go/packages: add ErrorKind field to differentiate error sources

Some applications (for example, diagnostics shown to a user in an
editor) may want to distinguish between errors generated by
the driver, parser, and type-checker. The Error struct did not have any
mechanism for doing this, so add an ErrorKind field and set it in
appendError.

Change-Id: If347163225d1e3a567e98610e9ba8a0930e4659c
Reviewed-on: https://go-review.googlesource.com/139317
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

go/packages/golist_fallback.go

@@ -194,15 +194,21 @@ func golistDriverFallback(cfg *Config, words ...string) (*driverResponse, error)
 				response.Packages = append(response.Packages, testmainPkg)
 				outdir, err := getOutdir()
 				if err != nil {
-					testmainPkg.Errors = append(testmainPkg.Errors,
+					testmainPkg.Errors = append(testmainPkg.Errors, Error{
-						Error{"-", fmt.Sprintf("failed to generate testmain: %v", err)})
+						Pos:  "-",
+						Msg:  fmt.Sprintf("failed to generate testmain: %v", err),
+						Kind: ListError,
+					})
 					return
 				}
 				testmain := filepath.Join(outdir, "testmain.go")
 				extraimports, extradeps, err := generateTestmain(testmain, testPkg, xtestPkg)
 				if err != nil {
-					testmainPkg.Errors = append(testmainPkg.Errors,
+					testmainPkg.Errors = append(testmainPkg.Errors, Error{
-						Error{"-", fmt.Sprintf("failed to generate testmain: %v", err)})
+						Pos:  "-",
+						Msg:  fmt.Sprintf("failed to generate testmain: %v", err),
+						Kind: ListError,
+					})
 				}
 				deps = append(deps, extradeps...)
 				for _, imp := range extraimports { // testing, testing/internal/testdeps, and maybe os

go/packages/packages.go

@@ -238,10 +238,23 @@ type Package struct {
 
 // An Error describes a problem with a package's metadata, syntax, or types.
 type Error struct {
-	Pos string // "file:line:col" or "file:line" or "" or "-"
+	Pos  string // "file:line:col" or "file:line" or "" or "-"
-	Msg string
+	Msg  string
+	Kind ErrorKind
 }
 
+// ErrorKind describes the source of the error, allowing the user to
+// differentiate between errors generated by the driver, the parser, or the
+// type-checker.
+type ErrorKind int
+
+const (
+	UnknownError ErrorKind = iota
+	ListError
+	ParserError
+	TypeError
+)
+
 func (err Error) Error() string {
 	pos := err.Pos
 	if pos == "" {
@@ -572,31 +585,35 @@ func (ld *loader) loadPackage(lpkg *loaderPackage) {
 		case *os.PathError:
 			// from parser
 			errs = append(errs, Error{
-				Pos: err.Path + ":1",
+				Pos:  err.Path + ":1",
-				Msg: err.Err.Error(),
+				Msg:  err.Err.Error(),
+				Kind: ParserError,
 			})
 
 		case scanner.ErrorList:
 			// from parser
 			for _, err := range err {
 				errs = append(errs, Error{
-					Pos: err.Pos.String(),
+					Pos:  err.Pos.String(),
-					Msg: err.Msg,
+					Msg:  err.Msg,
+					Kind: ParserError,
 				})
 			}
 
 		case types.Error:
 			// from type checker
 			errs = append(errs, Error{
-				Pos: err.Fset.Position(err.Pos).String(),
+				Pos:  err.Fset.Position(err.Pos).String(),
-				Msg: err.Msg,
+				Msg:  err.Msg,
+				Kind: TypeError,
 			})
 
 		default:
 			// unexpected impoverished error from parser?
 			errs = append(errs, Error{
-				Pos: "-",
+				Pos:  "-",
-				Msg: err.Error(),
+				Msg:  err.Error(),
+				Kind: UnknownError,
 			})
 
 			// If you see this error message, please file a bug.