go.tools/go/types: add mode to typecheck pkg-level decls only

This is useful for situations where one only cares about
exports, for instance (as in the goapi checker).

R=adonovan, bradfitz
CC=golang-dev
https://golang.org/cl/12292043

go/types/api.go

@@ -48,6 +48,10 @@ func Check(path string, fset *token.FileSet, files []*ast.File) (*Package, error
 // A Config specifies the configuration for type checking.
 // The zero value for Config is a ready-to-use default configuration.
 type Config struct {
+	// If IgnoreFuncBodies is set, function bodies are not
+	// type-checked.
+	IgnoreFuncBodies bool
+
 	// If Error != nil, it is called with each error found
 	// during type checking. The error strings of errors with
 	// detailed position information are formatted as follows:
@@ -99,7 +103,9 @@ type Info struct {
 	// If Objects != nil, it records the identifier and corresponding object
 	// for each identifier that is type-checked (including package names,
 	// dots "." of dot-imports, and blank "_" identifiers). For identifiers
-	// that were not declared due to an error, the corresponding object is nil.
+	// that are not declared (due to an error, or because they are symbolic
+	// as the t in t := x.(type) of a type switch header), the corresponding
+	// object is nil.
 	// BUG(gri) Label identifiers in break, continue, or goto statements
 	// are not recorded.
 	Objects map[*ast.Ident]Object

go/types/resolver.go

@@ -71,7 +71,7 @@ type funcInfo struct {
 // later appends a function with non-empty body to check.funcList.
 func (check *checker) later(f *Func, sig *Signature, body *ast.BlockStmt) {
 	// functions implemented elsewhere (say in assembly) have no body
-	if body != nil {
+	if !check.conf.IgnoreFuncBodies && body != nil {
 		check.funcList = append(check.funcList, funcInfo{f, sig, body})
 	}
 }

go/types/self_test.go

@@ -42,9 +42,15 @@ func TestSelf(t *testing.T) {
 		return // skip benchmark in short mode
 	}
 
+	benchmark(fset, files, false)
+	benchmark(fset, files, true)
+}
+
+func benchmark(fset *token.FileSet, files []*ast.File, full bool) {
 	b := testing.Benchmark(func(b *testing.B) {
 		for i := 0; i < b.N; i++ {
-			Check("go/types", fset, files)
+			conf := Config{IgnoreFuncBodies: !full}
+			conf.Check("go/types", fset, files, nil)
 		}
 	})