Adds allocation examples in Rust and TinyGo (#475)

Signed-off-by: Adrian Cole <adrian@tetrate.io>
Co-authored-by: Takeshi Yoneda <takeshi@tetrate.io>

examples/allocation/tinygo/testdata/greet.go

@@ -0,0 +1,75 @@
+package main
+
+import (
+	"fmt"
+	"reflect"
+	"unsafe"
+)
+
+// main is required for TinyGo to compile to Wasm.
+func main() {}
+
+// greet prints a greeting to the console.
+func greet(name string) {
+	log(fmt.Sprint("wasm >> ", greeting(name)))
+}
+
+// log a message to the console using _log.
+func log(message string) {
+	ptr, size := stringToPtr(message)
+	_log(ptr, size)
+}
+
+// _log is a WebAssembly import which prints a string (linear memory offset,
+// byteCount) to the console.
+//
+// Note: In TinyGo "//export" on a func is actually an import!
+//go:wasm-module env
+//export log
+func _log(ptr uint32, size uint32)
+
+// greeting gets a greeting for the name.
+func greeting(name string) string {
+	return fmt.Sprint("Hello, ", name, "!")
+}
+
+// _greet is a WebAssembly export that accepts a string pointer (linear
+// memory offset) and calls greet.
+//export greet
+func _greet(ptr, size uint32) {
+	name := ptrToString(ptr, size)
+	greet(name)
+}
+
+// _greet is a WebAssembly export that accepts a string pointer (linear
+// memory offset) and returns a pointer/size pair packed into a uint64.
+//
+// Note: This uses a uint64 instead of two result values for compatibility
+// with WebAssembly 1.0.
+//export greeting
+func _greeting(ptr, size uint32) (ptrSize uint64) {
+	name := ptrToString(ptr, size)
+	g := greeting(name)
+	ptr, size = stringToPtr(g)
+	return (uint64(ptr) << uint64(32)) | uint64(size)
+}
+
+// ptrToString returns a string from WebAssembly compatible numeric
+// types representing its pointer and length.
+func ptrToString(ptr uint32, size uint32) (ret string) {
+	// Here, we want to get a string represented by the ptr and size. If we
+	// wanted a []byte, we'd use reflect.SliceHeader instead.
+	strHdr := (*reflect.StringHeader)(unsafe.Pointer(&ret))
+	strHdr.Data = uintptr(ptr)
+	strHdr.Len = uintptr(size)
+	return
+}
+
+// stringToPtr returns a pointer and size pair for the given string
+// in a way that is compatible with WebAssembly numeric types.
+func stringToPtr(s string) (uint32, uint32) {
+	buf := []byte(s)
+	ptr := &buf[0]
+	unsafePtr := uintptr(unsafe.Pointer(ptr))
+	return uint32(unsafePtr), uint32(len(buf))
+}