math/big: add test that linker is able to remove unused code

(Follow-up to CL 228108.)

Change-Id: Ia6d119ee19c7aa923cdeead06d3cee87a1751105
Reviewed-on: https://go-review.googlesource.com/c/go/+/228109
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/link_test.go

@@ -0,0 +1,62 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package big
+
+import (
+	"bytes"
+	"internal/testenv"
+	"io/ioutil"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+// Tests that the linker is able to remove references to Float, Rat,
+// and Int if unused (notably, not used by init).
+func TestLinkerGC(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in short mode")
+	}
+	t.Parallel()
+	goBin := testenv.GoToolPath(t)
+	goFile := filepath.Join(t.TempDir(), "x.go")
+	file := []byte(`package main
+import _ "math/big"
+func main() {}
+`)
+	if err := ioutil.WriteFile(goFile, file, 0644); err != nil {
+		t.Fatal(err)
+	}
+	cmd := exec.Command(goBin, "build", "-o", "x.exe", "x.go")
+	cmd.Dir = t.TempDir()
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("compile: %v, %s", err, out)
+	}
+
+	cmd = exec.Command(goBin, "tool", "nm", "x.exe")
+	cmd.Dir = t.TempDir()
+	nm, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("nm: %v, %s", err, nm)
+	}
+	const want = "runtime.(*Frames).Next"
+	if !bytes.Contains(nm, []byte(want)) {
+		// Test the test.
+		t.Errorf("expected symbol %q not found", want)
+	}
+	bad := []string{
+		"math/big.(*Float)",
+		"math/big.(*Rat)",
+		"math/big.(*Int)",
+	}
+	for _, sym := range bad {
+		if bytes.Contains(nm, []byte(sym)) {
+			t.Errorf("unexpected symbol %q found", sym)
+		}
+	}
+	if t.Failed() {
+		t.Logf("Got: %s", nm)
+	}
+}