diff --git a/internal/analysisinternal/analysis.go b/internal/analysisinternal/analysis.go
index 78ee2c06bea..3f1e573342f 100644
--- a/internal/analysisinternal/analysis.go
+++ b/internal/analysisinternal/analysis.go
@@ -11,9 +11,8 @@ import (
 	"go/ast"
 	"go/token"
 	"go/types"
-	"strings"
+	"strconv"
 
-	"golang.org/x/tools/go/ast/astutil"
 	"golang.org/x/tools/internal/lsp/fuzzy"
 )
 
@@ -37,7 +36,7 @@ func TypeErrorEndPos(fset *token.FileSet, src []byte, start token.Pos) token.Pos
 	return end
 }
 
-func ZeroValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
+func ZeroValue(f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
 	under := typ
 	if n, ok := typ.(*types.Named); ok {
 		under = n.Underlying()
@@ -57,7 +56,7 @@ func ZeroValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.T
 	case *types.Chan, *types.Interface, *types.Map, *types.Pointer, *types.Signature, *types.Slice, *types.Array:
 		return ast.NewIdent("nil")
 	case *types.Struct:
-		texpr := TypeExpr(fset, f, pkg, typ) // typ because we want the name here.
+		texpr := TypeExpr(f, pkg, typ) // typ because we want the name here.
 		if texpr == nil {
 			return nil
 		}
@@ -81,7 +80,7 @@ func IsZeroValue(expr ast.Expr) bool {
 	}
 }
 
-func TypeExpr(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
+func TypeExpr(f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
 	switch t := typ.(type) {
 	case *types.Basic:
 		switch t.Kind() {
@@ -91,7 +90,7 @@ func TypeExpr(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Ty
 			return ast.NewIdent(t.Name())
 		}
 	case *types.Pointer:
-		x := TypeExpr(fset, f, pkg, t.Elem())
+		x := TypeExpr(f, pkg, t.Elem())
 		if x == nil {
 			return nil
 		}
@@ -100,7 +99,7 @@ func TypeExpr(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Ty
 			X:  x,
 		}
 	case *types.Array:
-		elt := TypeExpr(fset, f, pkg, t.Elem())
+		elt := TypeExpr(f, pkg, t.Elem())
 		if elt == nil {
 			return nil
 		}
@@ -112,7 +111,7 @@ func TypeExpr(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Ty
 			Elt: elt,
 		}
 	case *types.Slice:
-		elt := TypeExpr(fset, f, pkg, t.Elem())
+		elt := TypeExpr(f, pkg, t.Elem())
 		if elt == nil {
 			return nil
 		}
@@ -120,8 +119,8 @@ func TypeExpr(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Ty
 			Elt: elt,
 		}
 	case *types.Map:
-		key := TypeExpr(fset, f, pkg, t.Key())
-		value := TypeExpr(fset, f, pkg, t.Elem())
+		key := TypeExpr(f, pkg, t.Key())
+		value := TypeExpr(f, pkg, t.Elem())
 		if key == nil || value == nil {
 			return nil
 		}
@@ -134,7 +133,7 @@ func TypeExpr(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Ty
 		if t.Dir() == types.SendRecv {
 			dir = ast.SEND | ast.RECV
 		}
-		value := TypeExpr(fset, f, pkg, t.Elem())
+		value := TypeExpr(f, pkg, t.Elem())
 		if value == nil {
 			return nil
 		}
@@ -145,7 +144,7 @@ func TypeExpr(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Ty
 	case *types.Signature:
 		var params []*ast.Field
 		for i := 0; i < t.Params().Len(); i++ {
-			p := TypeExpr(fset, f, pkg, t.Params().At(i).Type())
+			p := TypeExpr(f, pkg, t.Params().At(i).Type())
 			if p == nil {
 				return nil
 			}
@@ -160,7 +159,7 @@ func TypeExpr(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Ty
 		}
 		var returns []*ast.Field
 		for i := 0; i < t.Results().Len(); i++ {
-			r := TypeExpr(fset, f, pkg, t.Results().At(i).Type())
+			r := TypeExpr(f, pkg, t.Results().At(i).Type())
 			if r == nil {
 				return nil
 			}
@@ -184,13 +183,12 @@ func TypeExpr(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Ty
 			return ast.NewIdent(t.Obj().Name())
 		}
 		pkgName := t.Obj().Pkg().Name()
+
 		// If the file already imports the package under another name, use that.
-		for _, group := range astutil.Imports(fset, f) {
-			for _, cand := range group {
-				if strings.Trim(cand.Path.Value, `"`) == t.Obj().Pkg().Path() {
-					if cand.Name != nil && cand.Name.Name != "" {
-						pkgName = cand.Name.Name
-					}
+		for _, cand := range f.Imports {
+			if path, _ := strconv.Unquote(cand.Path.Value); path == t.Obj().Pkg().Path() {
+				if cand.Name != nil && cand.Name.Name != "" {
+					pkgName = cand.Name.Name
 				}
 			}
 		}
diff --git a/internal/imports/imports.go b/internal/imports/imports.go
index 79026726a24..95a88383a79 100644
--- a/internal/imports/imports.go
+++ b/internal/imports/imports.go
@@ -103,12 +103,17 @@ func ApplyFixes(fixes []*ImportFix, filename string, src []byte, opt *Options, e
 	return formatFile(fileSet, file, src, nil, opt)
 }
 
