cgo: refactor constant expressions

Put them in a separate file for separation of concerns (making them
testable) and add some tests.

cgo/const_test.go

@@ -0,0 +1,45 @@
+package cgo
+
+import (
+	"bytes"
+	"go/format"
+	"go/token"
+	"testing"
+)
+
+func TestParseConst(t *testing.T) {
+	// Test converting a C constant to a Go constant.
+	for _, tc := range []struct {
+		C  string
+		Go string
+	}{
+		{`5`, `5`},
+		{`(5)`, `5`},
+		{`(((5)))`, `5`},
+		{`5.8f`, `5.8`},
+		{`foo`, `<invalid>`}, // identifiers unimplemented
+		{``, `<invalid>`},    // empty constants not allowed in Go
+		{`"foo"`, `"foo"`},
+		{`'a'`, `'a'`},
+		{`0b10`, `<invalid>`}, // binary number literals unimplemented
+	} {
+		fset := token.NewFileSet()
+		startPos := fset.AddFile("test.c", -1, 1000).Pos(0)
+		expr := parseConst(startPos, tc.C)
+		s := "<invalid>"
+		if expr != nil {
+			// Serialize the Go constant to a string, for more readable test
+			// cases.
+			buf := &bytes.Buffer{}
+			err := format.Node(buf, fset, expr)
+			if err != nil {
+				t.Errorf("could not format expr from C constant %#v: %v", tc.C, err)
+				continue
+			}
+			s = buf.String()
+		}
+		if s != tc.Go {
+			t.Errorf("C constant %#v was parsed to %#v while expecting %#v", tc.C, s, tc.Go)
+		}
+	}
+}