writer.go

		w.assignList(namesAsExpr(decl.NameList))
	}
}

func (w *writer) blockStmt(stmt *syntax.BlockStmt) {
	w.sync(syncBlockStmt)
	w.openScope(stmt.Pos())
	w.stmts(stmt.List)
	w.closeScope(stmt.Rbrace)
}

func (w *writer) forStmt(stmt *syntax.ForStmt) {
	w.sync(syncForStmt)
	w.openScope(stmt.Pos())

	if rang, ok := stmt.Init.(*syntax.RangeClause); w.bool(ok) {
		w.pos(rang)
		w.expr(rang.X)
		w.assignList(rang.Lhs)
	} else {
		w.pos(stmt)
		w.stmt(stmt.Init)
		w.expr(stmt.Cond)
		w.stmt(stmt.Post)
	}

	w.blockStmt(stmt.Body)
	w.closeAnotherScope()
}

func (w *writer) ifStmt(stmt *syntax.IfStmt) {
	w.sync(syncIfStmt)
	w.openScope(stmt.Pos())
	w.pos(stmt)
	w.stmt(stmt.Init)
	w.expr(stmt.Cond)
	w.blockStmt(stmt.Then)
	w.stmt(stmt.Else)
	w.closeAnotherScope()
}

func (w *writer) selectStmt(stmt *syntax.SelectStmt) {
	w.sync(syncSelectStmt)

	w.pos(stmt)
	w.len(len(stmt.Body))
	for i, clause := range stmt.Body {
		if i > 0 {
			w.closeScope(clause.Pos())
		}
		w.openScope(clause.Pos())

		w.pos(clause)
		w.stmt(clause.Comm)
		w.stmts(clause.Body)
	}
	if len(stmt.Body) > 0 {
		w.closeScope(stmt.Rbrace)
	}
}

func (w *writer) switchStmt(stmt *syntax.SwitchStmt) {
	w.sync(syncSwitchStmt)

	w.openScope(stmt.Pos())
	w.pos(stmt)
	w.stmt(stmt.Init)

	if guard, ok := stmt.Tag.(*syntax.TypeSwitchGuard); w.bool(ok) {
		w.pos(guard)
		if tag := guard.Lhs; w.bool(tag != nil) {
			w.pos(tag)
			w.string(tag.Value)
		}
		w.expr(guard.X)
	} else {
		w.expr(stmt.Tag)
	}

	w.len(len(stmt.Body))
	for i, clause := range stmt.Body {
		if i > 0 {
			w.closeScope(clause.Pos())
		}
		w.openScope(clause.Pos())

		w.pos(clause)
		w.exprList(clause.Cases)

		if obj, ok := w.p.info.Implicits[clause]; ok {
			// TODO(mdempsky): These pos details are quirkish, but also
			// necessary so the variable's position is correct for DWARF
			// scope assignment later. It would probably be better for us to
			// instead just set the variable's DWARF scoping info earlier so
			// we can give it the correct position information.
			pos := clause.Pos()
			if typs := unpackListExpr(clause.Cases); len(typs) != 0 {
				pos = typeExprEndPos(typs[len(typs)-1])
			}
			w.pos(pos)

			obj := obj.(*types2.Var)
			w.typ(obj.Type())
			w.addLocal(obj)
		}

		w.stmts(clause.Body)
	}
	if len(stmt.Body) > 0 {
		w.closeScope(stmt.Rbrace)
	}

	w.closeScope(stmt.Rbrace)
}

func (w *writer) label(label *syntax.Name) {
	w.sync(syncLabel)

	// TODO(mdempsky): Replace label strings with dense indices.
	w.string(label.Value)
}

func (w *writer) optLabel(label *syntax.Name) {
	w.sync(syncOptLabel)
	if w.bool(label != nil) {
		w.label(label)
	}
}

// @@@ Expressions

func (w *writer) expr(expr syntax.Expr) {
	expr = unparen(expr) // skip parens; unneeded after typecheck

	obj, inst := lookupObj(w.p.info, expr)
	targs := inst.TypeArgs

	if tv, ok := w.p.info.Types[expr]; ok {
		// TODO(mdempsky): Be more judicious about which types are marked as "needed".
		if inst.Type != nil {
			w.needType(inst.Type)
		} else {
			w.needType(tv.Type)
		}

		if tv.IsType() {
			w.code(exprType)
			w.typ(tv.Type)
			return
		}

		if tv.Value != nil {
			w.code(exprConst)
			w.pos(expr.Pos())
			w.typ(tv.Type)
			w.value(tv.Value)