-func formatFile(fileSet *token.FileSet, file *ast.File, src []byte, adjust func(orig []byte, src []byte) []byte, opt *Options) ([]byte, error) {
-	mergeImports(fileSet, file)
-	sortImports(opt.LocalPrefix, fileSet, file)
-	imps := astutil.Imports(fileSet, file)
+// formatFile formats the file syntax tree.
+// It may mutate the token.FileSet.
+//
+// If an adjust function is provided, it is called after formatting
+// with the original source (formatFile's src parameter) and the
+// formatted file, and returns the postpocessed result.
+func formatFile(fset *token.FileSet, file *ast.File, src []byte, adjust func(orig []byte, src []byte) []byte, opt *Options) ([]byte, error) {
+	mergeImports(file)
+	sortImports(opt.LocalPrefix, fset.File(file.Pos()), file)
 	var spacesBefore []string // import paths we need spaces before
-	for _, impSection := range imps {
+	for _, impSection := range astutil.Imports(fset, file) {
 		// Within each block of contiguous imports, see if any
 		// import lines are in different group numbers. If so,
 		// we'll need to put a space between them so it's
@@ -132,7 +137,7 @@ func formatFile(fileSet *token.FileSet, file *ast.File, src []byte, adjust func(
 	printConfig := &printer.Config{Mode: printerMode, Tabwidth: opt.TabWidth}
 
 	var buf bytes.Buffer
-	err := printConfig.Fprint(&buf, fileSet, file)
+	err := printConfig.Fprint(&buf, fset, file)
 	if err != nil {
 		return nil, err
 	}
diff --git a/internal/imports/sortimports.go b/internal/imports/sortimports.go
index c5a70c0bb36..85144db1dfa 100644
--- a/internal/imports/sortimports.go
+++ b/internal/imports/sortimports.go
@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Hacked up copy of go/ast/import.go
+// Modified to use a single token.File in preference to a FileSet.
 
