compiler: implement recover() built-in function

src/internal/task/task.go

@@ -20,6 +20,22 @@ type Task struct {
 
 	// state is the underlying running state of the task.
 	state state
+
+	// DeferFrame stores a pointer to the (stack allocated) defer frame of the
+	// goroutine that is used for the recover builtin.
+	DeferFrame *DeferFrame
+}
+
+// DeferFrame is a stack allocated object that stores information for the
+// current "defer frame", which is used in functions that use the `defer`
+// keyword.
+// The compiler knows the JumpPC struct offset.
+type DeferFrame struct {
+	JumpSP     unsafe.Pointer // stack pointer to return to
+	JumpPC     unsafe.Pointer // pc to return to
+	Previous   *DeferFrame    // previous recover buffer pointer
+	Panicking  bool           // true iff this defer frame is panicking
+	PanicValue interface{}    // panic value, might be nil for panic(nil) for example
 }
 
 // getGoroutineStackSize is a compiler intrinsic that returns the stack size for

src/internal/task/task_none.go

@@ -4,6 +4,9 @@ package task
 
 import "unsafe"
 
+// There is only one goroutine so the task struct can be a global.
+var mainTask Task
+
 //go:linkname runtimePanic runtime.runtimePanic
 func runtimePanic(str string)
 
@@ -12,8 +15,8 @@ func Pause() {
 }
 
 func Current() *Task {
-	runtimePanic("scheduler is disabled")
-	return nil
+	// Return a task struct, which is used for the recover builtin for example.
+	return &mainTask
 }
 
 //go:noinline