			// TODO(mdempsky): These details are only important for backend
			// diagnostics. Explore writing them out separately.
			w.op(constExprOp(expr))
			w.string(syntax.String(expr))
			return
		}
	}

	if obj != nil {
		if isGlobal(obj) {
			w.code(exprName)
			w.obj(obj, targs)
			return
		}

		obj := obj.(*types2.Var)
		assert(!obj.IsField())
		assert(targs.Len() == 0)

		w.code(exprLocal)
		w.useLocal(expr.Pos(), obj)
		return
	}

	switch expr := expr.(type) {
	default:
		w.p.unexpected("expression", expr)

	case nil: // absent slice index, for condition, or switch tag
		w.code(exprNone)

	case *syntax.Name:
		assert(expr.Value == "_")
		w.code(exprBlank)

	case *syntax.CompositeLit:
		w.code(exprCompLit)
		w.compLit(expr)

	case *syntax.FuncLit:
		w.code(exprFuncLit)
		w.funcLit(expr)

	case *syntax.SelectorExpr:
		sel, ok := w.p.info.Selections[expr]
		assert(ok)

		w.code(exprSelector)
		w.expr(expr.X)
		w.pos(expr)
		w.selector(sel.Obj())

	case *syntax.IndexExpr:
		tv, ok := w.p.info.Types[expr.Index]
		assert(ok && tv.IsValue())

		w.code(exprIndex)
		w.expr(expr.X)
		w.pos(expr)
		w.expr(expr.Index)

	case *syntax.SliceExpr:
		w.code(exprSlice)
		w.expr(expr.X)
		w.pos(expr)
		for _, n := range &expr.Index {
			w.expr(n)
		}

	case *syntax.AssertExpr:
		w.code(exprAssert)
		w.expr(expr.X)
		w.pos(expr)
		w.expr(expr.Type)

	case *syntax.Operation:
		if expr.Y == nil {
			w.code(exprUnaryOp)
			w.op(unOps[expr.Op])
			w.pos(expr)
			w.expr(expr.X)
			break
		}

		w.code(exprBinaryOp)
		w.op(binOps[expr.Op])
		w.expr(expr.X)
		w.pos(expr)
		w.expr(expr.Y)

	case *syntax.CallExpr:
		tv, ok := w.p.info.Types[expr.Fun]
		assert(ok)
		if tv.IsType() {
			assert(len(expr.ArgList) == 1)
			assert(!expr.HasDots)

			w.code(exprConvert)
			w.typ(tv.Type)
			w.pos(expr)
			w.expr(expr.ArgList[0])
			break
		}

		writeFunExpr := func() {
			if selector, ok := unparen(expr.Fun).(*syntax.SelectorExpr); ok {
				if sel, ok := w.p.info.Selections[selector]; ok && sel.Kind() == types2.MethodVal {
					w.expr(selector.X)
					w.bool(true) // method call
					w.pos(selector)
					w.selector(sel.Obj())
					return
				}
			}

			w.expr(expr.Fun)
			w.bool(false) // not a method call (i.e., normal function call)
		}

		w.code(exprCall)
		writeFunExpr()
		w.pos(expr)
		w.exprs(expr.ArgList)
		w.bool(expr.HasDots)
	}
}

func (w *writer) compLit(lit *syntax.CompositeLit) {
	tv, ok := w.p.info.Types[lit]
	assert(ok)

	w.sync(syncCompLit)
	w.pos(lit)
	w.typ(tv.Type)

	typ := tv.Type
	if ptr, ok := types2.CoreType(typ).(*types2.Pointer); ok {
		typ = ptr.Elem()
	}
	str, isStruct := types2.CoreType(typ).(*types2.Struct)

	w.len(len(lit.ElemList))
	for i, elem := range lit.ElemList {
		if isStruct {
			if kv, ok := elem.(*syntax.KeyValueExpr); ok {
				// use position of expr.Key rather than of elem (which has position of ':')
				w.pos(kv.Key)
				w.len(fieldIndex(w.p.info, str, kv.Key.(*syntax.Name)))
				elem = kv.Value
			} else {
				w.pos(elem)
				w.len(i)
			}
		} else {
			if kv, ok := elem.(*syntax.KeyValueExpr); w.bool(ok) {
				// use position of expr.Key rather than of elem (which has position of ':')
				w.pos(kv.Key)
				w.expr(kv.Key)
				elem = kv.Value
			}
		}
		w.pos(elem)
		w.expr(elem)
	}
}