 package imports
 
@@ -16,7 +17,9 @@ import (
 
 // sortImports sorts runs of consecutive import lines in import blocks in f.
 // It also removes duplicate imports when it is possible to do so without data loss.
-func sortImports(localPrefix string, fset *token.FileSet, f *ast.File) {
+//
+// It may mutate the token.File.
+func sortImports(localPrefix string, tokFile *token.File, f *ast.File) {
 	for i, d := range f.Decls {
 		d, ok := d.(*ast.GenDecl)
 		if !ok || d.Tok != token.IMPORT {
@@ -39,21 +42,21 @@ func sortImports(localPrefix string, fset *token.FileSet, f *ast.File) {
 		i := 0
 		specs := d.Specs[:0]
 		for j, s := range d.Specs {
-			if j > i && fset.Position(s.Pos()).Line > 1+fset.Position(d.Specs[j-1].End()).Line {
+			if j > i && tokFile.Line(s.Pos()) > 1+tokFile.Line(d.Specs[j-1].End()) {
 				// j begins a new run.  End this one.
-				specs = append(specs, sortSpecs(localPrefix, fset, f, d.Specs[i:j])...)
+				specs = append(specs, sortSpecs(localPrefix, tokFile, f, d.Specs[i:j])...)
 				i = j
 			}
 		}
-		specs = append(specs, sortSpecs(localPrefix, fset, f, d.Specs[i:])...)
+		specs = append(specs, sortSpecs(localPrefix, tokFile, f, d.Specs[i:])...)
 		d.Specs = specs
 
 		// Deduping can leave a blank line before the rparen; clean that up.
 		if len(d.Specs) > 0 {
 			lastSpec := d.Specs[len(d.Specs)-1]
-			lastLine := fset.PositionFor(lastSpec.Pos(), false).Line
-			if rParenLine := fset.PositionFor(d.Rparen, false).Line; rParenLine > lastLine+1 {
-				fset.File(d.Rparen).MergeLine(rParenLine - 1)
+			lastLine := tokFile.PositionFor(lastSpec.Pos(), false).Line
+			if rParenLine := tokFile.PositionFor(d.Rparen, false).Line; rParenLine > lastLine+1 {
+				tokFile.MergeLine(rParenLine - 1) // has side effects!
 			}
 		}
 	}
@@ -62,7 +65,7 @@ func sortImports(localPrefix string, fset *token.FileSet, f *ast.File) {
 // mergeImports merges all the import declarations into the first one.
 // Taken from golang.org/x/tools/ast/astutil.
 // This does not adjust line numbers properly
-func mergeImports(fset *token.FileSet, f *ast.File) {
+func mergeImports(f *ast.File) {
 	if len(f.Decls) <= 1 {
 		return
 	}
@@ -144,7 +147,9 @@ type posSpan struct {
 	End   token.Pos
 }
 
-func sortSpecs(localPrefix string, fset *token.FileSet, f *ast.File, specs []ast.Spec) []ast.Spec {
+// sortSpecs sorts the import specs within each import decl.
+// It may mutate the token.File.
+func sortSpecs(localPrefix string, tokFile *token.File, f *ast.File, specs []ast.Spec) []ast.Spec {
 	// Can't short-circuit here even if specs are already sorted,
 	// since they might yet need deduplication.
 	// A lone import, however, may be safely ignored.
@@ -160,7 +165,7 @@ func sortSpecs(localPrefix string, fset *token.FileSet, f *ast.File, specs []ast
 
 	// Identify comments in this range.
 	// Any comment from pos[0].Start to the final line counts.
-	lastLine := fset.Position(pos[len(pos)-1].End).Line
+	lastLine := tokFile.Line(pos[len(pos)-1].End)
 	cstart := len(f.Comments)
 	cend := len(f.Comments)
 	for i, g := range f.Comments {
@@ -170,7 +175,7 @@ func sortSpecs(localPrefix string, fset *token.FileSet, f *ast.File, specs []ast
 		if i < cstart {
 			cstart = i
 		}
-		if fset.Position(g.End()).Line > lastLine {
+		if tokFile.Line(g.End()) > lastLine {
 			cend = i
 			break
 		}
@@ -203,7 +208,7 @@ func sortSpecs(localPrefix string, fset *token.FileSet, f *ast.File, specs []ast
 			deduped = append(deduped, s)
 		} else {
 			p := s.Pos()
-			fset.File(p).MergeLine(fset.Position(p).Line)
+			tokFile.MergeLine(tokFile.Line(p)) // has side effects!
 		}
 	}
 	specs = deduped
@@ -234,21 +239,21 @@ func sortSpecs(localPrefix string, fset *token.FileSet, f *ast.File, specs []ast
 
