Print descriptive error message on startup when TinyGo flags are not …

…set (#12)

finalizer.go

@@ -1,7 +1,7 @@
 // Copyright wasilibs authors
 // SPDX-License-Identifier: MIT
 
-//go:build nottinygc_finalizer
+//go:build gc.custom && nottinygc_finalizer
 
 package nottinygc
 

gc.go

@@ -1,6 +1,8 @@
 // Copyright wasilibs authors
 // SPDX-License-Identifier: MIT
 
+//go:build gc.custom
+
 package nottinygc
 
 import (

gc_notcustom.go

@@ -0,0 +1,10 @@
+// Copyright wasilibs authors
+// SPDX-License-Identifier: MIT
+
+//go:build !gc.custom
+
+package nottinygc
+
+func init() {
+	panic("nottinygc requires passing -gc=custom and -tags=custommalloc to TinyGo when compiling.\nhttps://github.com/wasilibs/nottinygc#usage")
+}

magefiles/magefile.go

@@ -4,6 +4,7 @@
 package main
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"io"
@@ -27,7 +28,19 @@ func Test() error {
 		tags = append(tags, "nottinygc_finalizer")
 	}
 
-	return sh.RunV("tinygo", "test", "-gc=custom", fmt.Sprintf("-tags='%s'", strings.Join(tags, " ")), "-target=wasi", "-v", "-scheduler=none", "./...")
+	if err := sh.RunV("tinygo", "test", "-gc=custom", fmt.Sprintf("-tags='%s'", strings.Join(tags, " ")), "-target=wasi", "-v", "-scheduler=none", "./..."); err != nil {
+		return err
+	}
+
+	var stdout bytes.Buffer
+	if _, err := sh.Exec(map[string]string{}, &stdout, io.Discard, "tinygo", "test", "-target=wasi", "-v", "-scheduler=none", "./..."); err == nil {
+		return errors.New("expected tinygo test to fail without -gc=custom")
+	}
+	if s := stdout.String(); !strings.Contains(s, "nottinygc requires passing -gc=custom and -tags=custommalloc to TinyGo when compiling") {
+		return fmt.Errorf("unexpected error message: %s", s)
+	}
+
+	return nil
 }
 
 func Format() error {