func (w *writer) funcLit(expr *syntax.FuncLit) {
	tv, ok := w.p.info.Types[expr]
	assert(ok)
	sig := tv.Type.(*types2.Signature)

	body, closureVars := w.p.bodyIdx(w.p.curpkg, sig, expr.Body, w.dict)

	w.sync(syncFuncLit)
	w.pos(expr)
	w.signature(sig)

	w.len(len(closureVars))
	for _, cv := range closureVars {
		w.pos(cv.pos)
		w.useLocal(cv.pos, cv.obj)
	}

	w.reloc(relocBody, body)
}

type posObj struct {
	pos syntax.Pos
	obj *types2.Var
}

func (w *writer) exprList(expr syntax.Expr) {
	w.sync(syncExprList)
	w.exprs(unpackListExpr(expr))
}

func (w *writer) exprs(exprs []syntax.Expr) {
	if len(exprs) == 0 {
		assert(exprs == nil)
	}

	w.sync(syncExprs)
	w.len(len(exprs))
	for _, expr := range exprs {
		w.expr(expr)
	}
}

func (w *writer) op(op ir.Op) {
	// TODO(mdempsky): Remove in favor of explicit codes? Would make
	// export data more stable against internal refactorings, but low
	// priority at the moment.
	assert(op != 0)
	w.sync(syncOp)
	w.len(int(op))
}

func (w *writer) needType(typ types2.Type) {
	// Decompose tuple into component element types.
	if typ, ok := typ.(*types2.Tuple); ok {
		for i := 0; i < typ.Len(); i++ {
			w.needType(typ.At(i).Type())
		}
		return
	}

	if info := w.p.typIdx(typ, w.dict); info.derived {
		w.dict.derived[info.idx].needed = true
	}
}

// @@@ Package initialization

// Caution: This code is still clumsy, because toolstash -cmp is
// particularly sensitive to it.

type typeDeclGen struct {
	*syntax.TypeDecl
	gen int

	// Implicit type parameters in scope at this type declaration.
	implicits []*types2.TypeName
}

type fileImports struct {
	importedEmbed, importedUnsafe bool
}

type declCollector struct {
	pw         *pkgWriter
	typegen    *int
	file       *fileImports
	withinFunc bool
	implicits  []*types2.TypeName
}

func (c *declCollector) withTParams(obj types2.Object) *declCollector {
	tparams := objTypeParams(obj)
	n := tparams.Len()
	if n == 0 {
		return c
	}

	copy := *c
	copy.implicits = copy.implicits[:len(copy.implicits):len(copy.implicits)]
	for i := 0; i < n; i++ {
		copy.implicits = append(copy.implicits, tparams.At(i).Obj())
	}
	return &copy
}

func (c *declCollector) Visit(n syntax.Node) syntax.Visitor {
	pw := c.pw

	switch n := n.(type) {
	case *syntax.File:
		pw.checkPragmas(n.Pragma, ir.GoBuildPragma, false)

	case *syntax.ImportDecl:
		pw.checkPragmas(n.Pragma, 0, false)

		switch pkgNameOf(pw.info, n).Imported().Path() {
		case "embed":
			c.file.importedEmbed = true
		case "unsafe":
			c.file.importedUnsafe = true
		}

	case *syntax.ConstDecl:
		pw.checkPragmas(n.Pragma, 0, false)

	case *syntax.FuncDecl:
		pw.checkPragmas(n.Pragma, funcPragmas, false)

		obj := pw.info.Defs[n.Name].(*types2.Func)
		pw.funDecls[obj] = n

		return c.withTParams(obj)

	case *syntax.TypeDecl:
		obj := pw.info.Defs[n.Name].(*types2.TypeName)
		d := typeDeclGen{TypeDecl: n, implicits: c.implicits}

		if n.Alias {
			pw.checkPragmas(n.Pragma, 0, false)
		} else {
			pw.checkPragmas(n.Pragma, typePragmas, false)

