gopls/internal/analysis: add fill switch cases code action

This PR adds a code action to fill missing cases on type switches and
switches on named types.

Rules are defined here: golang/go#65411 (comment).

Edit: I added some tests, but I'm sure there are still things to fix so
sharing to get some feedback.

Fixes golang/go#65411

https://github.com/golang/tools/assets/4250565/1e67c404-e24f-478e-a3df-60a3adfaa9b1

Change-Id: Ie4ef0955d0e7ca130af8980a488b738c812aae4d
GitHub-Last-Rev: a04dc69
GitHub-Pull-Request: #476
Reviewed-on: https://go-review.googlesource.com/c/tools/+/561416
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

gopls/internal/analysis/fillswitch/doc.go

@@ -0,0 +1,66 @@
+// Copyright 2024 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 identifies switches with missing cases.
+//
+// It reports a diagnostic for each type switch or 'enum' switch that
+// has missing cases, and suggests a fix to fill them in.
+//
+// The possible cases are: for a type switch, each accessible named
+// type T or pointer *T that is assignable to the interface type; and
+// for an 'enum' switch, each accessible named constant of the same
+// type as the switch value.
+//
+// For an 'enum' switch, it will suggest cases for all possible values of the
+// type.
+//
+//	type Suit int8
+//	const (
+//		 Spades Suit = iota
+//		 Hearts
+//		 Diamonds
+//		 Clubs
+//	)
+//
+//	var s Suit
+//	switch s {
+//	case Spades:
+//	}
+//
+// It will report a diagnostic with a suggested fix to fill in the remaining
+// cases:
+//
+//	var s Suit
+//	switch s {
+//	case Spades:
+//	case Hearts:
+//	case Diamonds:
+//	case Clubs:
+//	default:
+//		 panic(fmt.Sprintf("unexpected Suit: %v", s))
+//	}
+//
+// For a type switch, it will suggest cases for all types that implement the
+// interface.
+//
+//	var stmt ast.Stmt
+//	switch stmt.(type) {
+//	case *ast.IfStmt:
+//	}
+//
+// It will report a diagnostic with a suggested fix to fill in the remaining
+// cases:
+//
+//	var stmt ast.Stmt
+//	switch stmt.(type) {
+//	case *ast.IfStmt:
+//	case *ast.ForStmt:
+//	case *ast.RangeStmt:
+//	case *ast.AssignStmt:
+//	case *ast.GoStmt:
+//	...
+//	default:
+//		 panic(fmt.Sprintf("unexpected ast.Stmt: %T", stmt))
+//	}
+package fillswitch

