feature: add support for custom build tags

_test/ct/ct1.go

@@ -0,0 +1,3 @@
+package ct
+
+func init() { println("hello from ct1") }

_test/ct/ct2.go

@@ -0,0 +1,5 @@
+// +build !dummy
+
+package ct
+
+func init() { println("hello from ct2") }

_test/ct/ct3.go

@@ -0,0 +1,5 @@
+// +build dummy
+
+package ct
+
+func init() { println("hello from ct3") }

_test/tag0.go

@@ -0,0 +1,15 @@
+// The following comment line has the same effect as 'go run -tags=dummy'
+//yaegi:tags dummy
+
+package main
+
+import _ "github.com/containous/yaegi/_test/ct"
+
+func main() {
+	println("bye")
+}
+
+// Output:
+// hello from ct1
+// hello from ct3
+// bye

cmd/yaegi/yaegi.go

@@ -18,7 +18,10 @@ at global level in an implicit main package.
 
 Options:
     -i
-	   start an interactive REPL after file execution
+	   start an interactive REPL after file execution.
+	-tags tag,list
+	   a comma-separated list of build tags to consider satisfied during
+	   the interpretation.
 
 Debugging support (may be removed at any time):
   YAEGI_AST_DOT=1
@@ -43,7 +46,9 @@ import (
 
 func main() {
 	var interactive bool
+	var tags string
 	flag.BoolVar(&interactive, "i", false, "start an interactive REPL")
+	flag.StringVar(&tags, "tags", "", "set a list of build tags")
 	flag.Usage = func() {
 		fmt.Println("Usage:", os.Args[0], "[options] [script] [args]")
 		fmt.Println("Options:")
@@ -53,7 +58,7 @@ func main() {
 	args := flag.Args()
 	log.SetFlags(log.Lshortfile)
 
-	i := interp.New(interp.Options{GoPath: build.Default.GOPATH})
+	i := interp.New(interp.Options{GoPath: build.Default.GOPATH, BuildTags: strings.Split(tags, ",")})
 	i.Use(stdlib.Symbols)
 	i.Use(interp.Symbols)
 

interp/ast.go

@@ -320,6 +320,7 @@ func (interp *Interpreter) firstToken(src string) token.Token {
 func (interp *Interpreter) ast(src, name string) (string, *node, error) {
 	inRepl := name == ""
 	var inFunc bool
+	var mode parser.Mode
 
 	// Allow incremental parsing of declarations or statements, by inserting
 	// them in a pseudo file package or function. Those statements or
@@ -334,17 +335,21 @@ func (interp *Interpreter) ast(src, name string) (string, *node, error) {
 			inFunc = true
 			src = "package main; func main() {" + src + "}"
 		}
+		// Parse comments in REPL mode, to allow tag setting
+		mode |= parser.ParseComments
 	}
 
-	if ok, err := interp.buildOk(interp.context, name, src); !ok || err != nil {
+	if ok, err := interp.buildOk(&interp.context, name, src); !ok || err != nil {
 		return "", nil, err // skip source not matching build constraints
 	}
 
-	f, err := parser.ParseFile(interp.fset, name, src, 0)
+	f, err := parser.ParseFile(interp.fset, name, src, mode)
 	if err != nil {
 		return "", nil, err
 	}
 
+	setYaegiTags(&interp.context, f.Comments)
+
 	var root *node
 	var anc astNode
 	var st nodestack

interp/build.go

@@ -1,6 +1,7 @@
 package interp
 
 import (
+	"go/ast"
 	"go/build"
 	"go/parser"
 	"path"
@@ -11,7 +12,7 @@ import (
 // buildOk returns true if a file or script matches build constraints
 // as specified in https://golang.org/pkg/go/build/#hdr-Build_Constraints.
 // An error from parser is returned as well.
-func (interp *Interpreter) buildOk(ctx build.Context, name, src string) (bool, error) {
+func (interp *Interpreter) buildOk(ctx *build.Context, name, src string) (bool, error) {
 	// Extract comments before the first clause
 	f, err := parser.ParseFile(interp.fset, name, src, parser.PackageClauseOnly|parser.ParseComments)
 	if err != nil {
@@ -25,12 +26,13 @@ func (interp *Interpreter) buildOk(ctx build.Context, name, src string) (bool, e
 			}
 		}
 	}
+	setYaegiTags(ctx, f.Comments)
 	return true, nil
 }
 
 // buildLineOk returns true if line is not a build constraint or
 // if build constraint is satisfied
-func buildLineOk(ctx build.Context, line string) (ok bool) {
+func buildLineOk(ctx *build.Context, line string) (ok bool) {
 	if len(line) < 7 || line[:7] != "+build " {
 		return true
 	}
@@ -45,7 +47,7 @@ func buildLineOk(ctx build.Context, line string) (ok bool) {
 }
 
 // buildOptionOk return true if all comma separated tags match, false otherwise
-func buildOptionOk(ctx build.Context, tag string) bool {
+func buildOptionOk(ctx *build.Context, tag string) bool {
 	// in option, evaluate the AND of individual tags
 	for _, t := range strings.Split(tag, ",") {
 		if !buildTagOk(ctx, t) {
@@ -57,7 +59,7 @@ func buildOptionOk(ctx build.Context, tag string) bool {
 