			// Assign a unique ID to function-scoped defined types.
			if c.withinFunc {
				*c.typegen++
				d.gen = *c.typegen
			}
		}

		pw.typDecls[obj] = d

		// TODO(mdempsky): Omit? Not strictly necessary; only matters for
		// type declarations within function literals within parameterized
		// type declarations, but types2 the function literals will be
		// constant folded away.
		return c.withTParams(obj)

	case *syntax.VarDecl:
		pw.checkPragmas(n.Pragma, 0, true)

		if p, ok := n.Pragma.(*pragmas); ok && len(p.Embeds) > 0 {
			if err := checkEmbed(n, c.file.importedEmbed, c.withinFunc); err != nil {
				pw.errorf(p.Embeds[0].Pos, "%s", err)
			}
		}

	case *syntax.BlockStmt:
		if !c.withinFunc {
			copy := *c
			copy.withinFunc = true
			return &copy
		}
	}

	return c
}

func (pw *pkgWriter) collectDecls(noders []*noder) {
	var typegen int
	for _, p := range noders {
		var file fileImports

		syntax.Walk(p.file, &declCollector{
			pw:      pw,
			typegen: &typegen,
			file:    &file,
		})

		pw.cgoPragmas = append(pw.cgoPragmas, p.pragcgobuf...)

		for _, l := range p.linknames {
			if !file.importedUnsafe {
				pw.errorf(l.pos, "//go:linkname only allowed in Go files that import \"unsafe\"")
				continue
			}

			switch obj := pw.curpkg.Scope().Lookup(l.local).(type) {
			case *types2.Func, *types2.Var:
				if _, ok := pw.linknames[obj]; !ok {
					pw.linknames[obj] = l.remote
				} else {
					pw.errorf(l.pos, "duplicate //go:linkname for %s", l.local)
				}

			default:
				// TODO(mdempsky): Enable after #42938 is fixed.
				if false {
					pw.errorf(l.pos, "//go:linkname must refer to declared function or variable")
				}
			}
		}
	}
}

func (pw *pkgWriter) checkPragmas(p syntax.Pragma, allowed ir.PragmaFlag, embedOK bool) {
	if p == nil {
		return
	}
	pragma := p.(*pragmas)

	for _, pos := range pragma.Pos {
		if pos.Flag&^allowed != 0 {
			pw.errorf(pos.Pos, "misplaced compiler directive")
		}
	}

	if !embedOK {
		for _, e := range pragma.Embeds {
			pw.errorf(e.Pos, "misplaced go:embed directive")
		}
	}
}

func (w *writer) pkgInit(noders []*noder) {
	w.len(len(w.p.cgoPragmas))
	for _, cgoPragma := range w.p.cgoPragmas {
		w.strings(cgoPragma)
	}

	w.sync(syncDecls)
	for _, p := range noders {
		for _, decl := range p.file.DeclList {
			w.pkgDecl(decl)
		}
	}
	w.code(declEnd)

	w.sync(syncEOF)
}

func (w *writer) pkgDecl(decl syntax.Decl) {
	switch decl := decl.(type) {
	default:
		w.p.unexpected("declaration", decl)

	case *syntax.ImportDecl:

	case *syntax.ConstDecl:
		w.code(declOther)
		w.pkgObjs(decl.NameList...)

	case *syntax.FuncDecl:
		if decl.Name.Value == "_" {
			break // skip blank functions
		}

		obj := w.p.info.Defs[decl.Name].(*types2.Func)
		sig := obj.Type().(*types2.Signature)

		if sig.RecvTypeParams() != nil || sig.TypeParams() != nil {
			break // skip generic functions
		}

		if recv := sig.Recv(); recv != nil {
			w.code(declMethod)
			w.typ(recvBase(recv))
			w.selector(obj)
			break
		}

		w.code(declFunc)
		w.pkgObjs(decl.Name)

	case *syntax.TypeDecl:
		if len(decl.TParamList) != 0 {
			break // skip generic type decls
		}

		if decl.Name.Value == "_" {
			break // skip blank type decls
		}

		name := w.p.info.Defs[decl.Name].(*types2.TypeName)
		// Skip type declarations for interfaces that are only usable as
		// type parameter bounds.
		if iface, ok := name.Type().Underlying().(*types2.Interface); ok && !iface.IsMethodSet() {
			break
		}

