writer.go

	fun := sel.Obj().(*types2.Func)
	sig := fun.Type().(*types2.Signature)

	w.typ(recv)
	w.typ(sig)
	w.pos(expr)
	w.selector(fun)

	// Method on a type parameter. These require an indirect call
	// through the current function's runtime dictionary.
	if typeParam, ok := recv.(*types2.TypeParam); w.Bool(ok) {
		typeParamIdx := w.dict.typeParamIndex(typeParam)
		methodInfo := w.p.selectorIdx(fun)

		w.Len(w.dict.typeParamMethodExprIdx(typeParamIdx, methodInfo))
		return
	}

	if isInterface(recv) != isInterface(sig.Recv().Type()) {
		w.p.fatalf(expr, "isInterface inconsistency: %v and %v", recv, sig.Recv().Type())
	}

	if !isInterface(recv) {
		if named, ok := deref2(recv).(*types2.Named); ok {
			obj, targs := splitNamed(named)
			info := w.p.objInstIdx(obj, targs, w.dict)

			// Method on a derived receiver type. These can be handled by a
			// static call to the shaped method, but require dynamically
			// looking up the appropriate dictionary argument in the current
			// function's runtime dictionary.
			if w.p.hasImplicitTypeParams(obj) || info.anyDerived() {
				w.Bool(true) // dynamic subdictionary
				w.Len(w.dict.subdictIdx(info))
				return
			}

			// Method on a fully known receiver type. These can be handled
			// by a static call to the shaped method, and with a static
			// reference to the receiver type's dictionary.
			if targs.Len() != 0 {
				w.Bool(false) // no dynamic subdictionary
				w.Bool(true)  // static dictionary
				w.objInfo(info)
				return
			}
		}
	}

	w.Bool(false) // no dynamic subdictionary
	w.Bool(false) // no static dictionary
}

// multiExpr writes a sequence of expressions, where the i'th value is
// implicitly converted to dstType(i). It also handles when exprs is a
// single, multi-valued expression (e.g., the multi-valued argument in
// an f(g()) call, or the RHS operand in a comma-ok assignment).
func (w *writer) multiExpr(pos poser, dstType func(int) types2.Type, exprs []syntax.Expr) {
	w.Sync(pkgbits.SyncMultiExpr)

	if len(exprs) == 1 {
		expr := exprs[0]
		if tuple, ok := w.p.typeOf(expr).(*types2.Tuple); ok {
			assert(tuple.Len() > 1)
			w.Bool(true) // N:1 assignment
			w.pos(pos)
			w.expr(expr)

			w.Len(tuple.Len())
			for i := 0; i < tuple.Len(); i++ {
				src := tuple.At(i).Type()
				// TODO(mdempsky): Investigate not writing src here. I think
				// the reader should be able to infer it from expr anyway.
				w.typ(src)
				if dst := dstType(i); w.Bool(dst != nil && !types2.Identical(src, dst)) {
					if src == nil || dst == nil {
						w.p.fatalf(pos, "src is %v, dst is %v", src, dst)
					}
					if !types2.AssignableTo(src, dst) {
						w.p.fatalf(pos, "%v is not assignable to %v", src, dst)
					}
					w.typ(dst)
					w.convRTTI(src, dst)
				}
			}
			return
		}
	}

	w.Bool(false) // N:N assignment
	w.Len(len(exprs))
	for i, expr := range exprs {
		w.implicitConvExpr(dstType(i), expr)
	}
}

// implicitConvExpr is like expr, but if dst is non-nil and different
// from expr's type, then an implicit conversion operation is inserted
// at expr's position.
func (w *writer) implicitConvExpr(dst types2.Type, expr syntax.Expr) {
	w.convertExpr(dst, expr, true)
}

func (w *writer) convertExpr(dst types2.Type, expr syntax.Expr, implicit bool) {
	src := w.p.typeOf(expr)