 	// Fixup comments can insert blank lines, because import specs are on different lines.
 	// We remove those blank lines here by merging import spec to the first import spec line.
-	firstSpecLine := fset.Position(specs[0].Pos()).Line
+	firstSpecLine := tokFile.Line(specs[0].Pos())
 	for _, s := range specs[1:] {
 		p := s.Pos()
-		line := fset.File(p).Line(p)
+		line := tokFile.Line(p)
 		for previousLine := line - 1; previousLine >= firstSpecLine; {
 			// MergeLine can panic. Avoid the panic at the cost of not removing the blank line
 			// golang/go#50329
-			if previousLine > 0 && previousLine < fset.File(p).LineCount() {
-				fset.File(p).MergeLine(previousLine)
+			if previousLine > 0 && previousLine < tokFile.LineCount() {
+				tokFile.MergeLine(previousLine) // has side effects!
 				previousLine--
 			} else {
 				// try to gather some data to diagnose how this could happen
 				req := "Please report what the imports section of your go file looked like."
 				log.Printf("panic avoided: first:%d line:%d previous:%d max:%d. %s",
-					firstSpecLine, line, previousLine, fset.File(p).LineCount(), req)
+					firstSpecLine, line, previousLine, tokFile.LineCount(), req)
 			}
 		}
 	}
diff --git a/internal/lsp/analysis/fillreturns/fillreturns.go b/internal/lsp/analysis/fillreturns/fillreturns.go
index 3299f74e989..72fe65d79ca 100644
--- a/internal/lsp/analysis/fillreturns/fillreturns.go
+++ b/internal/lsp/analysis/fillreturns/fillreturns.go
@@ -193,8 +193,7 @@ outer:
 				// generate a zero value.
 				value := analysisinternal.FindBestMatch(retTyp.String(), idents)
 				if value == nil {
-					value = analysisinternal.ZeroValue(
-						pass.Fset, file, pass.Pkg, retTyp)
+					value = analysisinternal.ZeroValue(file, pass.Pkg, retTyp)
 				}
 				if value == nil {
 					return nil, nil
diff --git a/internal/lsp/analysis/fillstruct/fillstruct.go b/internal/lsp/analysis/fillstruct/fillstruct.go
index 6987f786d2b..f160d4422ae 100644
--- a/internal/lsp/analysis/fillstruct/fillstruct.go
+++ b/internal/lsp/analysis/fillstruct/fillstruct.go
@@ -256,7 +256,7 @@ func SuggestedFix(fset *token.FileSet, rng span.Range, content []byte, file *ast
 			// NOTE: We currently match on the name of the field key rather than the field type.
 			value := analysisinternal.FindBestMatch(obj.Field(i).Name(), idents)
 			if value == nil {
-				value = populateValue(fset, file, pkg, fieldTyp)
+				value = populateValue(file, pkg, fieldTyp)
 			}
 			if value == nil {
 				return nil, nil
@@ -354,7 +354,7 @@ func indent(str, ind []byte) []byte {
 //
 // The reasoning here is that users will call fillstruct with the intention of
 // initializing the struct, in which case setting these fields to nil has no effect.
-func populateValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
+func populateValue(f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
 	under := typ
 	if n, ok := typ.(*types.Named); ok {
 		under = n.Underlying()
@@ -374,8 +374,8 @@ func populateValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ typ
