go/doc: avoid panic on references to functions with no body

This change guards a call to ast.Inspect with a nil check on the first
argument. This avoids a panic when inspecting a reference to a function
with a nil body. This can only happen when a function body is defined outside Go.

Fixes #42706

Change-Id: I91bc607b24b6224920c24cfd07e76ce7737a98d4
GitHub-Last-Rev: 08072b9ce5
GitHub-Pull-Request: golang/go#43011
Reviewed-on: https://go-review.googlesource.com/c/go/+/275516
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/doc/example.go

@@ -237,7 +237,10 @@ func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
 				}
 			}
 
-			ast.Inspect(d.Body, inspectFunc)
+			// Functions might not have a body. See #42706.
+			if d.Body != nil {
+				ast.Inspect(d.Body, inspectFunc)
+			}
 		case *ast.GenDecl:
 			for _, spec := range d.Specs {
 				switch s := spec.(type) {

src/go/doc/example_test.go

@@ -352,6 +352,25 @@ func main() {
 }
 `
 
+const exampleWholeFileExternalFunction = `package foo_test
+
+func foo(int)
+
+func Example() {
+	foo(42)
+	// Output:
+}
+`
+
+const exampleWholeFileExternalFunctionOutput = `package main
+
+func foo(int)
+
+func main() {
+	foo(42)
+}
+`
+
 var exampleWholeFileTestCases = []struct {
 	Title, Source, Play, Output string
 }{
@@ -367,6 +386,12 @@ var exampleWholeFileTestCases = []struct {
 		exampleWholeFileFunctionOutput,
 		"Hello, world!\n",
 	},
+	{
+		"ExternalFunction",
+		exampleWholeFileExternalFunction,
+		exampleWholeFileExternalFunctionOutput,
+		"",
+	},
 }
 
 func TestExamplesWholeFile(t *testing.T) {