starlark: add debug API for Locals and FreeVars (#539)

This change adds the following new API to allow debugging tools
and built-in functions access to the internals of Function values
and call frames:

package starlark

type Binding struct {
       Name string
       Pos  syntax.Position
}

func (fr *frame) NumLocals() int
func (fr *frame) Local(i int) (Binding, Value)

type DebugFrame interface {
    ...
    NumLocals() int
    Local(i int) (Binding, Value)
}

func (fn *Function) NumFreeVars() int
func (fn *Function) FreeVar(i int) (Binding, Value)

This is strictly a breaking change, but the changed functions
(the Local methods) were previously documented as experimental.
The fix is straightforward.

Also, a test of DebugFrame to write an 'env' function in
a similar vein to Python's 'dir' function.

Fixes #538
diff --git a/internal/compile/compile.go b/internal/compile/compile.go
index ecf689f..b257d70 100644
--- a/internal/compile/compile.go
+++ b/internal/compile/compile.go
@@ -335,7 +335,7 @@
 	pclinetab             []uint16        // mapping from pc to linenum
 	Locals                []Binding       // locals, parameters first
 	Cells                 []int           // indices of Locals that require cells
-	Freevars              []Binding       // for tracing
+	FreeVars              []Binding       // for tracing
 	MaxStack              int
 	NumParams             int
 	NumKwonlyParams       int
@@ -520,7 +520,7 @@
 			Name:     name,
 			Doc:      docStringFromBody(stmts),
 			Locals:   bindings(locals),
-			Freevars: bindings(freevars),
+			FreeVars: bindings(freevars),
 		},
 	}
 
@@ -887,7 +887,7 @@
 	case ATTR, SETFIELD, PREDECLARED, UNIVERSAL:
 		comment = fn.Prog.Names[arg]
 	case FREE:
-		comment = fn.Freevars[arg].Name
+		comment = fn.FreeVars[arg].Name
 	case CALL, CALL_VAR, CALL_KW, CALL_VAR_KW:
 		comment = fmt.Sprintf("%d pos, %d named", arg>>8, arg&0xff)
 	default:
diff --git a/internal/compile/serial.go b/internal/compile/serial.go
index 4d71738..0dbae47 100644
--- a/internal/compile/serial.go
+++ b/internal/compile/serial.go
@@ -195,7 +195,7 @@
 	for _, index := range fn.Cells {
 		e.int(index)
 	}
-	e.bindings(fn.Freevars)
+	e.bindings(fn.FreeVars)
 	e.int(fn.MaxStack)
 	e.int(fn.NumParams)
 	e.int(fn.NumKwonlyParams)
@@ -389,7 +389,7 @@
 		pclinetab:       pclinetab,
 		Locals:          locals,
 		Cells:           cells,
-		Freevars:        freevars,
+		FreeVars:        freevars,
 		MaxStack:        maxStack,
 		NumParams:       numParams,
 		NumKwonlyParams: numKwonlyParams,
diff --git a/starlark/debug.go b/starlark/debug.go
index 22a2124..bbb37b5 100644
--- a/starlark/debug.go
+++ b/starlark/debug.go
@@ -1,41 +1,59 @@
 package starlark
 
-import "go.starlark.net/syntax"
+import (
+	"go.starlark.net/syntax"
+)
 
 // This file defines an experimental API for the debugging tools.
 // Some of these declarations expose details of internal packages.
 // (The debugger makes liberal use of exported fields of unexported types.)
 // Breaking changes may occur without notice.
 
-// Local returns the value of the i'th local variable.
-// It may be nil if not yet assigned.
+// A Binding is the name and position of a binding identifier.
+type Binding struct {
+	Name string
+	Pos  syntax.Position
+}
+
+// NumLocals returns the number of local variables of this frame.
+// It is zero unless fr.Callable() is a *Function.
+func (fr *frame) NumLocals() int { return len(fr.locals) }
+
+// Local returns the binding (name and binding position) and value of
+// the i'th local variable of the frame's function.
+// Beware: the value may be nil if it has not yet been assigned!
 //
-// Local may be called only for frames whose Callable is a *Function (a
-// function defined by Starlark source code), and only while the frame
-// is active; it will panic otherwise.
+// The index i must be less than [NumLocals].
+// Local may be called only while the frame is active.
 //
 // This function is provided only for debugging tools.
-//
-// THIS API IS EXPERIMENTAL AND MAY CHANGE WITHOUT NOTICE.
-func (fr *frame) Local(i int) Value { return fr.locals[i] }
+func (fr *frame) Local(i int) (Binding, Value) {
+	return Binding(fr.callable.(*Function).funcode.Locals[i]), fr.locals[i]
+}
 
 // DebugFrame is the debugger API for a frame of the interpreter's call stack.
 //
 // Most applications have no need for this API; use CallFrame instead.
 //
+// It may be tempting to use this interface when implementing built-in
+// functions. Beware that reflection over the call stack is easily
+// abused, leading to built-in functions whose behavior is mysterious
+// and unpredictable.
+//
 // Clients must not retain a DebugFrame nor call any of its methods once
 // the current built-in call has returned or execution has resumed
 // after a breakpoint as this may have unpredictable effects, including
 // but not limited to retention of object that would otherwise be garbage.
 type DebugFrame interface {
-	Callable() Callable        // returns the frame's function
-	Local(i int) Value         // returns the value of the (Starlark) frame's ith local variable
-	Position() syntax.Position // returns the current position of execution in this frame
+	Callable() Callable           // returns the frame's function
+	NumLocals() int               // returns the number of local variables in this frame
+	Local(i int) (Binding, Value) // returns the binding and value of the (Starlark) frame's ith local variable
+	Position() syntax.Position    // returns the current position of execution in this frame
 }
 