 			panic("unknown basic type")
 		}
 	case *types.Map:
-		k := analysisinternal.TypeExpr(fset, f, pkg, u.Key())
-		v := analysisinternal.TypeExpr(fset, f, pkg, u.Elem())
+		k := analysisinternal.TypeExpr(f, pkg, u.Key())
+		v := analysisinternal.TypeExpr(f, pkg, u.Elem())
 		if k == nil || v == nil {
 			return nil
 		}
@@ -386,7 +386,7 @@ func populateValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ typ
 			},
 		}
 	case *types.Slice:
-		s := analysisinternal.TypeExpr(fset, f, pkg, u.Elem())
+		s := analysisinternal.TypeExpr(f, pkg, u.Elem())
 		if s == nil {
 			return nil
 		}
@@ -396,7 +396,7 @@ func populateValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ typ
 			},
 		}
 	case *types.Array:
-		a := analysisinternal.TypeExpr(fset, f, pkg, u.Elem())
+		a := analysisinternal.TypeExpr(f, pkg, u.Elem())
 		if a == nil {
 			return nil
 		}
@@ -409,7 +409,7 @@ func populateValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ typ
 			},
 		}
 	case *types.Chan:
-		v := analysisinternal.TypeExpr(fset, f, pkg, u.Elem())
+		v := analysisinternal.TypeExpr(f, pkg, u.Elem())
 		if v == nil {
 			return nil
 		}
@@ -427,7 +427,7 @@ func populateValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ typ
 			},
 		}
 	case *types.Struct:
-		s := analysisinternal.TypeExpr(fset, f, pkg, typ)
+		s := analysisinternal.TypeExpr(f, pkg, typ)
 		if s == nil {
 			return nil
 		}
@@ -437,7 +437,7 @@ func populateValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ typ
 	case *types.Signature:
 		var params []*ast.Field
 		for i := 0; i < u.Params().Len(); i++ {
-			p := analysisinternal.TypeExpr(fset, f, pkg, u.Params().At(i).Type())
+			p := analysisinternal.TypeExpr(f, pkg, u.Params().At(i).Type())
 			if p == nil {
 				return nil
 			}
@@ -452,7 +452,7 @@ func populateValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ typ
 		}
 		var returns []*ast.Field
 		for i := 0; i < u.Results().Len(); i++ {
-			r := analysisinternal.TypeExpr(fset, f, pkg, u.Results().At(i).Type())
+			r := analysisinternal.TypeExpr(f, pkg, u.Results().At(i).Type())
 			if r == nil {
 				return nil
 			}
@@ -487,7 +487,7 @@ func populateValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ typ
 		default:
 			return &ast.UnaryExpr{
 				Op: token.AND,
-				X:  populateValue(fset, f, pkg, u.Elem()),
+				X:  populateValue(f, pkg, u.Elem()),
 			}
 		}
 	case *types.Interface:
diff --git a/internal/lsp/analysis/undeclaredname/undeclared.go b/internal/lsp/analysis/undeclaredname/undeclared.go
index 22b552c374d..faa14091aee 100644
--- a/internal/lsp/analysis/undeclaredname/undeclared.go
+++ b/internal/lsp/analysis/undeclaredname/undeclared.go
@@ -268,7 +268,7 @@ func newFunctionDeclaration(path []ast.Node, file *ast.File, pkg *types.Package,
 			Names: []*ast.Ident{
 				ast.NewIdent(name),
 			},
-			Type: analysisinternal.TypeExpr(fset, file, pkg, paramTypes[i]),
+			Type: analysisinternal.TypeExpr(file, pkg, paramTypes[i]),
 		})
 	}
 
