gopls: add fill switch cases code action

gopls/internal/analysis/fillswitch/fillswitch.go

@@ -0,0 +1,350 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fillswitch defines an Analyzer that automatically
+// fills the missing cases in type switches or switches over named types.
+//
+// The analyzer's diagnostic is merely a prompt.
+// The actual fix is created by a separate direct call from gopls to
+// the SuggestedFixes function.
+// Tests of Analyzer.Run can be found in ./testdata/src.
+// Tests of the SuggestedFixes logic live in ../../testdata/fillswitch.
+package fillswitch
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"go/ast"
+	"go/token"
+	"go/types"
+	"slices"
+	"strings"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/ast/astutil"
+	"golang.org/x/tools/go/ast/inspector"
+	"golang.org/x/tools/gopls/internal/cache"
+	"golang.org/x/tools/gopls/internal/cache/parsego"
+)
+
+const FixCategory = "fillswitch" // recognized by gopls ApplyFix
+
+// errNoSuggestedFix is returned when no suggested fix is available. This could
+// be because all cases are already covered, or (in the case of a type switch)
+// because the remaining cases are for types not accessible by the current
+// package.
+var errNoSuggestedFix = errors.New("no suggested fix")
+
+// Diagnose computes diagnostics for switch statements with missing cases
+// overlapping with the provided start and end position.
+//
+// The diagnostic contains a lazy fix; the actual patch is computed
+// (via the ApplyFix command) by a call to [SuggestedFix].
+//
+// If either start or end is invalid, the entire package is inspected.
+func Diagnose(inspect *inspector.Inspector, start, end token.Pos, pkg *types.Package, info *types.Info) []analysis.Diagnostic {
+	var diags []analysis.Diagnostic
+	nodeFilter := []ast.Node{(*ast.SwitchStmt)(nil), (*ast.TypeSwitchStmt)(nil)}
+	inspect.Preorder(nodeFilter, func(n ast.Node) {
+		if expr, ok := n.(*ast.SwitchStmt); ok {
+			if (start.IsValid() && expr.End() < start) || (end.IsValid() && expr.Pos() > end) {
+				return // non-overlapping
+			}
+
+			if defaultHandled(expr.Body) {
+				return
+			}
+
+			namedType, err := namedTypeFromSwitch(expr, info)
+			if err != nil {
+				return
+			}
+
+			if _, err := suggestedFixSwitch(expr, pkg, info); err != nil {
+				return
+			}
+
+			diags = append(diags, analysis.Diagnostic{
+				Message:  "Switch has missing cases",
+				Pos:      expr.Pos(),
+				End:      expr.End(),
+				Category: FixCategory,
+				SuggestedFixes: []analysis.SuggestedFix{{
+					Message: fmt.Sprintf("Add cases for %s", namedType.Obj().Name()),
+					// No TextEdits => computed later by gopls.
+				}},
+			})
+		}
+
+		if expr, ok := n.(*ast.TypeSwitchStmt); ok {
+			if (start.IsValid() && expr.End() < start) || (end.IsValid() && expr.Pos() > end) {
+				return // non-overlapping
+			}
+
+			if defaultHandled(expr.Body) {
+				return
+			}
+
+			namedType, err := namedTypeFromTypeSwitch(expr, info)
+			if err != nil {
+				return
+			}
+
+			if _, err := suggestedFixTypeSwitch(expr, pkg, info); err != nil {
+				return
+			}
+
+			diags = append(diags, analysis.Diagnostic{
+				Message:  "Switch has missing cases",
+				Pos:      expr.Pos(),
+				End:      expr.End(),
+				Category: FixCategory,
+				SuggestedFixes: []analysis.SuggestedFix{{
+					Message: fmt.Sprintf("Add cases for %v", namedType.Obj().Name()),
+					// No TextEdits => computed later by gopls.