 // DebugFrame returns the debugger interface for
 // the specified frame of the interpreter's call stack.
-// Frame numbering is as for Thread.CallFrame.
+// Frame numbering is as for Thread.CallFrame: 0 <= depth < thread.CallStackDepth().
 //
 // This function is intended for use in debugging tools.
 // Most applications should have no need for it; use CallFrame instead.
diff --git a/starlark/eval_test.go b/starlark/eval_test.go
index 6678671..3bf3592 100644
--- a/starlark/eval_test.go
+++ b/starlark/eval_test.go
@@ -824,7 +824,8 @@
 						buf.WriteString(", ")
 					}
 					name, _ := fn.Param(i)
-					fmt.Fprintf(buf, "%s=%s", name, fr.Local(i))
+					_, v := fr.Local(i)
+					fmt.Fprintf(buf, "%s=%s", name, v)
 				}
 			} else {
 				buf.WriteString("...") // a built-in function
@@ -1056,3 +1057,55 @@
 		}()
 	}
 }
+
+func TestDebugFrame(t *testing.T) {
+	predeclared := starlark.StringDict{
+		"env": starlark.NewBuiltin("env", func(thread *starlark.Thread, b *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
+			if thread.CallStackDepth() < 2 {
+				return nil, fmt.Errorf("env must not be called directly")
+			}
+			fr := thread.DebugFrame(1) // parent
+			fn, ok := fr.Callable().(*starlark.Function)
+			if !ok {
+				return nil, fmt.Errorf("env must be called from a Starlark function")
+			}
+			dict := starlark.NewDict(0)
+			for i := 0; i < fr.NumLocals(); i++ {
+				bind, val := fr.Local(i)
+				if val == nil {
+					continue
+				}
+				dict.SetKey(starlark.String(bind.Name), val) // ignore error
+			}
+			for i := 0; i < fn.NumFreeVars(); i++ {
+				bind, val := fn.FreeVar(i)
+				dict.SetKey(starlark.String(bind.Name), val) // ignore error
+			}
+			dict.Freeze()
+			return dict, nil
+		}),
+	}
+	const src = `
+e = [None]
+
+def f(p):
+    outer = 3
+    def g(q):
+        inner = outer + 1
+        e[0] = env() # {"q": 2, "inner": 4, "outer": 3}
+	inner2 = None # not defined at call to env()
+    g(2)
+
+f(1)
+`
+	thread := new(starlark.Thread)
+	m, err := starlark.ExecFile(thread, "env.star", src, predeclared)
+	if err != nil {
+		t.Fatalf("ExecFile returned error %q, expected panic", err)
+	}
+	got := m["e"].(*starlark.List).Index(0).String()
+	want := `{"q": 2, "inner": 4, "outer": 3}`
+	if got != want {
+		t.Errorf("env() returned %s, want %s", got, want)
+	}
+}
diff --git a/starlark/interp.go b/starlark/interp.go
index d29e525..261077f 100644
--- a/starlark/interp.go
+++ b/starlark/interp.go
@@ -541,7 +541,7 @@
 		case compile.MAKEFUNC:
 			funcode := f.Prog.Functions[arg]
 			tuple := stack[sp-1].(Tuple)
-			n := len(tuple) - len(funcode.Freevars)
+			n := len(tuple) - len(funcode.FreeVars)
 			defaults := tuple[:n:n]
 			freevars := tuple[n:]
 			stack[sp-1] = &Function{
@@ -622,7 +622,7 @@
 		case compile.FREECELL:
 			v := fn.freevars[arg].(*cell).v
 			if v == nil {
-				err = fmt.Errorf("local variable %s referenced before assignment", f.Freevars[arg].Name)
+				err = fmt.Errorf("local variable %s referenced before assignment", f.FreeVars[arg].Name)
 				break loop
 			}
 			stack[sp] = v
diff --git a/starlark/value.go b/starlark/value.go
index f24a3c8..94e200a 100644
--- a/starlark/value.go
+++ b/starlark/value.go
@@ -775,6 +775,15 @@
 func (fn *Function) HasVarargs() bool { return fn.funcode.HasVarargs }
 func (fn *Function) HasKwargs() bool  { return fn.funcode.HasKwargs }
 
+// NumFreeVars returns the number of free variables of this function.
+func (fn *Function) NumFreeVars() int { return len(fn.funcode.FreeVars) }
+
+// FreeVar returns the binding (name and binding position) and value
+// of the i'th free variable of function fn.
+func (fn *Function) FreeVar(i int) (Binding, Value) {
+	return Binding(fn.funcode.FreeVars[i]), fn.freevars[i].(*cell).v
+}
+
 // A Builtin is a function implemented in Go.
 type Builtin struct {
 	name string