	// Omit implicit no-op conversions.
	identical := dst == nil || types2.Identical(src, dst)
	if implicit && identical {
		w.expr(expr)
		return
	}

	if implicit && !types2.AssignableTo(src, dst) {
		w.p.fatalf(expr, "%v is not assignable to %v", src, dst)
	}

	w.Code(exprConvert)
	w.Bool(implicit)
	w.typ(dst)
	w.pos(expr)
	w.convRTTI(src, dst)
	w.Bool(isTypeParam(dst))
	w.Bool(identical)
	w.expr(expr)
}

func (w *writer) compLit(lit *syntax.CompositeLit) {
	typ := w.p.typeOf(lit)

	w.Sync(pkgbits.SyncCompLit)
	w.pos(lit)
	w.typ(typ)

	if ptr, ok := types2.CoreType(typ).(*types2.Pointer); ok {
		typ = ptr.Elem()
	}
	var keyType, elemType types2.Type
	var structType *types2.Struct
	switch typ0 := typ; typ := types2.CoreType(typ).(type) {
	default:
		w.p.fatalf(lit, "unexpected composite literal type: %v", typ)
	case *types2.Array:
		elemType = typ.Elem()
	case *types2.Map:
		w.rtype(typ0)
		keyType, elemType = typ.Key(), typ.Elem()
	case *types2.Slice:
		elemType = typ.Elem()
	case *types2.Struct:
		structType = typ
	}

	w.Len(len(lit.ElemList))
	for i, elem := range lit.ElemList {
		elemType := elemType
		if structType != nil {
			if kv, ok := elem.(*syntax.KeyValueExpr); ok {
				// use position of expr.Key rather than of elem (which has position of ':')
				w.pos(kv.Key)
				i = fieldIndex(w.p.info, structType, kv.Key.(*syntax.Name))
				elem = kv.Value
			} else {
				w.pos(elem)
			}
			elemType = structType.Field(i).Type()
			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.implicitConvExpr(keyType, kv.Key)
				elem = kv.Value
			}
		}
		w.pos(elem)
		w.implicitConvExpr(elemType, elem)
	}
}

func (w *writer) funcLit(expr *syntax.FuncLit) {
	sig := w.p.typeOf(expr).(*types2.Signature)

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

	w.Sync(pkgbits.SyncFuncLit)
	w.pos(expr)
	w.signature(sig)

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

	w.Reloc(pkgbits.RelocBody, body)
}

type posVar struct {
	pos  syntax.Pos
	var_ *types2.Var
}

func (w *writer) exprList(expr syntax.Expr) {
	w.Sync(pkgbits.SyncExprList)
	w.exprs(unpackListExpr(expr))
}

func (w *writer) exprs(exprs []syntax.Expr) {
	w.Sync(pkgbits.SyncExprs)
	w.Len(len(exprs))
	for _, expr := range exprs {
		w.expr(expr)
	}
}

// rtype writes information so that the reader can construct an
// expression of type *runtime._type representing typ.
func (w *writer) rtype(typ types2.Type) {
	typ = types2.Default(typ)

	info := w.p.typIdx(typ, w.dict)
	w.rtypeInfo(info)
}

func (w *writer) rtypeInfo(info typeInfo) {
	w.Sync(pkgbits.SyncRType)

	if w.Bool(info.derived) {
		w.Len(w.dict.rtypeIdx(info))
	} else {
		w.typInfo(info)
	}
}

// varDictIndex writes out information for populating DictIndex for
// the ir.Name that will represent obj.
func (w *writer) varDictIndex(obj *types2.Var) {
	info := w.p.typIdx(obj.Type(), w.dict)
	if w.Bool(info.derived) {
		w.Len(w.dict.rtypeIdx(info))
	}
}

func isUntyped(typ types2.Type) bool {
	basic, ok := typ.(*types2.Basic)
	return ok && basic.Info()&types2.IsUntyped != 0
}

func isTuple(typ types2.Type) bool {
	_, ok := typ.(*types2.Tuple)
	return ok
}

func (w *writer) itab(typ, iface types2.Type) {
	typ = types2.Default(typ)
	iface = types2.Default(iface)

	typInfo := w.p.typIdx(typ, w.dict)
	ifaceInfo := w.p.typIdx(iface, w.dict)

	w.rtypeInfo(typInfo)
	w.rtypeInfo(ifaceInfo)
	if w.Bool(typInfo.derived || ifaceInfo.derived) {
		w.Len(w.dict.itabIdx(typInfo, ifaceInfo))
	}
}