gopls/internal/analysis/fillswitch/fillswitch.go

@@ -0,0 +1,301 @@
+// Copyright 2024 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
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/token"
+	"go/types"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/ast/inspector"
+)
+
+// Diagnose computes diagnostics for switch statements with missing cases
+// overlapping with the provided start and end position.
+//
+// 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 start.IsValid() && n.End() < start ||
+			end.IsValid() && n.Pos() > end {
+			return // non-overlapping
+		}
+
+		var fix *analysis.SuggestedFix
+		switch n := n.(type) {
+		case *ast.SwitchStmt:
+			fix = suggestedFixSwitch(n, pkg, info)
+		case *ast.TypeSwitchStmt:
+			fix = suggestedFixTypeSwitch(n, pkg, info)
+		}
+
+		if fix == nil {
+			return
+		}
+
+		diags = append(diags, analysis.Diagnostic{
+			Message:        fix.Message,
+			Pos:            n.Pos(),
+			End:            n.Pos() + token.Pos(len("switch")),
+			SuggestedFixes: []analysis.SuggestedFix{*fix},
+		})
+	})
+
+	return diags
+}
+
+func suggestedFixTypeSwitch(stmt *ast.TypeSwitchStmt, pkg *types.Package, info *types.Info) *analysis.SuggestedFix {
+	if hasDefaultCase(stmt.Body) {
+		return nil
+	}
+
+	namedType := namedTypeFromTypeSwitch(stmt, info)
+	if namedType == nil {
+		return nil
+	}
+
+	existingCases := caseTypes(stmt.Body, info)
+	// Gather accessible package-level concrete types
+	// that implement the switch interface type.
+	scope := namedType.Obj().Pkg().Scope()
+	var buf bytes.Buffer
+	for _, name := range scope.Names() {
+		obj := scope.Lookup(name)
+		if tname, ok := obj.(*types.TypeName); !ok || tname.IsAlias() {
+			continue // not a defined type
+		}
+
+		if types.IsInterface(obj.Type()) {
+			continue
+		}
+
+		samePkg := obj.Pkg() == pkg
+		if !samePkg && !obj.Exported() {
+			continue // inaccessible
+		}
+
+		var key caseType
+		if types.AssignableTo(obj.Type(), namedType.Obj().Type()) {
+			key.named = obj.Type().(*types.Named)
+		} else if ptr := types.NewPointer(obj.Type()); types.AssignableTo(ptr, namedType.Obj().Type()) {
+			key.named = obj.Type().(*types.Named)
+			key.ptr = true
+		}
+
+		if key.named != nil {
+			if existingCases[key] {
+				continue
+			}
+
+			if buf.Len() > 0 {
+				buf.WriteString("\t")
+			}
+
+			buf.WriteString("case ")
+			if key.ptr {
+				buf.WriteByte('*')
+			}
+
+			if p := key.named.Obj().Pkg(); p != pkg {
+				// TODO: use the correct package name when the import is renamed
+				buf.WriteString(p.Name())
+				buf.WriteByte('.')
+			}
+			buf.WriteString(key.named.Obj().Name())
+			buf.WriteString(":\n")
+		}
+	}
+
+	if buf.Len() == 0 {
+		return nil
+	}
+
+	switch assign := stmt.Assign.(type) {
+	case *ast.AssignStmt:
+		addDefaultCase(&buf, namedType, assign.Lhs[0])
+	case *ast.ExprStmt:
+		if assert, ok := assign.X.(*ast.TypeAssertExpr); ok {
+			addDefaultCase(&buf, namedType, assert.X)
+		}
+	}
+
+	return &analysis.SuggestedFix{
+		Message: fmt.Sprintf("Add cases for %s", namedType.Obj().Name()),
+		TextEdits: []analysis.TextEdit{{
+			Pos:     stmt.End() - token.Pos(len("}")),
+			End:     stmt.End() - token.Pos(len("}")),
+			NewText: buf.Bytes(),
+		}},
+	}
+}
+
+func suggestedFixSwitch(stmt *ast.SwitchStmt, pkg *types.Package, info *types.Info) *analysis.SuggestedFix {
+	if hasDefaultCase(stmt.Body) {
+		return nil
+	}
+
+	namedType, ok := info.TypeOf(stmt.Tag).(*types.Named)
+	if !ok {
+		return nil
+	}
+
+	existingCases := caseConsts(stmt.Body, info)
+	// Gather accessible named constants of the same type as the switch value.
+	scope := namedType.Obj().Pkg().Scope()
+	var buf bytes.Buffer
