go assignments 源码
golang assignments 代码
文件路径：/src/cmd/compile/internal/types2/assignments.go
// Copyright 2013 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.
// This file implements initialization and assignment checks.
package types2
import (
"cmd/compile/internal/syntax"
"fmt"
"strings"
)
// assignment reports whether x can be assigned to a variable of type T,
// if necessary by attempting to convert untyped values to the appropriate
// type. context describes the context in which the assignment takes place.
// Use T == nil to indicate assignment to an untyped blank identifier.
// x.mode is set to invalid if the assignment failed.
func (check *Checker) assignment(x *operand, T Type, context string) {
check.singleValue(x)
switch x.mode {
case invalid:
return // error reported before
case constant_, variable, mapindex, value, nilvalue, commaok, commaerr:
// ok
default:
// we may get here because of other problems (issue #39634, crash 12)
check.errorf(x, "cannot assign %s to %s in %s", x, T, context)
return
}
if isUntyped(x.typ) {
target := T
// spec: "If an untyped constant is assigned to a variable of interface
// type or the blank identifier, the constant is first converted to type
// bool, rune, int, float64, complex128 or string respectively, depending
// on whether the value is a boolean, rune, integer, floating-point,
// complex, or string constant."
if x.isNil() {
if T == nil {
check.errorf(x, "use of untyped nil in %s", context)
x.mode = invalid
return
}
} else if T == nil || isNonTypeParamInterface(T) {
target = Default(x.typ)
}
newType, val, code := check.implicitTypeAndValue(x, target)
if code != 0 {
msg := check.sprintf("cannot use %s as %s value in %s", x, target, context)
switch code {
case _TruncatedFloat:
msg += " (truncated)"
case _NumericOverflow:
msg += " (overflows)"
}
check.error(x, msg)
x.mode = invalid
return
}
if val != nil {
x.val = val
check.updateExprVal(x.expr, val)
}
if newType != x.typ {
x.typ = newType
check.updateExprType(x.expr, newType, false)
}
}
// x.typ is typed
// A generic (non-instantiated) function value cannot be assigned to a variable.
if sig, _ := under(x.typ).(*Signature); sig != nil && sig.TypeParams().Len() > 0 {
check.errorf(x, "cannot use generic function %s without instantiation in %s", x, context)
}
// spec: "If a left-hand side is the blank identifier, any typed or
// non-constant value except for the predeclared identifier nil may
// be assigned to it."
if T == nil {
return
}
reason := ""
if ok, _ := x.assignableTo(check, T, &reason); !ok {
if check.conf.CompilerErrorMessages {
if reason != "" {
check.errorf(x, "cannot use %s as type %s in %s:\n\t%s", x, T, context, reason)
} else {
check.errorf(x, "cannot use %s as type %s in %s", x, T, context)
}
} else {
if reason != "" {
check.errorf(x, "cannot use %s as %s value in %s: %s", x, T, context, reason)
} else {
check.errorf(x, "cannot use %s as %s value in %s", x, T, context)
}
}
x.mode = invalid
}
}
func (check *Checker) initConst(lhs *Const, x *operand) {
if x.mode == invalid || x.typ == Typ[Invalid] || lhs.typ == Typ[Invalid] {
if lhs.typ == nil {
lhs.typ = Typ[Invalid]
}
return
}
// rhs must be a constant
if x.mode != constant_ {
check.errorf(x, "%s is not constant", x)
if lhs.typ == nil {
lhs.typ = Typ[Invalid]
}
return
}
assert(isConstType(x.typ))
// If the lhs doesn't have a type yet, use the type of x.
if lhs.typ == nil {
lhs.typ = x.typ
}
check.assignment(x, lhs.typ, "constant declaration")
if x.mode == invalid {
return
}
lhs.val = x.val
}
func (check *Checker) initVar(lhs *Var, x *operand, context string) Type {
if x.mode == invalid || x.typ == Typ[Invalid] || lhs.typ == Typ[Invalid] {
if lhs.typ == nil {
lhs.typ = Typ[Invalid]
}
// Note: This was reverted in go/types (https://golang.org/cl/292751).
// TODO(gri): decide what to do (also affects test/run.go exclusion list)
lhs.used = true // avoid follow-on "declared but not used" errors
return nil
}
// If the lhs doesn't have a type yet, use the type of x.
if lhs.typ == nil {
typ := x.typ
if isUntyped(typ) {
// convert untyped types to default types
if typ == Typ[UntypedNil] {
check.errorf(x, "use of untyped nil in %s", context)
lhs.typ = Typ[Invalid]
return nil
}
typ = Default(typ)
}
lhs.typ = typ
}
check.assignment(x, lhs.typ, context)
if x.mode == invalid {
lhs.used = true // avoid follow-on "declared but not used" errors
return nil
}
return x.typ
}
func (check *Checker) assignVar(lhs syntax.Expr, x *operand) Type {
if x.mode == invalid || x.typ == Typ[Invalid] {
check.use(lhs)
return nil
}
// Determine if the lhs is a (possibly parenthesized) identifier.