+				}},
+			})
+		}
+	})
+
+	return diags
+}
+
+func suggestedFixTypeSwitch(stmt *ast.TypeSwitchStmt, pkg *types.Package, info *types.Info) (*analysis.SuggestedFix, error) {
+	namedType, err := namedTypeFromTypeSwitch(stmt, info)
+	if err != nil {
+		return nil, err
+	}
+
+	scope := namedType.Obj().Pkg().Scope()
+	variants := make([]string, 0)
+	for _, name := range scope.Names() {
+		obj := scope.Lookup(name)
+		if _, ok := obj.(*types.TypeName); !ok {
+			continue
+		}
+
+		if types.Identical(obj.Type(), namedType.Obj().Type()) {
+			continue
+		}
+
+		if types.AssignableTo(obj.Type(), namedType.Obj().Type()) {
+			if obj.Pkg().Name() != pkg.Name() {
+				if !obj.Exported() {
+					continue
+				}
+
+				variants = append(variants, obj.Pkg().Name()+"."+obj.Name())
+			} else {
+				variants = append(variants, obj.Name())
+			}
+		} else if types.AssignableTo(types.NewPointer(obj.Type()), namedType.Obj().Type()) {
+			if obj.Pkg().Name() != pkg.Name() {
+				if !obj.Exported() {
+					continue
+				}
+
+				variants = append(variants, "*"+obj.Pkg().Name()+"."+obj.Name())
+			} else {
+				variants = append(variants, "*"+obj.Name())
+			}
+		}
+	}
+
+	handledVariants := getHandledVariants(stmt.Body)
+	if len(variants) == 0 || len(variants) == len(handledVariants) {
+		return nil, errNoSuggestedFix
+	}
+
+	newText := buildNewText(variants, handledVariants)
+	return &analysis.SuggestedFix{
+		Message: fmt.Sprintf("Add cases for %s", namedType.Obj().Name()),
+		TextEdits: []analysis.TextEdit{{
+			Pos:     stmt.End() - 1,
+			End:     stmt.End() - 1,
+			NewText: indent([]byte(newText), []byte{'\t'}),
+		}},
+	}, nil
+}
+
+func suggestedFixSwitch(stmt *ast.SwitchStmt, pkg *types.Package, info *types.Info) (*analysis.SuggestedFix, error) {
+	namedType, err := namedTypeFromSwitch(stmt, info)
+	if err != nil {
+		return nil, err
+	}
+
+	scope := namedType.Obj().Pkg().Scope()
+	variants := make([]string, 0)
+	for _, name := range scope.Names() {
+		obj := scope.Lookup(name)
+		if obj.Id() == namedType.Obj().Id() {
+			continue
+		}
+
+		if types.Identical(obj.Type(), namedType.Obj().Type()) {
+			// TODO: comparing the package name like this feels wrong, is it?
+			if obj.Pkg().Name() != pkg.Name() {
+				if !obj.Exported() {
+					continue
+				}
+
+				variants = append(variants, obj.Pkg().Name()+"."+obj.Name())
+			} else {
+				variants = append(variants, obj.Name())
+			}
+		}
+	}
+
+	handledVariants := getHandledVariants(stmt.Body)
+	if len(variants) == 0 || len(variants) == len(handledVariants) {
+		return nil, errNoSuggestedFix
+	}
+
+	newText := buildNewText(variants, handledVariants)
+	return &analysis.SuggestedFix{
+		Message: fmt.Sprintf("Add cases for %s", namedType.Obj().Name()),
+		TextEdits: []analysis.TextEdit{{
+			Pos:     stmt.End() - 1,
+			End:     stmt.End() - 1,
+			NewText: indent([]byte(newText), []byte{'\t'}),
+		}},
+	}, nil
+}
+
+func namedTypeFromSwitch(stmt *ast.SwitchStmt, info *types.Info) (*types.Named, error) {
+	typ := info.TypeOf(stmt.Tag)
+	if typ == nil {
+		return nil, errors.New("expected switch statement to have a tag")
+	}
+
+	namedType, ok := typ.(*types.Named)
+	if !ok {
+		return nil, errors.New("switch statement is not on a named type")
+	}
+
+	return namedType, nil
+}
+
+func namedTypeFromTypeSwitch(stmt *ast.TypeSwitchStmt, info *types.Info) (*types.Named, error) {
+	switch s := stmt.Assign.(type) {
+	case *ast.ExprStmt:
+		typ := s.X.(*ast.TypeAssertExpr)
+		namedType, ok := info.TypeOf(typ.X).(*types.Named)
+		if !ok {
+			return nil, errors.New("type switch expression is not on a named type")
+		}
+
+		return namedType, nil
+	case *ast.AssignStmt:
+		for _, expr := range s.Rhs {
+			typ, ok := expr.(*ast.TypeAssertExpr)
+			if !ok {
+				continue
+			}
+
