Inital work on Rust support (#333)

* Inital work on Rust support

* update accepted runtimes

* fn: fix rust build

* Last tweaks and README.md

* fix typo

* update comment on rust.go

fn/common.go

@@ -235,6 +235,7 @@ var acceptableFnRuntimes = map[string]string{
 	"python":    "iron/python:2",
 	"ruby":      "iron/ruby",
 	"scala":     "iron/scala",
+	"rust":      "corey/rust-alpine",
 }
 
 const tplDockerfile = `FROM {{ .BaseImage }}

fn/init.go

@@ -27,6 +27,7 @@ var (
 		".js": "node",
 		".rb": "ruby",
 		".py": "python",
+		".rs": "rust",
 	}
 
 	fnInitRuntimes []string

fn/langs/base.go

@@ -13,6 +13,8 @@ func GetLangHelper(lang string) (LangHelper, error) {
 		return &RubyLangHelper{}, nil
 	case "python":
 		return &PythonHelper{}, nil
+	case "rust":
+		return &RustLangHelper{}, nil
 	}
 	return nil, fmt.Errorf("No language helper found for %v", lang)
 }

fn/langs/rust.go

@@ -0,0 +1,42 @@
+package langs
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+type RustLangHelper struct{}
+
+func (lh *RustLangHelper) Entrypoint() string {
+	return "/function/target/release/func"
+}
+
+func (lh *RustLangHelper) HasPreBuild() bool {
+	return true
+}
+
+// PreBuild for rust builds the binary so the final image can be as small as possible
+func (lh *RustLangHelper) PreBuild() error {
+	wd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+
+	cmd := exec.Command(
+		"docker", "run",
+		"--rm", "-v",
+		wd+":/app", "-w", "/app", "corey/rust-alpine",
+		"/bin/sh", "-c", "cargo build --release",
+	)
+	cmd.Stderr = os.Stderr
+	cmd.Stdout = os.Stdout
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("error running docker build: %v", err)
+	}
+	return nil
+}
+
+func (lh *RustLangHelper) AfterBuild() error {
+	return os.RemoveAll("target")
+}