		// Skip aliases to uninstantiated generic types.
		// TODO(mdempsky): Revisit after #46477 is resolved.
		if name.IsAlias() {
			named, ok := name.Type().(*types2.Named)
			if ok && named.TypeParams().Len() != 0 && named.TypeArgs().Len() == 0 {
				break
			}
		}

		w.code(declOther)
		w.pkgObjs(decl.Name)

	case *syntax.VarDecl:
		w.code(declVar)
		w.pos(decl)
		w.pkgObjs(decl.NameList...)
		w.exprList(decl.Values)

		var embeds []pragmaEmbed
		if p, ok := decl.Pragma.(*pragmas); ok {
			embeds = p.Embeds
		}
		w.len(len(embeds))
		for _, embed := range embeds {
			w.pos(embed.Pos)
			w.strings(embed.Patterns)
		}
	}
}

func (w *writer) pkgObjs(names ...*syntax.Name) {
	w.sync(syncDeclNames)
	w.len(len(names))

	for _, name := range names {
		obj, ok := w.p.info.Defs[name]
		assert(ok)

		w.sync(syncDeclName)
		w.obj(obj, nil)
	}
}

// @@@ Helpers

// isDefinedType reports whether obj is a defined type.
func isDefinedType(obj types2.Object) bool {
	if obj, ok := obj.(*types2.TypeName); ok {
		return !obj.IsAlias()
	}
	return false
}

// isGlobal reports whether obj was declared at package scope.
//
// Caveat: blank objects are not declared.
func isGlobal(obj types2.Object) bool {
	return obj.Parent() == obj.Pkg().Scope()
}

// lookupObj returns the object that expr refers to, if any. If expr
// is an explicit instantiation of a generic object, then the instance
// object is returned as well.
func lookupObj(info *types2.Info, expr syntax.Expr) (obj types2.Object, inst types2.Instance) {
	if index, ok := expr.(*syntax.IndexExpr); ok {
		args := unpackListExpr(index.Index)
		if len(args) == 1 {
			tv, ok := info.Types[args[0]]
			assert(ok)
			if tv.IsValue() {
				return // normal index expression
			}
		}

		expr = index.X
	}

	// Strip package qualifier, if present.
	if sel, ok := expr.(*syntax.SelectorExpr); ok {
		if !isPkgQual(info, sel) {
			return // normal selector expression
		}
		expr = sel.Sel
	}

	if name, ok := expr.(*syntax.Name); ok {
		obj = info.Uses[name]
		inst = info.Instances[name]
	}
	return
}

// isPkgQual reports whether the given selector expression is a
// package-qualified identifier.
func isPkgQual(info *types2.Info, sel *syntax.SelectorExpr) bool {
	if name, ok := sel.X.(*syntax.Name); ok {
		_, isPkgName := info.Uses[name].(*types2.PkgName)
		return isPkgName
	}
	return false
}

// recvBase returns the base type for the given receiver parameter.
func recvBase(recv *types2.Var) *types2.Named {
	typ := recv.Type()
	if ptr, ok := typ.(*types2.Pointer); ok {
		typ = ptr.Elem()
	}
	return typ.(*types2.Named)
}

// namesAsExpr returns a list of names as a syntax.Expr.
func namesAsExpr(names []*syntax.Name) syntax.Expr {
	if len(names) == 1 {
		return names[0]
	}

	exprs := make([]syntax.Expr, len(names))
	for i, name := range names {
		exprs[i] = name
	}
	return &syntax.ListExpr{ElemList: exprs}
}

// fieldIndex returns the index of the struct field named by key.
func fieldIndex(info *types2.Info, str *types2.Struct, key *syntax.Name) int {
	field := info.Uses[key].(*types2.Var)

	for i := 0; i < str.NumFields(); i++ {
		if str.Field(i) == field {
			return i
		}
	}

	panic(fmt.Sprintf("%s: %v is not a field of %v", key.Pos(), field, str))
}

// objTypeParams returns the type parameters on the given object.
func objTypeParams(obj types2.Object) *types2.TypeParamList {
	switch obj := obj.(type) {
	case *types2.Func:
		sig := obj.Type().(*types2.Signature)
		if sig.Recv() != nil {
			return sig.RecvTypeParams()
		}
		return sig.TypeParams()
	case *types2.TypeName:
		if !obj.IsAlias() {
			return obj.Type().(*types2.Named).TypeParams()
		}
	}
	return nil
}

func asPragmaFlag(p syntax.Pragma) ir.PragmaFlag {
	if p == nil {
		return 0
	}
	return p.(*pragmas).Flag
}