ident, _ := unparen(lhs).(*syntax.Name)
// Don't evaluate lhs if it is the blank identifier.
if ident != nil && ident.Value == "_" {
check.recordDef(ident, nil)
check.assignment(x, nil, "assignment to _ identifier")
if x.mode == invalid {
return nil
}
return x.typ
}
// If the lhs is an identifier denoting a variable v, this assignment
// is not a 'use' of v. Remember current value of v.used and restore
// after evaluating the lhs via check.expr.
var v *Var
var v_used bool
if ident != nil {
if obj := check.lookup(ident.Value); obj != nil {
// It's ok to mark non-local variables, but ignore variables
// from other packages to avoid potential race conditions with
// dot-imported variables.
if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
v = w
v_used = v.used
}
}
}
var z operand
check.expr(&z, lhs)
if v != nil {
v.used = v_used // restore v.used
}
if z.mode == invalid || z.typ == Typ[Invalid] {
return nil
}
// spec: "Each left-hand side operand must be addressable, a map index
// expression, or the blank identifier. Operands may be parenthesized."
switch z.mode {
case invalid:
return nil
case variable, mapindex:
// ok
default:
if sel, ok := z.expr.(*syntax.SelectorExpr); ok {
var op operand
check.expr(&op, sel.X)
if op.mode == mapindex {
check.errorf(&z, "cannot assign to struct field %s in map", syntax.String(z.expr))
return nil
}
}
check.errorf(&z, "cannot assign to %s", &z)
return nil
}
check.assignment(x, z.typ, "assignment")
if x.mode == invalid {
return nil
}
return x.typ
}
// operandTypes returns the list of types for the given operands.
func operandTypes(list []*operand) (res []Type) {
for _, x := range list {
res = append(res, x.typ)
}
return res
}
// varTypes returns the list of types for the given variables.
func varTypes(list []*Var) (res []Type) {
for _, x := range list {
res = append(res, x.typ)
}
return res
}
// typesSummary returns a string of the form "(t1, t2, ...)" where the
// ti's are user-friendly string representations for the given types.
// If variadic is set and the last type is a slice, its string is of
// the form "...E" where E is the slice's element type.
func (check *Checker) typesSummary(list []Type, variadic bool) string {
var res []string
for i, t := range list {
var s string
switch {
case t == nil:
fallthrough // should not happen but be cautious
case t == Typ[Invalid]:
s = "<T>"
case isUntyped(t):
if isNumeric(t) {
// Do not imply a specific type requirement:
// "have number, want float64" is better than
// "have untyped int, want float64" or
// "have int, want float64".
s = "number"
} else {
// If we don't have a number, omit the "untyped" qualifier
// for compactness.
s = strings.Replace(t.(*Basic).name, "untyped ", "", -1)
}
case variadic && i == len(list)-1:
s = check.sprintf("...%s", t.(*Slice).elem)
}
if s == "" {
s = check.sprintf("%s", t)
}
res = append(res, s)
}
return "(" + strings.Join(res, ", ") + ")"
}
func measure(x int, unit string) string {
if x != 1 {
unit += "s"
}
return fmt.Sprintf("%d %s", x, unit)
}
func (check *Checker) assignError(rhs []syntax.Expr, nvars, nvals int) {
vars := measure(nvars, "variable")
vals := measure(nvals, "value")
rhs0 := rhs[0]
if len(rhs) == 1 {
if call, _ := unparen(rhs0).(*syntax.CallExpr); call != nil {
check.errorf(rhs0, "assignment mismatch: %s but %s returns %s", vars, call.Fun, vals)
return
}
}
check.errorf(rhs0, "assignment mismatch: %s but %s", vars, vals)
}
// If returnStmt != nil, initVars is called to type-check the assignment
// of return expressions, and returnStmt is the return statement.