+	for _, name := range scope.Names() {
+		obj := scope.Lookup(name)
+		if c, ok := obj.(*types.Const); ok &&
+			(obj.Pkg() == pkg || obj.Exported()) && // accessible
+			types.Identical(obj.Type(), namedType.Obj().Type()) &&
+			!existingCases[c] {
+
+			if buf.Len() > 0 {
+				buf.WriteString("\t")
+			}
+
+			buf.WriteString("case ")
+			if c.Pkg() != pkg {
+				buf.WriteString(c.Pkg().Name())
+				buf.WriteByte('.')
+			}
+			buf.WriteString(c.Name())
+			buf.WriteString(":\n")
+		}
+	}
+
+	if buf.Len() == 0 {
+		return nil
+	}
+
+	addDefaultCase(&buf, namedType, stmt.Tag)
+
+	return &analysis.SuggestedFix{
+		Message: fmt.Sprintf("Add cases for %s", namedType.Obj().Name()),
+		TextEdits: []analysis.TextEdit{{
+			Pos:     stmt.End() - token.Pos(len("}")),
+			End:     stmt.End() - token.Pos(len("}")),
+			NewText: buf.Bytes(),
+		}},
+	}
+}
+
+func addDefaultCase(buf *bytes.Buffer, named *types.Named, expr ast.Expr) {
+	var dottedBuf bytes.Buffer
+	// writeDotted emits a dotted path a.b.c.
+	var writeDotted func(e ast.Expr) bool
+	writeDotted = func(e ast.Expr) bool {
+		switch e := e.(type) {
+		case *ast.SelectorExpr:
+			if !writeDotted(e.X) {
+				return false
+			}
+			dottedBuf.WriteByte('.')
+			dottedBuf.WriteString(e.Sel.Name)
+			return true
+		case *ast.Ident:
+			dottedBuf.WriteString(e.Name)
+			return true
+		}
+		return false
+	}
+
+	buf.WriteString("\tdefault:\n")
+	typeName := fmt.Sprintf("%s.%s", named.Obj().Pkg().Name(), named.Obj().Name())
+	if writeDotted(expr) {
+		// Switch tag expression is a dotted path.
+		// It is safe to re-evaluate it in the default case.
+		format := fmt.Sprintf("unexpected %s: %%#v", typeName)
+		fmt.Fprintf(buf, "\t\tpanic(fmt.Sprintf(%q, %s))\n\t", format, dottedBuf.String())
+	} else {
+		// Emit simpler message, without re-evaluating tag expression.
+		fmt.Fprintf(buf, "\t\tpanic(%q)\n\t", "unexpected "+typeName)
+	}
+}
+
+func namedTypeFromTypeSwitch(stmt *ast.TypeSwitchStmt, info *types.Info) *types.Named {
+	switch assign := stmt.Assign.(type) {
+	case *ast.ExprStmt:
+		if typ, ok := assign.X.(*ast.TypeAssertExpr); ok {
+			if named, ok := info.TypeOf(typ.X).(*types.Named); ok {
+				return named
+			}
+		}
+
+	case *ast.AssignStmt:
+		if typ, ok := assign.Rhs[0].(*ast.TypeAssertExpr); ok {
+			if named, ok := info.TypeOf(typ.X).(*types.Named); ok {
+				return named
+			}
+		}
+	}
+
+	return nil
+}
+
+func hasDefaultCase(body *ast.BlockStmt) bool {
+	for _, clause := range body.List {
+		if len(clause.(*ast.CaseClause).List) == 0 {
+			return true
+		}
+	}
+
+	return false
+}
+
+func caseConsts(body *ast.BlockStmt, info *types.Info) map[*types.Const]bool {
+	out := map[*types.Const]bool{}
+	for _, stmt := range body.List {
+		for _, e := range stmt.(*ast.CaseClause).List {
+			if info.Types[e].Value == nil {
+				continue // not a constant
+			}
+
+			if sel, ok := e.(*ast.SelectorExpr); ok {
+				e = sel.Sel // replace pkg.C with C
+			}
+
+			if e, ok := e.(*ast.Ident); ok {
+				if c, ok := info.Uses[e].(*types.Const); ok {
+					out[c] = true
+				}
+			}
+		}
+	}
+
+	return out
+}
+
+type caseType struct {
+	named *types.Named
+	ptr   bool
+}
+
+func caseTypes(body *ast.BlockStmt, info *types.Info) map[caseType]bool {
+	out := map[caseType]bool{}
+	for _, stmt := range body.List {
+		for _, e := range stmt.(*ast.CaseClause).List {
+			if tv, ok := info.Types[e]; ok && tv.IsType() {
+				t := tv.Type
+				ptr := false
+				if p, ok := t.(*types.Pointer); ok {
+					t = p.Elem()
+					ptr = true
+				}
+
+				if named, ok := t.(*types.Named); ok {
+					out[caseType{named, ptr}] = true
+				}
+			}
+		}
+	}
+
+	return out
+}