Allow to override toolchain from rust-toolchain file (#35)

src/args.ts

@@ -1,5 +1,6 @@
-import * as core from '@actions/core';
 import {input} from '@actions-rs/core';
+import {info, debug} from "@actions/core";
+import {existsSync, readFileSync} from 'fs';
 
 export interface ToolchainOptions {
     name: string,
@@ -10,13 +11,14 @@ export interface ToolchainOptions {
     components: string[] | undefined,
 }
 
-export function toolchain_args(): ToolchainOptions {
+export function toolchain_args(overrideFile: string): ToolchainOptions {
     let components: string[] | undefined = input.getInputList('components');
     if (components && components.length === 0) {
         components = undefined;
     }
+
     return {
-        name: input.getInput('toolchain', {required: true}),
+        name: determineToolchain(overrideFile),
         target: input.getInput('target') || undefined,
         default: input.getInputBool('default'),
         override: input.getInputBool('override'),
@@ -24,3 +26,17 @@ export function toolchain_args(): ToolchainOptions {
         components: components,
     };
 }
+
+function determineToolchain(overrideFile: string): string {
+    if (existsSync(overrideFile)) {
+        debug(`using toolchain override from ${overrideFile}`);
+        const content = readFileSync(overrideFile, {
+            encoding: "utf-8",
+            flag: "r"
+        });
+        return content.trim();
+    } else {
+        debug(`toolchain override file ${overrideFile} does not exist, falling back to input variable`);
+        return input.getInput('toolchain', {required: true})
+    }
+}

src/main.ts

@@ -1,12 +1,16 @@
 import * as core from '@actions/core';
 import * as exec from '@actions/exec';
 import * as io from '@actions/io';
+import path from "path";
 
 import * as args from './args';
 import {RustUp, ToolchainOptions} from '@actions-rs/core';
 
 async function run() {
-    const opts = args.toolchain_args();
+    // we use path.join to make sure this works on Windows, Linux and MacOS
+    let toolchainOverrideFile = path.join(process.cwd(), "rust-toolchain");
+
+    const opts = args.toolchain_args(toolchainOverrideFile);
     const rustup = await RustUp.getOrInstall();
     await rustup.call(['show']);