go/importer: support importing directly from source

For #11415.

Change-Id: I5da39dad059113cfc4276152390aa4925bd18862
Reviewed-on: https://go-review.googlesource.com/37405
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/build/deps_test.go

@@ -217,7 +217,7 @@ var pkgDeps = map[string][]string{
 
 	// Go type checking.
 	"go/constant":               {"L4", "go/token", "math/big"},
-	"go/importer":               {"L4", "go/internal/gcimporter", "go/internal/gccgoimporter", "go/types"},
+	"go/importer":               {"L4", "go/build", "go/internal/gccgoimporter", "go/internal/gcimporter", "go/internal/srcimporter", "go/token", "go/types"},
 	"go/internal/gcimporter":    {"L4", "OS", "go/build", "go/constant", "go/token", "go/types", "text/scanner"},
 	"go/internal/gccgoimporter": {"L4", "OS", "debug/elf", "go/constant", "go/token", "go/types", "text/scanner"},
 	"go/internal/srcimporter":   {"L4", "fmt", "go/ast", "go/build", "go/parser", "go/token", "go/types", "path/filepath"},

src/go/importer/importer.go

@@ -6,8 +6,11 @@
 package importer
 
 import (
+	"go/build"
 	"go/internal/gccgoimporter"
 	"go/internal/gcimporter"
+	"go/internal/srcimporter"
+	"go/token"
 	"go/types"
 	"io"
 	"runtime"
@@ -17,22 +20,30 @@ import (
 // a given import path, or an error if no matching package is found.
 type Lookup func(path string) (io.ReadCloser, error)
 
-// For returns an Importer for the given compiler and lookup interface,
-// or nil. Supported compilers are "gc", and "gccgo". If lookup is nil,
-// the default package lookup mechanism for the given compiler is used.
+// For returns an Importer for importing from installed packages
+// for the compilers "gc" and "gccgo", or for importing directly
+// from the source if the compiler argument is "source". In this
+// latter case, importing may fail under circumstances where the
+// exported API is not entirely defined in pure Go source code
+// (if the package API depends on cgo-defined entities, the type
+// checker won't have access to those).
+//
+// If lookup is nil, the default package lookup mechanism for the
+// given compiler is used.
+//
 // BUG(issue13847): For does not support non-nil lookup functions.
 func For(compiler string, lookup Lookup) types.Importer {
 	switch compiler {
 	case "gc":
 		if lookup != nil {
-			panic("gc importer for custom import path lookup not yet implemented")
+			panic("gc importer for custom import path lookup not supported (issue #13847).")
 		}
 
 		return make(gcimports)
 
 	case "gccgo":
 		if lookup != nil {
-			panic("gccgo importer for custom import path lookup not yet implemented")
+			panic("gccgo importer for custom import path lookup not supported (issue #13847).")
 		}
 
 		var inst gccgoimporter.GccgoInstallation
@@ -43,6 +54,13 @@ func For(compiler string, lookup Lookup) types.Importer {
 			packages: make(map[string]*types.Package),
 			importer: inst.GetImporter(nil, nil),
 		}
+
+	case "source":
+		if lookup != nil {
+			panic("source importer for custom import path lookup not supported (issue #13847).")
+		}
+
+		return srcimporter.New(&build.Default, token.NewFileSet(), make(map[string]*types.Package))
 	}
 
 	// compiler not supported
@@ -55,7 +73,7 @@ func Default() types.Importer {
 	return For(runtime.Compiler, nil)
 }
 
-// gc support
+// gc importer
 
 type gcimports map[string]*types.Package
 
@@ -70,7 +88,7 @@ func (m gcimports) ImportFrom(path, srcDir string, mode types.ImportMode) (*type
 	return gcimporter.Import(m, path, srcDir)
 }
 
-// gccgo support
+// gccgo importer
 
 type gccgoimports struct {
 	packages map[string]*types.Package