go/buildutil: ExpandPatterns: ignore a trailing slash in a pattern

Previously, a trailing slash would cause "foo/" to be treated as the
name of a package, with comical results.

Fixed golang/go#14584

Change-Id: I660f8a079bbd63d3645812e516a9264c8e080e61
Reviewed-on: https://go-review.googlesource.com/30452
Reviewed-by: Robert Griesemer <gri@golang.org>

go/buildutil/allpackages.go

@@ -137,6 +137,9 @@ func allPackages(ctxt *build.Context, root string, ch chan<- item) {
 //
 // 	encoding/... -encoding/xml
 //
+// A trailing slash in a pattern is ignored.  (Path components of Go
+// package names are separated by slash, not the platform's path separator.)
+//
 func ExpandPatterns(ctxt *build.Context, patterns []string) map[string]bool {
 	// TODO(adonovan): support other features of 'go list':
 	// - "std"/"cmd"/"all" meta-packages
@@ -187,7 +190,7 @@ func ExpandPatterns(ctxt *build.Context, patterns []string) map[string]bool {
 			}
 		} else {
 			// single package
-			doPkg(arg, neg)
+			doPkg(strings.TrimSuffix(arg, "/"), neg)
 		}
 	}
 

go/buildutil/allpackages_test.go

@@ -61,6 +61,8 @@ func TestExpandPatterns(t *testing.T) {
 		{"...", "encoding encoding/hex encoding/json encoding/xml fmt"},
 		{"encoding/... -encoding/xml", "encoding encoding/hex encoding/json"},
 		{"... -encoding/...", "fmt"},
+		{"encoding", "encoding"},
+		{"encoding/", "encoding"},
 	} {
 		var pkgs []string
 		for pkg := range buildutil.ExpandPatterns(ctxt, strings.Fields(test.patterns)) {