func (check *Checker) initVars(lhs []*Var, orig_rhs []syntax.Expr, returnStmt syntax.Stmt) {
rhs, commaOk := check.exprList(orig_rhs, len(lhs) == 2 && returnStmt == nil)
if len(lhs) != len(rhs) {
// invalidate lhs
for _, obj := range lhs {
obj.used = true // avoid declared but not used errors
if obj.typ == nil {
obj.typ = Typ[Invalid]
}
}
// don't report an error if we already reported one
for _, x := range rhs {
if x.mode == invalid {
return
}
}
if returnStmt != nil {
var at poser = returnStmt
qualifier := "not enough"
if len(rhs) > len(lhs) {
at = rhs[len(lhs)].expr // report at first extra value
qualifier = "too many"
} else if len(rhs) > 0 {
at = rhs[len(rhs)-1].expr // report at last value
}
var err error_
err.errorf(at, "%s return values", qualifier)
err.errorf(nopos, "have %s", check.typesSummary(operandTypes(rhs), false))
err.errorf(nopos, "want %s", check.typesSummary(varTypes(lhs), false))
check.report(&err)
return
}
if check.conf.CompilerErrorMessages {
check.assignError(orig_rhs, len(lhs), len(rhs))
} else {
check.errorf(rhs[0], "cannot initialize %d variables with %d values", len(lhs), len(rhs))
}
return
}
context := "assignment"
if returnStmt != nil {
context = "return statement"
}
if commaOk {
var a [2]Type
for i := range a {
a[i] = check.initVar(lhs[i], rhs[i], context)
}
check.recordCommaOkTypes(orig_rhs[0], a)
return
}
ok := true
for i, lhs := range lhs {
if check.initVar(lhs, rhs[i], context) == nil {
ok = false
}
}
// avoid follow-on "declared but not used" errors if any initialization failed
if !ok {
for _, lhs := range lhs {
lhs.used = true
}
}
}
func (check *Checker) assignVars(lhs, orig_rhs []syntax.Expr) {
rhs, commaOk := check.exprList(orig_rhs, len(lhs) == 2)
if len(lhs) != len(rhs) {
check.use(lhs...)
// don't report an error if we already reported one
for _, x := range rhs {
if x.mode == invalid {
return
}
}
if check.conf.CompilerErrorMessages {
check.assignError(orig_rhs, len(lhs), len(rhs))
} else {
check.errorf(rhs[0], "cannot assign %d values to %d variables", len(rhs), len(lhs))
}
return
}
if commaOk {
var a [2]Type
for i := range a {
a[i] = check.assignVar(lhs[i], rhs[i])
}
check.recordCommaOkTypes(orig_rhs[0], a)
return
}
ok := true
for i, lhs := range lhs {
if check.assignVar(lhs, rhs[i]) == nil {
ok = false
}
}
// avoid follow-on "declared but not used" errors if any assignment failed
if !ok {
// don't call check.use to avoid re-evaluation of the lhs expressions
for _, lhs := range lhs {
if name, _ := unparen(lhs).(*syntax.Name); name != nil {
if obj := check.lookup(name.Value); obj != nil {
// see comment in assignVar
if v, _ := obj.(*Var); v != nil && v.pkg == check.pkg {
v.used = true
}
}
}
}
}
}
// unpack unpacks a *syntax.ListExpr into a list of syntax.Expr.
// Helper introduced for the go/types -> types2 port.
// TODO(gri) Should find a more efficient solution that doesn't
// require introduction of a new slice for simple
// expressions.
func unpackExpr(x syntax.Expr) []syntax.Expr {
if x, _ := x.(*syntax.ListExpr); x != nil {
return x.ElemList
}
if x != nil {
return []syntax.Expr{x}
}
return nil
}
func (check *Checker) shortVarDecl(pos syntax.Pos, lhs, rhs []syntax.Expr) {
top := len(check.delayed)
scope := check.scope
// collect lhs variables
seen := make(map[string]bool, len(lhs))
lhsVars := make([]*Var, len(lhs))
newVars := make([]*Var, 0, len(lhs))
hasErr := false
for i, lhs := range lhs {
ident, _ := lhs.(*syntax.Name)
if ident == nil {
check.use(lhs)
check.errorf(lhs, "non-name %s on left side of :=", lhs)
hasErr = true
continue
}
name := ident.Value
if name != "_" {
if seen[name] {
check.errorf(lhs, "%s repeated on left side of :=", lhs)
hasErr = true
continue
}
seen[name] = true
}
// Use the correct obj if the ident is redeclared. The
// variable's scope starts after the declaration; so we
// must use Scope.Lookup here and call Scope.Insert
// (via check.declare) later.
if alt := scope.Lookup(name); alt != nil {
check.recordUse(ident, alt)
// redeclared object must be a variable
if obj, _ := alt.(*Var); obj != nil {
lhsVars[i] = obj
} else {
check.errorf(lhs, "cannot assign to %s", lhs)
hasErr = true
}
continue
}
// declare new variable
obj := NewVar(ident.Pos(), check.pkg, name, nil)
lhsVars[i] = obj
if name != "_" {
newVars = append(newVars, obj)
}
check.recordDef(ident, obj)
}
// create dummy variables where the lhs is invalid
for i, obj := range lhsVars {
if obj == nil {
lhsVars[i] = NewVar(lhs[i].Pos(), check.pkg, "_", nil)
}
}
check.initVars(lhsVars, rhs, nil)
// process function literals in rhs expressions before scope changes
check.processDelayed(top)
if len(newVars) == 0 && !hasErr {
check.softErrorf(pos, "no new variables on left side of :=")
return
}
// declare new variables
// spec: "The scope of a constant or variable identifier declared inside
// a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl
// for short variable declarations) and ends at the end of the innermost
// containing block."
scopePos := syntax.EndPos(rhs[len(rhs)-1])
for _, obj := range newVars {
check.declare(scope, nil, obj, scopePos) // id = nil: recordDef already called
}
}
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