+			namedType, ok := info.TypeOf(typ.X).(*types.Named)
+			if !ok {
+				continue
+			}
+
+			return namedType, nil
+		}
+
+		return nil, errors.New("expected type switch expression to have a named type")
+	default:
+		return nil, errors.New("node is not a type switch statement")
+	}
+}
+
+func defaultHandled(body *ast.BlockStmt) bool {
+	for _, bl := range body.List {
+		if len(bl.(*ast.CaseClause).List) == 0 {
+			return true
+		}
+	}
+
+	return false
+}
+
+func buildNewText(variants []string, handledVariants []string) string {
+	var textBuilder strings.Builder
+	for _, c := range variants {
+		if slices.Contains(handledVariants, c) {
+			continue
+		}
+
+		textBuilder.WriteString("case ")
+		textBuilder.WriteString(c)
+		textBuilder.WriteString(":\n")
+	}
+
+	return textBuilder.String()
+}
+
+func getHandledVariants(body *ast.BlockStmt) []string {
+	out := make([]string, 0)
+	for _, bl := range body.List {
+		for _, c := range bl.(*ast.CaseClause).List {
+			switch v := c.(type) {
+			case *ast.Ident:
+				out = append(out, v.Name)
+			case *ast.SelectorExpr:
+				out = append(out, v.X.(*ast.Ident).Name+"."+v.Sel.Name)
+			case *ast.StarExpr:
+				out = append(out, "*"+v.X.(*ast.Ident).Name)
+			}
+		}
+	}
+
+	return out
+}
+
+// SuggestedFix computes the suggested fix for the kinds of
+// diagnostics produced by the Analyzer above.
+func SuggestedFix(ctx context.Context, snapshot *cache.Snapshot, pkg *cache.Package, pgf *parsego.File, start, end token.Pos) (*token.FileSet, *analysis.SuggestedFix, error) {
+	pos := start // don't use the end
+	path, _ := astutil.PathEnclosingInterval(pgf.File, pos, pos)
+	if len(path) < 2 {
+		return nil, nil, fmt.Errorf("no expression found")
+	}
+
+	switch stmt := path[0].(type) {
+	case *ast.SwitchStmt:
+		fix, err := suggestedFixSwitch(stmt, pkg.GetTypes(), pkg.GetTypesInfo())
+		if err != nil {
+			return nil, nil, err
+		}
+
+		return pkg.FileSet(), fix, nil
+	case *ast.TypeSwitchStmt:
+		fix, err := suggestedFixTypeSwitch(stmt, pkg.GetTypes(), pkg.GetTypesInfo())
+		if err != nil {
+			return nil, nil, err
+		}
+
+		return pkg.FileSet(), fix, nil
+	default:
+		return nil, nil, fmt.Errorf("no switch statement found")
+	}
+}
+
+// indent works line by line through str, prefixing each line with
+// prefix.
+func indent(str, prefix []byte) []byte {
+	split := bytes.Split(str, []byte("\n"))
+	newText := bytes.NewBuffer(nil)
+	for i, s := range split {
+		if i != 0 {
+			newText.Write(prefix)
+		}
+
+		newText.Write(s)
+		if i < len(split)-1 {
+			newText.WriteByte('\n')
+		}
+	}
+	return newText.Bytes()
+}

gopls/internal/analysis/fillswitch/fillswitch_test.go

@@ -0,0 +1,38 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fillswitch_test
+
+import (
+	"go/token"
+	"testing"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/analysis/analysistest"
+	"golang.org/x/tools/go/analysis/passes/inspect"
+	"golang.org/x/tools/go/ast/inspector"
+	"golang.org/x/tools/gopls/internal/analysis/fillswitch"
+)
+
+// analyzer allows us to test the fillswitch code action using the analysistest
+// harness. (fillswitch used to be a gopls analyzer.)
+var analyzer = &analysis.Analyzer{
+	Name:     "fillswitch",
+	Doc:      "test only",
+	Requires: []*analysis.Analyzer{inspect.Analyzer},
+	Run: func(pass *analysis.Pass) (any, error) {
+		inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
+		for _, d := range fillswitch.Diagnose(inspect, token.NoPos, token.NoPos, pass.Pkg, pass.TypesInfo) {
+			pass.Report(d)
+		}
+		return nil, nil
+	},
+	URL:              "https://pkg.go.dev/golang.org/x/tools/gopls/internal/analysis/fillswitch",
+	RunDespiteErrors: true,
+}
+
+func Test(t *testing.T) {
+	testdata := analysistest.TestData()
+	analysistest.Run(t, testdata, analyzer, "a")
+}