buildtags: go:build support (#208)

Implements formatting of constraints according to the current Go version
supported syntax. This is achieved by delegating to go/format and extracting
out the resulting comments.

Also provides functions to query for constraint syntax support, which are
mainly intended for writing version-dependent tests.

Updates #183

buildtags/syntax.go

@@ -0,0 +1,45 @@
+package buildtags
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"go/format"
+	"strings"
+)
+
+// PlusBuildSyntaxSupported reports whether the current Go version supports the
+// "// +build" constraint syntax.
+func PlusBuildSyntaxSupported() bool { return plusbuild }
+
+// GoBuildSyntaxSupported reports whether the current Go version supports the
+// "//go:build" constraint syntax.
+func GoBuildSyntaxSupported() bool { return gobuild }
+
+// Format constraints according to the syntax supported by the current Go version.
+func Format(t ConstraintsConvertable) (string, error) {
+	// Print build tags to minimal Go source that can be passed to go/format.
+	src := t.ToConstraints().GoString() + "\npackage stub"
+
+	// Format them.
+	formatted, err := format.Source([]byte(src))
+	if err != nil {
+		return "", fmt.Errorf("format build constraints: %w", err)
+	}
+
+	// Extract the comment lines.
+	buf := bytes.NewReader(formatted)
+	scanner := bufio.NewScanner(buf)
+	output := ""
+	for scanner.Scan() {
+		line := scanner.Text()
+		if strings.HasPrefix(line, "//") {
+			output += line + "\n"
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return "", fmt.Errorf("parse formatted build constraints: %w", err)
+	}
+
+	return output, nil
+}

buildtags/syntax_go116.go

@@ -0,0 +1,9 @@
+//go:build !go1.17
+// +build !go1.17
+
+package buildtags
+
+const (
+	plusbuild = true
+	gobuild   = false
+)

buildtags/syntax_go117.go

@@ -0,0 +1,9 @@
+//go:build go1.17 && !go1.18
+// +build go1.17,!go1.18
+
+package buildtags
+
+const (
+	plusbuild = true
+	gobuild   = true
+)

buildtags/syntax_go118.go

@@ -0,0 +1,9 @@
+//go:build go1.18
+// +build go1.18
+
+package buildtags
+
+const (
+	plusbuild = false
+	gobuild   = true
+)

buildtags/syntax_test.go

@@ -0,0 +1,71 @@
+package buildtags
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+)
+
+func TestFormat(t *testing.T) {
+	cases := []struct {
+		PlusBuild []string
+		GoBuild   string
+	}{
+		{
+			PlusBuild: []string{"amd64"},
+			GoBuild:   "amd64",
+		},
+		{
+			PlusBuild: []string{
+				"linux darwin",
+				"amd64 arm64 mips64x ppc64x",
+			},
+			GoBuild: "(linux || darwin) && (amd64 || arm64 || mips64x || ppc64x)",
+		},
+		{
+			PlusBuild: []string{"!linux,!darwin !amd64,!arm64,!mips64x,!ppc64x"},
+			GoBuild:   "(!linux && !darwin) || (!amd64 && !arm64 && !mips64x && !ppc64x)",
+		},
+		{
+			PlusBuild: []string{
+				"linux,386 darwin,!cgo",
+				"!noasm",
+			},
+			GoBuild: "((linux && 386) || (darwin && !cgo)) && !noasm",
+		},
+	}
+
+	for _, c := range cases {
+		// Parse constraints.
+		var cs Constraints
+		for _, expr := range c.PlusBuild {
+			constraint, err := ParseConstraint(expr)
+			if err != nil {
+				t.Fatal(err)
+			}
+			cs = append(cs, constraint)
+		}
+
+		// Build expected output.
+		var buf bytes.Buffer
+		if GoBuildSyntaxSupported() {
+			fmt.Fprintf(&buf, "//go:build %s\n", c.GoBuild)
+		}
+		if PlusBuildSyntaxSupported() {
+			for _, expr := range c.PlusBuild {
+				fmt.Fprintf(&buf, "// +build %s\n", expr)
+			}
+		}
+		expect := buf.String()
+
+		// Format and check.
+		got, err := Format(cs)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if got != expect {
+			t.Errorf("got=\n%s\nexpect=\n%s\n", got, expect)
+		}
+	}
+}