go/types, go/importer: importing package unsafe is responsibility of importer

TBR adonovan

Fixes #13882.

Change-Id: I8664669f5d6adfec6f16e154263b1f0ea8988175
Reviewed-on: https://go-review.googlesource.com/18445
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/internal/gcimporter/gcimporter.go

@@ -112,13 +112,11 @@ func ImportData(packages map[string]*types.Package, filename, id string, data io
 // The packages map must contain all packages already imported.
 //
 func Import(packages map[string]*types.Package, path, srcDir string) (pkg *types.Package, err error) {
-	// package "unsafe" is handled by the type checker
-	if path == "unsafe" {
-		panic(`gcimporter.Import called for package "unsafe"`)
-	}
-
 	filename, id := FindPkg(path, srcDir)
 	if filename == "" {
+		if path == "unsafe" {
+			return types.Unsafe, nil
+		}
 		err = fmt.Errorf("can't find import: %s", id)
 		return
 	}

src/go/types/api.go

@@ -112,11 +112,12 @@ type Config struct {
 	// error found.
 	Error func(err error)
 
-	// Importer.Import is called for each import declaration except when
-	// importing package "unsafe". An error is reported if an importer is
-	// needed but none was installed.
-	// If the installed Importer implements Importer2, the Import2 method
+	// An importer is used to import packages referred to from
+	// import declarations.
+	// If the installed importer implements Importer2, Import2
 	// is called instead of Import.
+	// An error is reported if an importer is needed but none
+	// was installed.
 	Importer Importer
 
 	// If Sizes != nil, it provides the sizing functions for package unsafe.

src/go/types/resolver.go

@@ -183,9 +183,6 @@ func (check *Checker) collectObjects() {
 							// TODO(gri) shouldn't create a new one each time
 							imp = NewPackage("C", "C")
 							imp.fake = true
-						} else if path == "unsafe" {
-							// package "unsafe" is known to the language
-							imp = Unsafe
 						} else {
 							// ordinary import
 							if importer := check.conf.Importer; importer == nil {