cmd/go: add $DOLLAR to the predefined variables for go generate

Without some hook like this, it's impossible to get a $ into the generate
command, which is necessary if you're trying to do some shell scripting
or regular expressions.

We could use backslash escaping but that's already tricky enough
because the strings are processed as Go strings. Using $ like this
means we need no more mechanism, just a predefined variable.

We may need to revisit this but I hope we can avoid new quoting rules.

Change-Id: Ieb478c8cc767a866765282472239ed3c1e5669a8
Reviewed-on: https://go-review.googlesource.com/8091
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/generate.go

@@ -64,6 +64,8 @@ Go generate sets several variables when it runs the generator:
 		The base name of the file.
 	$GOPACKAGE
 		The name of the package of the file containing the directive.
+	$DOLLAR
+		A dollar sign.
 
 Other than variable substitution and quoted-string evaluation, no
 special processing such as "globbing" is performed on the command
@@ -348,6 +350,8 @@ func (g *Generator) expandEnv(word string) string {
 			sub = g.file
 		case "GOPACKAGE":
 			sub = g.pkg
+		case "DOLLAR":
+			sub = "$"
 		default:
 			sub = os.Getenv(envVar)
 		}

src/cmd/go/generate_test.go

@@ -26,6 +26,7 @@ var splitTests = []splitTest{
 	{"$GOPACKAGE", []string{"sys"}},
 	{"a $XXNOTDEFINEDXX b", []string{"a", "", "b"}},
 	{"/$XXNOTDEFINED/", []string{"//"}},
+	{"/$DOLLAR/", []string{"/$/"}},
 	{"yacc -o $GOARCH/yacc_$GOFILE", []string{"go", "tool", "yacc", "-o", runtime.GOARCH + "/yacc_proc.go"}},
 }