transform: show better error message in coroutines lowering

A common error is when someone tries to export a blocking function. This
is not possible with the coroutines scheduler. Previously, it resulted
in an error like this:

    panic: trying to make exported function async: messageHandler

With this change, the error is much better and shows where it comes from
exactly:

    /home/ayke/tmp/export-async.go:8: blocking operation in exported function: messageHandler

    traceback:
    messageHandler
            /home/ayke/tmp/export-async.go:9:5
    main.foo
            /home/ayke/tmp/export-async.go:15:2
    runtime.chanSend
            /home/ayke/src/github.com/tinygo-org/tinygo/src/runtime/chan.go:494:12

This should make it easier to identify and fix the problem. And it
avoids a compiler panic, which is a really bad way of showing
diagnostics.

main.go

@@ -25,6 +25,7 @@ import (
 	"github.com/tinygo-org/tinygo/goenv"
 	"github.com/tinygo-org/tinygo/interp"
 	"github.com/tinygo-org/tinygo/loader"
+	"github.com/tinygo-org/tinygo/transform"
 	"tinygo.org/x/go-llvm"
 
 	"go.bug.st/serial"
@@ -776,6 +777,15 @@ func printCompilerError(logln func(...interface{}), err error) {
 				logln()
 			}
 		}
+	case transform.CoroutinesError:
+		logln(err.Pos.String() + ": " + err.Msg)
+		logln("\ntraceback:")
+		for _, line := range err.Traceback {
+			logln(line.Name)
+			if line.Position.IsValid() {
+				logln("\t" + line.Position.String())
+			}
+		}
 	case loader.Errors:
 		logln("#", err.Pkg.ImportPath)
 		for _, err := range err.Errs {

transform/coroutines.go

@@ -5,6 +5,7 @@ package transform
 
 import (
 	"errors"
+	"go/token"
 	"strconv"
 
 	"github.com/tinygo-org/tinygo/compiler/llvmutil"
@@ -104,6 +105,31 @@ func LowerCoroutines(mod llvm.Module, needStackSlots bool) error {
 	return nil
 }
 
+// CoroutinesError is an error returned when coroutine lowering failed, for
+// example because an async function is exported.
+type CoroutinesError struct {
+	Msg       string
+	Pos       token.Position
+	Traceback []CoroutinesErrorLine
+}
+
+// CoroutinesErrorLine is a single line of a CoroutinesError traceback.
+type CoroutinesErrorLine struct {
+	Name     string         // function name
+	Position token.Position // position in the function
+}
+
+// Error implements the error interface by returning a simple error message
+// without the stack.
+func (err CoroutinesError) Error() string {
+	return err.Msg
+}
+
+type asyncCallInfo struct {
+	fn   llvm.Value
+	call llvm.Value
+}
+
 // asyncFunc is a metadata container for an asynchronous function.
 type asyncFunc struct {
 	// fn is the underlying function pointer.
@@ -168,10 +194,11 @@ type coroutineLoweringPass struct {
 
 // findAsyncFuncs finds all asynchronous functions.
 // A function is considered asynchronous if it calls an asynchronous function or intrinsic.
-func (c *coroutineLoweringPass) findAsyncFuncs() {
+func (c *coroutineLoweringPass) findAsyncFuncs() error {
 	asyncs := map[llvm.Value]*asyncFunc{}
 	asyncsOrdered := []llvm.Value{}
 	calls := []llvm.Value{}
+	callsAsyncFunction := map[llvm.Value]asyncCallInfo{}
 
 	// Use a breadth-first search to find all async functions.
 	worklist := []llvm.Value{c.pause}
@@ -183,7 +210,18 @@ func (c *coroutineLoweringPass) findAsyncFuncs() {
 		// Get task pointer argument.
 		task := fn.LastParam()
 		if fn != c.pause && (task.IsNil() || task.Name() != "parentHandle") {
-			panic("trying to make exported function async: " + fn.Name())
+			// Exported functions must not do async operations.
+			err := CoroutinesError{
+				Msg: "blocking operation in exported function: " + fn.Name(),
+				Pos: getPosition(fn),
+			}
+			f := fn
+			for !f.IsNil() && f != c.pause {
+				data := callsAsyncFunction[f]
+				err.Traceback = append(err.Traceback, CoroutinesErrorLine{f.Name(), getPosition(data.call)})
+				f = data.fn
+			}
+			return err
 		}
 
 		// Search all uses of the function while collecting callers.
@@ -218,6 +256,13 @@ func (c *coroutineLoweringPass) findAsyncFuncs() {
 			asyncs[caller] = nil
 			asyncsOrdered = append(asyncsOrdered, caller)
 
+			// Track which calls caused this function to be marked async (for
+			// better diagnostics).
+			callsAsyncFunction[caller] = asyncCallInfo{
+				fn:   fn,
+				call: user,
+			}
+
 			// Put the caller on the worklist.
 			worklist = append(worklist, caller)
 		}
@@ -243,6 +288,8 @@ func (c *coroutineLoweringPass) findAsyncFuncs() {
 	c.asyncFuncs = asyncs
 	c.asyncFuncsOrdered = asyncFuncsOrdered
 	c.calls = calls
+
+	return nil
 }
 
 func (c *coroutineLoweringPass) load() error {
@@ -302,7 +349,10 @@ func (c *coroutineLoweringPass) load() error {
 	}
 
 	// Find async functions.
-	c.findAsyncFuncs()
+	err := c.findAsyncFuncs()
+	if err != nil {
+		return err
+	}
 
 	// Get i8* type.
 	c.i8ptr = llvm.PointerType(c.ctx.Int8Type(), 0)