main: calculate default output path if -o is not specified

This matches the Go command and is generally convenient to have.

compileopts/config.go

@@ -245,6 +245,29 @@ func (c *Config) LibcPath(name string) (path string, precompiled bool) {
 	return filepath.Join(goenv.Get("GOCACHE"), name+"-"+archname), false
 }
 
+// DefaultBinaryExtension returns the default extension for binaries, such as
+// .exe, .wasm, or no extension (depending on the target).
+func (c *Config) DefaultBinaryExtension() string {
+	parts := strings.Split(c.Triple(), "-")
+	if parts[0] == "wasm32" {
+		// WebAssembly files always have the .wasm file extension.
+		return ".wasm"
+	}
+	if len(parts) >= 3 && parts[2] == "windows" {
+		// Windows uses .exe.
+		return ".exe"
+	}
+	if len(parts) >= 3 && parts[2] == "unknown" {
+		// There appears to be a convention to use the .elf file extension for
+		// ELF files intended for microcontrollers. I'm not aware of the origin
+		// of this, it's just something that is used by many projects.
+		// I think it's a good tradition, so let's keep it.
+		return ".elf"
+	}
+	// Linux, MacOS, etc, don't use a file extension. Use it as a fallback.
+	return ""
+}
+
 // CFlags returns the flags to pass to the C compiler. This is necessary for CGo
 // preprocessing.
 func (c *Config) CFlags() []string {

main.go

@@ -149,6 +149,17 @@ func Build(pkgName, outpath string, options *compileopts.Options) error {
 	}
 
 	return builder.Build(pkgName, outpath, config, func(result builder.BuildResult) error {
+		if outpath == "" {
+			if strings.HasSuffix(pkgName, ".go") {
+				// A Go file was specified directly on the command line.
+				// Base the binary name off of it.
+				outpath = filepath.Base(pkgName[:len(pkgName)-3]) + config.DefaultBinaryExtension()
+			} else {
+				// Pick a default output path based on the main directory.
+				outpath = filepath.Base(result.MainDir) + config.DefaultBinaryExtension()
+			}
+		}
+
 		if err := os.Rename(result.Binary, outpath); err != nil {
 			// Moving failed. Do a file copy.
 			inf, err := os.Open(result.Binary)
@@ -1319,11 +1330,6 @@ func main() {
 
 	switch command {
 	case "build":
-		if outpath == "" {
-			fmt.Fprintln(os.Stderr, "No output filename supplied (-o).")
-			usage(command)
-			os.Exit(1)
-		}
 		pkgName := "."
 		if flag.NArg() == 1 {
 			pkgName = filepath.ToSlash(flag.Arg(0))