Invert the priority order of the input and the rust-toolchain file

1. Not doing so would be a breaking change for users who have
a rust-toolchain file with a different version from what they
specify in the input.
2. Not doing so makes it overly complicated to install a specific
version.
Imagine you want to test your software against the version in the
rust-toolchain file and nightly as-well. Without this patch, users
will have the replace the content of the rust-toolchain file because
it is prioritized over the input.
It makes a lot more sense to prioritize the input over the file
because that one is easier changed.

CHANGELOG.md

@@ -4,6 +4,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+- Support for the `rust-toolchain` file: If the toolchain input is not given, we will try and install the version specified in the `rust-toolchain` file.
+
 ## [1.0.3] - 2019-10-19
 
 ### Added

__tests__/args.test.ts

@@ -40,6 +40,16 @@ describe('actions-rs/toolchain', () => {
             'INPUT_TOOLCHAIN': 'nightly',
         });
 
+        expect(args.name).toBe("nightly")
+    });
+
+    it('uses rust-toolchain file if input does not exist', function () {
+        let rustToolchainFile = tempWriteSync("1.39.0");
+
+        let args = morph(() => {
+            return toolchain_args(rustToolchainFile);
+        }, {});
+
         expect(args.name).toBe("1.39.0")
     });
 
@@ -48,9 +58,7 @@ describe('actions-rs/toolchain', () => {
 
         let args = morph(() => {
             return toolchain_args(rustToolchainFile);
-        }, {
-            'INPUT_TOOLCHAIN': 'nightly',
-        });
+        }, {});
 
         expect(args.name).toBe("1.39.0")
     });

action.yml

@@ -10,6 +10,7 @@ inputs:
       Rust toolchain name.
 
       See https://github.com/rust-lang/rustup.rs#toolchain-specification
+    required: false
   target:
     description: Target triple to install for this toolchain
     required: false

src/args.ts

@@ -28,15 +28,24 @@ export function toolchain_args(overrideFile: string): ToolchainOptions {
 }
 
 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})
+
+    const toolchainInput = input.getInput('toolchain', {required: false});
+
+    if (toolchainInput) {
+        debug(`using toolchain from input: ${toolchainInput}`);
+        return toolchainInput
     }
+
+    if (!existsSync(overrideFile)) {
+        throw new Error("toolchain input was not given and repository does not have a rust-toolchain file")
+    }
+
+    const rustToolchainFile = readFileSync(overrideFile, {
+        encoding: "utf-8",
+        flag: "r"
+    }).trim();
+
+    debug(`using toolchain from rust-toolchain file: ${rustToolchainFile}`);
+
+    return rustToolchainFile;
 }