cmd/compile/internal/syntax: implement Pos.FileBase method (cleanup)

Factor out file base computation into a method.

Change-Id: Ia6de100459b6df2919f2320872890320aa88866d
Reviewed-on: https://go-review.googlesource.com/c/go/+/586156
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/noder/irgen.go

@@ -29,9 +29,9 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 
 	// setup and syntax error reporting
 	files := make([]*syntax.File, len(noders))
-	// posBaseMap maps all file pos bases back to *syntax.File
+	// fileBaseMap maps all file pos bases back to *syntax.File
 	// for checking Go version mismatched.
-	posBaseMap := make(map[*syntax.PosBase]*syntax.File)
+	fileBaseMap := make(map[*syntax.PosBase]*syntax.File)
 	for i, p := range noders {
 		files[i] = p.file
 		// The file.Pos() is the position of the package clause.
@@ -40,7 +40,7 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 		// Make sure to consistently map back to file base, here and
 		// when we look for a file in the conf.Error handler below,
 		// otherwise the file may not be found (was go.dev/issue/67141).
-		posBaseMap[fileBase(p.file.Pos())] = p.file
+		fileBaseMap[p.file.Pos().FileBase()] = p.file
 	}
 
 	// typechecking
@@ -75,9 +75,9 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 		terr := err.(types2.Error)
 		msg := terr.Msg
 		if versionErrorRx.MatchString(msg) {
-			posBase := fileBase(terr.Pos)
-			fileVersion := info.FileVersions[posBase]
-			file := posBaseMap[posBase]
+			fileBase := terr.Pos.FileBase()
+			fileVersion := info.FileVersions[fileBase]
+			file := fileBaseMap[fileBase]
 			if file == nil {
 				// This should never happen, but be careful and don't crash.
 			} else if file.GoVersion == fileVersion {
@@ -155,15 +155,6 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 	return pkg, info
 }
 
-// fileBase returns a file's position base given a position in the file.
-func fileBase(pos syntax.Pos) *syntax.PosBase {
-	base := pos.Base()
-	for !base.IsFileBase() { // line directive base
-		base = base.Pos().Base()
-	}
-	return base
-}
-
 // A cycleFinder detects anonymous interface cycles (go.dev/issue/56103).
 type cycleFinder struct {
 	cyclic map[*types2.Interface]bool

src/cmd/compile/internal/syntax/pos.go

@@ -34,6 +34,18 @@ func (pos Pos) Base() *PosBase { return pos.base }
 func (pos Pos) Line() uint     { return uint(pos.line) }
 func (pos Pos) Col() uint      { return uint(pos.col) }
 
+// FileBase returns the PosBase of the file containing pos,
+// skipping over intermediate PosBases from //line directives.
+// The result is nil if pos doesn't have a file base.
+func (pos Pos) FileBase() *PosBase {
+	b := pos.base
+	for b != nil && b != b.pos.base {
+		b = b.pos.base
+	}
+	// b == nil || b == b.pos.base
+	return b
+}
+
 func (pos Pos) RelFilename() string { return pos.base.Filename() }
 
 func (pos Pos) RelLine() uint {

src/cmd/compile/internal/types2/check.go

@@ -373,7 +373,7 @@ func (check *Checker) initFiles(files []*syntax.File) {
 				check.errorf(file.PkgName, TooNew, "file requires newer Go version %v", fileVersion)
 			}
 		}
-		versions[base(file.Pos())] = v // base(file.Pos()) may be nil for tests
+		versions[file.Pos().FileBase()] = v // file.Pos().FileBase() may be nil for tests
 	}
 }
 

src/cmd/compile/internal/types2/version.go

@@ -5,7 +5,6 @@
 package types2
 
 import (
-	"cmd/compile/internal/syntax"
 	"fmt"
 	"go/version"
 	"internal/goversion"
@@ -56,7 +55,7 @@ var (
 func (check *Checker) allowVersion(at poser, v goVersion) bool {
 	fileVersion := check.conf.GoVersion
 	if pos := at.Pos(); pos.IsKnown() {
-		fileVersion = check.versions[base(pos)]
+		fileVersion = check.versions[pos.FileBase()]
 	}
 
 	// We need asGoVersion (which calls version.Lang) below
@@ -76,19 +75,3 @@ func (check *Checker) verifyVersionf(at poser, v goVersion, format string, args
 	}
 	return true
 }
-
-// base finds the underlying PosBase of the source file containing pos,
-// skipping over intermediate PosBase layers created by //line directives.
-// The positions must be known.
-func base(pos syntax.Pos) *syntax.PosBase {
-	assert(pos.IsKnown())
-	b := pos.Base()
-	for {
-		bb := b.Pos().Base()
-		if bb == nil || bb == b {
-			break
-		}
-		b = bb
-	}
-	return b
-}