diff --git a/internal/lsp/source/extract.go b/internal/lsp/source/extract.go
index 1272ff37fe5..90999d821a6 100644
--- a/internal/lsp/source/extract.go
+++ b/internal/lsp/source/extract.go
@@ -330,7 +330,7 @@ func extractFunctionMethod(fset *token.FileSet, rng span.Range, src []byte, file
 			// The blank identifier is always a local variable
 			continue
 		}
-		typ := analysisinternal.TypeExpr(fset, file, pkg, v.obj.Type())
+		typ := analysisinternal.TypeExpr(file, pkg, v.obj.Type())
 		if typ == nil {
 			return nil, fmt.Errorf("nil AST expression for type: %v", v.obj.Name())
 		}
@@ -1130,7 +1130,7 @@ func generateReturnInfo(enclosing *ast.FuncType, pkg *types.Package, path []ast.
 				return nil, nil, fmt.Errorf(
 					"failed type conversion, AST expression: %T", field.Type)
 			}
-			expr := analysisinternal.TypeExpr(fset, file, pkg, typ)
+			expr := analysisinternal.TypeExpr(file, pkg, typ)
 			if expr == nil {
 				return nil, nil, fmt.Errorf("nil AST expression")
 			}
@@ -1138,10 +1138,9 @@ func generateReturnInfo(enclosing *ast.FuncType, pkg *types.Package, path []ast.
 			name, idx = generateAvailableIdentifier(pos, file,
 				path, info, "returnValue", idx)
 			retVars = append(retVars, &returnVariable{
-				name: ast.NewIdent(name),
-				decl: &ast.Field{Type: expr},
-				zeroVal: analysisinternal.ZeroValue(
-					fset, file, pkg, typ),
+				name:    ast.NewIdent(name),
+				decl:    &ast.Field{Type: expr},
+				zeroVal: analysisinternal.ZeroValue(file, pkg, typ),
 			})
 		}
 	}
@@ -1170,7 +1169,7 @@ func adjustReturnStatements(returnTypes []*ast.Field, seenVars map[types.Object]
 			if typ != returnType.Type {
 				continue
 			}
-			val = analysisinternal.ZeroValue(fset, file, pkg, obj.Type())
+			val = analysisinternal.ZeroValue(file, pkg, obj.Type())
 			break
 		}
 		if val == nil {
diff --git a/internal/lsp/source/stub.go b/internal/lsp/source/stub.go
index f09bbab03ef..0d1981795f2 100644
--- a/internal/lsp/source/stub.go
+++ b/internal/lsp/source/stub.go
@@ -19,6 +19,7 @@ import (
 	"golang.org/x/tools/go/ast/astutil"
 	"golang.org/x/tools/internal/lsp/analysis/stubmethods"
 	"golang.org/x/tools/internal/lsp/protocol"
+	"golang.org/x/tools/internal/lsp/safetoken"
 	"golang.org/x/tools/internal/span"
 	"golang.org/x/tools/internal/typeparams"
 )
@@ -57,8 +58,8 @@ func stubSuggestedFixFunc(ctx context.Context, snapshot Snapshot, fh VersionedFi
 	if err != nil {
 		return nil, fmt.Errorf("error reading concrete file source: %w", err)
 	}
-	insertPos := snapshot.FileSet().Position(nodes[1].End()).Offset
-	if insertPos >= len(concreteSrc) {
+	insertPos, err := safetoken.Offset(parsedConcreteFile.Tok, nodes[1].End())
+	if err != nil || insertPos >= len(concreteSrc) {
 		return nil, fmt.Errorf("insertion position is past the end of the file")
 	}
 	var buf bytes.Buffer
diff --git a/internal/lsp/source/util.go b/internal/lsp/source/util.go
index 06720d7085b..9cb2ee69482 100644
--- a/internal/lsp/source/util.go
+++ b/internal/lsp/source/util.go
@@ -113,15 +113,11 @@ func IsGenerated(ctx context.Context, snapshot Snapshot, uri span.URI) bool {
 	if err != nil {
 		return false
 	}
-	tok := snapshot.FileSet().File(pgf.File.Pos())
-	if tok == nil {
-		return false
-	}
 	for _, commentGroup := range pgf.File.Comments {
 		for _, comment := range commentGroup.List {
 			if matched := generatedRx.MatchString(comment.Text); matched {
 				// Check if comment is at the beginning of the line in source.
-				if pos := tok.Position(comment.Slash); pos.Column == 1 {
+				if pgf.Tok.Position(comment.Slash).Column == 1 {
 					return true
 				}
 			}