diff --git a/go/buildutil/allpackages.go b/go/buildutil/allpackages.go
index bee5fc17..4d4366c1 100644
--- a/go/buildutil/allpackages.go
+++ b/go/buildutil/allpackages.go
@@ -123,3 +123,73 @@ func allPackages(ctxt *build.Context, root string, ch chan<- item) {
 	walkDir(root)
 	wg.Wait()
 }
+
+// ExpandPatterns returns the set of packages matched by patterns,
+// which may have the following forms:
+//
+//		golang.org/x/tools/cmd/guru     # a single package
+//		golang.org/x/tools/...          # all packages beneath dir
+//		...                             # the entire workspace.
+//
+// Order is significant: a pattern preceded by '-' removes matching
+// packages from the set.  For example, these patterns match all encoding
+// packages except encoding/xml:
+//
+// 	encoding/... -encoding/xml
+//
+func ExpandPatterns(ctxt *build.Context, patterns []string) map[string]bool {
+	// TODO(adonovan): support other features of 'go list':
+	// - "std"/"cmd"/"all" meta-packages
+	// - "..." not at the end of a pattern
+	// - relative patterns using "./" or "../" prefix
+
+	pkgs := make(map[string]bool)
+	doPkg := func(pkg string, neg bool) {
+		if neg {
+			delete(pkgs, pkg)
+		} else {
+			pkgs[pkg] = true
+		}
+	}
+
+	// Scan entire workspace if wildcards are present.
+	// TODO(adonovan): opt: scan only the necessary subtrees of the workspace.
+	var all []string
+	for _, arg := range patterns {
+		if strings.HasSuffix(arg, "...") {
+			all = AllPackages(ctxt)
+			break
+		}
+	}
+
+	for _, arg := range patterns {
+		if arg == "" {
+			continue
+		}
+
+		neg := arg[0] == '-'
+		if neg {
+			arg = arg[1:]
+		}
+
+		if arg == "..." {
+			// ... matches all packages
+			for _, pkg := range all {
+				doPkg(pkg, neg)
+			}
+		} else if dir := strings.TrimSuffix(arg, "/..."); dir != arg {
+			// dir/... matches all packages beneath dir
+			for _, pkg := range all {
+				if strings.HasPrefix(pkg, dir) &&
+					(len(pkg) == len(dir) || pkg[len(dir)] == '/') {
+					doPkg(pkg, neg)
+				}
+			}
+		} else {
+			// single package
+			doPkg(arg, neg)
+		}
+	}
+
+	return pkgs
+}
diff --git a/go/buildutil/allpackages_test.go b/go/buildutil/allpackages_test.go
index d5bd964b..96bf77a6 100644
--- a/go/buildutil/allpackages_test.go
+++ b/go/buildutil/allpackages_test.go
@@ -10,6 +10,8 @@ package buildutil_test
 
 import (
 	"go/build"
+	"sort"
+	"strings"
 	"testing"
 
 	"golang.org/x/tools/go/buildutil"
@@ -34,3 +36,41 @@ func TestAllPackages(t *testing.T) {
 		}
 	}
 }
+
+func TestExpandPatterns(t *testing.T) {
+	tree := make(map[string]map[string]string)
+	for _, pkg := range []string{
+		"encoding",
+		"encoding/xml",
+		"encoding/hex",
+		"encoding/json",
+		"fmt",
+	} {
+		tree[pkg] = make(map[string]string)
+	}
+	ctxt := buildutil.FakeContext(tree)
+
+	for _, test := range []struct {
+		patterns string
+		want     string
+	}{
+		{"", ""},
+		{"fmt", "fmt"},
+		{"nosuchpkg", "nosuchpkg"},
+		{"nosuchdir/...", ""},
+		{"...", "encoding encoding/hex encoding/json encoding/xml fmt"},
+		{"encoding/... -encoding/xml", "encoding encoding/hex encoding/json"},
+		{"... -encoding/...", "fmt"},
+	} {
+		var pkgs []string
+		for pkg := range buildutil.ExpandPatterns(ctxt, strings.Fields(test.patterns)) {
+			pkgs = append(pkgs, pkg)
+		}
+		sort.Strings(pkgs)
+		got := strings.Join(pkgs, " ")
+		if got != test.want {
+			t.Errorf("ExpandPatterns(%s) = %s, want %s",
+				test.patterns, got, test.want)
+		}
+	}
+}