 // buildTagOk returns true if a build tag matches, false otherwise
 // if first character is !, result is negated
-func buildTagOk(ctx build.Context, s string) (r bool) {
+func buildTagOk(ctx *build.Context, s string) (r bool) {
 	not := s[0] == '!'
 	if not {
 		s = s[1:]
@@ -82,6 +84,25 @@ func buildTagOk(ctx build.Context, s string) (r bool) {
 	return
 }
 
+// setYaegiTags scans a comment group for "yaegi:tags tag1 tag2 ..." lines
+// and adds the corresponding tags to the interpreter build tags.
+func setYaegiTags(ctx *build.Context, comments []*ast.CommentGroup) {
+	for _, g := range comments {
+		for _, line := range strings.Split(strings.TrimSpace(g.Text()), "\n") {
+			if len(line) < 11 || line[:11] != "yaegi:tags " {
+				continue
+			}
+
+			tags := strings.Split(strings.TrimSpace(line[10:]), " ")
+			for _, tag := range tags {
+				if !contains(ctx.BuildTags, tag) {
+					ctx.BuildTags = append(ctx.BuildTags, tag)
+				}
+			}
+		}
+	}
+}
+
 func contains(tags []string, tag string) bool {
 	for _, t := range tags {
 		if t == tag {
@@ -92,7 +113,7 @@ func contains(tags []string, tag string) bool {
 }
 
 // goMinorVersion returns the go minor version number
-func goMinorVersion(ctx build.Context) int {
+func goMinorVersion(ctx *build.Context) int {
 	current := ctx.ReleaseTags[len(ctx.ReleaseTags)-1]
 
 	v := strings.Split(current, ".")
@@ -108,7 +129,7 @@ func goMinorVersion(ctx build.Context) int {
 }
 
 // skipFile returns true if file should be skipped
-func skipFile(ctx build.Context, p string) bool {
+func skipFile(ctx *build.Context, p string) bool {
 	if !strings.HasSuffix(p, ".go") {
 		return true
 	}

interp/build_test.go

@@ -44,7 +44,7 @@ func TestBuildTag(t *testing.T) {
 		test := test
 		src := test.src + "\npackage x"
 		t.Run(test.src, func(t *testing.T) {
-			if r, _ := i.buildOk(ctx, "", src); r != test.res {
+			if r, _ := i.buildOk(&ctx, "", src); r != test.res {
 				t.Errorf("got %v, want %v", r, test.res)
 			}
 		})
@@ -74,7 +74,7 @@ func TestBuildFile(t *testing.T) {
 	for _, test := range tests {
 		test := test
 		t.Run(test.src, func(t *testing.T) {
-			if r := skipFile(ctx, test.src); r != test.res {
+			if r := skipFile(&ctx, test.src); r != test.res {
 				t.Errorf("got %v, want %v", r, test.res)
 			}
 		})
@@ -106,7 +106,7 @@ func Test_goMinorVersion(t *testing.T) {
 	for _, test := range tests {
 		test := test
 		t.Run(test.desc, func(t *testing.T) {
-			minor := goMinorVersion(test.context)
+			minor := goMinorVersion(&test.context)
 
 			if minor != test.expected {
 				t.Errorf("got %v, want %v", minor, test.expected)

interp/doc.go

@@ -4,6 +4,31 @@ Package interp provides a complete Go interpreter
 For the Go language itself, refer to the official Go specification
 https://golang.org/ref/spec.
 
+Custom build tags
+
+Custom build tags allow to control which files in imported source
+packages are interpreted, in the same way as the "-tags" option of the
+"go build" command. Setting a custom build tag spans globally for all
+future imports of the session.
+
+A build tag is a line comment that begins
+
+	// yaegi:tags
+
+that lists the build constraints to be satisfied by the further
+imports of source packages.
+
+For example the following custom build tag
+
+	// yaegi:tags noasm
+
+Will ensure that an import of a package will exclude files containing
+
+	// +build !noasm
+
+And include files containing
+
+	// +build noasm
 */
 package interp
 

interp/interp_consistent_test.go

@@ -104,7 +104,7 @@ func TestInterpConsistencyBuild(t *testing.T) {
 
 			bin := filepath.Join(dir, strings.TrimSuffix(file.Name(), ".go"))
 
-			cmdBuild := exec.Command("go", "build", "-o", bin, filePath)
+			cmdBuild := exec.Command("go", "build", "-tags=dummy", "-o", bin, filePath)
 			outBuild, err := cmdBuild.CombinedOutput()
 			if err != nil {
 				t.Log(string(outBuild))

interp/src.go

@@ -49,7 +49,7 @@ func (interp *Interpreter) importSrc(rPath, path, alias string) error {
 	// Parse source files
 	for _, file := range files {
 		name := file.Name()
-		if skipFile(interp.context, name) {
+		if skipFile(&interp.context, name) {
 			continue
 		}