// convRTTI writes information so that the reader can construct
// expressions for converting from src to dst.
func (w *writer) convRTTI(src, dst types2.Type) {
	w.Sync(pkgbits.SyncConvRTTI)
	w.itab(src, dst)
}

func (w *writer) exprType(iface types2.Type, typ syntax.Expr) {
	base.Assertf(iface == nil || isInterface(iface), "%v must be nil or an interface type", iface)

	tv := w.p.typeAndValue(typ)
	assert(tv.IsType())

	w.Sync(pkgbits.SyncExprType)
	w.pos(typ)

	if w.Bool(iface != nil && !iface.Underlying().(*types2.Interface).Empty()) {
		w.itab(tv.Type, iface)
	} else {
		w.rtype(tv.Type)

		info := w.p.typIdx(tv.Type, w.dict)
		w.Bool(info.derived)
	}
}

// isInterface reports whether typ is known to be an interface type.
// If typ is a type parameter, then isInterface reports an internal
// compiler error instead.
func isInterface(typ types2.Type) bool {
	if _, ok := typ.(*types2.TypeParam); ok {
		// typ is a type parameter and may be instantiated as either a
		// concrete or interface type, so the writer can't depend on
		// knowing this.
		base.Fatalf("%v is a type parameter", typ)
	}

	_, ok := typ.Underlying().(*types2.Interface)
	return ok
}

// op writes an Op into the bitstream.
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(pkgbits.SyncOp)
	w.Len(int(op))
}

// @@@ 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
}

// declCollector is a visitor type that collects compiler-needed
// information about declarations that types2 doesn't track.
//
// Notably, it maps declared types and functions back to their
// declaration statement, keeps track of implicit type parameters, and
// assigns unique type "generation" numbers to local defined types.
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, 0, 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:
				if types.AllowsGoVersion(1, 18) {
					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(pkgbits.SyncDecls)
	for _, p := range noders {
		for _, decl := range p.file.DeclList {
			w.pkgDecl(decl)
		}
	}
	w.Code(declEnd)

	w.Sync(pkgbits.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
		}

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

	case *syntax.VarDecl:
		w.Code(declVar)
		w.pos(decl)
		w.pkgObjs(decl.NameList...)

		// TODO(mdempsky): It would make sense to use multiExpr here, but
		// that results in IR that confuses pkginit/initorder.go. So we
		// continue using exprList, and let typecheck handle inserting any
		// implicit conversions. That's okay though, because package-scope
		// assignments never require dictionaries.
		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(pkgbits.SyncDeclNames)
	w.Len(len(names))

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

		w.Sync(pkgbits.SyncDeclName)
		w.obj(obj, nil)
	}
}

// @@@ Helpers

// hasImplicitTypeParams reports whether obj is a defined type with
// implicit type parameters (e.g., declared within a generic function
// or method).
func (p *pkgWriter) hasImplicitTypeParams(obj *types2.TypeName) bool {
	if obj.Pkg() == p.curpkg {
		decl, ok := p.typDecls[obj]
		assert(ok)
		if len(decl.implicits) != 0 {
			return true
		}
	}
	return false
}

// 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(p *pkgWriter, 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 := p.typeAndValue(args[0])
			if tv.IsValue() {
				return // normal index expression
			}
		}

		expr = index.X
	}

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

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

// isNil reports whether expr is a (possibly parenthesized) reference
// to the predeclared nil value.
func isNil(p *pkgWriter, expr syntax.Expr) bool {
	tv := p.typeAndValue(expr)
	return tv.IsNil()
}

// 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
}

// splitNamed decomposes a use of a defined type into its original
// type definition and the type arguments used to instantiate it.
func splitNamed(typ *types2.Named) (*types2.TypeName, *types2.TypeList) {
	base.Assertf(typ.TypeParams().Len() == typ.TypeArgs().Len(), "use of uninstantiated type: %v", typ)

	orig := typ.Origin()
	base.Assertf(orig.TypeArgs() == nil, "origin %v of %v has type arguments", orig, typ)
	base.Assertf(typ.Obj() == orig.Obj(), "%v has object %v, but %v has object %v", typ, typ.Obj(), orig, orig.Obj())

	return typ.Obj(), typ.TypeArgs()
}

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

func asWasmImport(p syntax.Pragma) *WasmImport {
	if p == nil {
		return nil
	}
	return p.(*pragmas).WasmImport
}

// isPtrTo reports whether from is the type *to.
func isPtrTo(from, to types2.Type) bool {
	ptr, ok := from.(*types2.Pointer)
	return ok && types2.Identical(ptr.Elem(), to)
}