path from 'path';
+import * as os from 'os';
+import {chmodSync} from 'fs';
+
+let osPlat: string = os.platform();
+let osArch: string = os.arch();
+let tempDirectory = process.env['RUNNER_TEMPDIRECTORY'] || '';
+const WINDOWS: boolean = osPlat == 'win32';
+
+export async function install(version: string) {
+  let toolPath = toolCache.find('rust', version);
+
+  if (!toolPath) {
+    toolPath = await acquireRust(version);
+    core.debug('Rust toolchain is cached under ' + toolPath);
+  }
+
+  toolPath = path.join(toolPath, 'bin');
+  core.addPath(toolPath);
+
+  await exec.exec('rustup', ['update']);
+}
+
+async function acquireRust(version: string): Promise<string> {
+  let script: string;
+
+  try {
+    script = await toolCache.downloadTool(rustupUrl());
+  } catch (error) {
+    core.debug(error);
+    throw `Failed to download rustup: ${error}`;
+  }
+
+  if(WINDOWS) {
+    // TODO: Fix this
+    // It currently fails with:
+    // error: 'rustup-init.exe' is not installed for the toolchain 'stable-x86_64-pc-windows-msvc'
+    await io.cp(script, script + '.exe');
+    script += '.exe';
+
+    const powershell = await io.which('powershell', true);
+
+    await exec.exec(
+      `"${script}"`,
+      [
+        '--default-host',
+        'gnu',
+        '--default-toolchain',
+        version,
+      ]
+    );
+  } else {
+    chmodSync(script, '777');
+
+    await exec.exec(`"${script}"`, ['-y', '--default-toolchain', version]);
+  }
+
+  return await toolCache.cacheDir(binRoot(), 'rust', version);
+}
+
+function rustupUrl(): string {
+  if(WINDOWS) {
+    return "https://static.rust-lang.org/rustup/dist/x86_64-pc-windows-gnu/rustup-init.exe"
+  } else {
+    return "https://sh.rustup.rs"
+  }
+}
+
+function binRoot(): string {
+  if(WINDOWS) {
+    return path.join(process.env['USERPROFILE'] || '', '.cargo');
+  } else {
+    return path.join(process.env['HOME'] || '', '.cargo');
+  }
+}