assignments.go

Documentation: cmd/compile/internal/types2

     1  // Copyright 2013 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // This file implements initialization and assignment checks.
     6  
     7  package types2
     8  
     9  import (
    10  	"cmd/compile/internal/syntax"
    11  	"fmt"
    12  	. "internal/types/errors"
    13  	"strings"
    14  )
    15  
    16  // assignment reports whether x can be assigned to a variable of type T,
    17  // if necessary by attempting to convert untyped values to the appropriate
    18  // type. context describes the context in which the assignment takes place.
    19  // Use T == nil to indicate assignment to an untyped blank identifier.
    20  // If the assignment check fails, x.mode is set to invalid.
    21  func (check *Checker) assignment(x *operand, T Type, context string) {
    22  	check.singleValue(x)
    23  
    24  	switch x.mode {
    25  	case invalid:
    26  		return // error reported before
    27  	case nilvalue:
    28  		assert(isTypes2)
    29  		// ok
    30  	case constant_, variable, mapindex, value, commaok, commaerr:
    31  		// ok
    32  	default:
    33  		// we may get here because of other problems (go.dev/issue/39634, crash 12)
    34  		// TODO(gri) do we need a new "generic" error code here?
    35  		check.errorf(x, IncompatibleAssign, "cannot assign %s to %s in %s", x, T, context)
    36  		x.mode = invalid
    37  		return
    38  	}
    39  
    40  	if isUntyped(x.typ) {
    41  		target := T
    42  		// spec: "If an untyped constant is assigned to a variable of interface
    43  		// type or the blank identifier, the constant is first converted to type
    44  		// bool, rune, int, float64, complex128 or string respectively, depending
    45  		// on whether the value is a boolean, rune, integer, floating-point,
    46  		// complex, or string constant."
    47  		if isTypes2 {
    48  			if x.isNil() {
    49  				if T == nil {
    50  					check.errorf(x, UntypedNilUse, "use of untyped nil in %s", context)
    51  					x.mode = invalid
    52  					return
    53  				}
    54  			} else if T == nil || isNonTypeParamInterface(T) {
    55  				target = Default(x.typ)
    56  			}
    57  		} else { // go/types
    58  			if T == nil || isNonTypeParamInterface(T) {
    59  				if T == nil && x.typ == Typ[UntypedNil] {
    60  					check.errorf(x, UntypedNilUse, "use of untyped nil in %s", context)
    61  					x.mode = invalid
    62  					return
    63  				}
    64  				target = Default(x.typ)
    65  			}
    66  		}
    67  		newType, val, code := check.implicitTypeAndValue(x, target)
    68  		if code != 0 {
    69  			msg := check.sprintf("cannot use %s as %s value in %s", x, target, context)
    70  			switch code {
    71  			case TruncatedFloat:
    72  				msg += " (truncated)"
    73  			case NumericOverflow:
    74  				msg += " (overflows)"
    75  			default:
    76  				code = IncompatibleAssign
    77  			}
    78  			check.error(x, code, msg)
    79  			x.mode = invalid
    80  			return
    81  		}
    82  		if val != nil {
    83  			x.val = val
    84  			check.updateExprVal(x.expr, val)
    85  		}
    86  		if newType != x.typ {
    87  			x.typ = newType
    88  			check.updateExprType(x.expr, newType, false)
    89  		}
    90  	}
    91  	// x.typ is typed
    92  
    93  	// A generic (non-instantiated) function value cannot be assigned to a variable.
    94  	if sig, _ := under(x.typ).(*Signature); sig != nil && sig.TypeParams().Len() > 0 {
    95  		check.errorf(x, WrongTypeArgCount, "cannot use generic function %s without instantiation in %s", x, context)
    96  		x.mode = invalid
    97  		return
    98  	}
    99  
   100  	// spec: "If a left-hand side is the blank identifier, any typed or
   101  	// non-constant value except for the predeclared identifier nil may
   102  	// be assigned to it."
   103  	if T == nil {
   104  		return
   105  	}
   106  
   107  	cause := ""
   108  	if ok, code := x.assignableTo(check, T, &cause); !ok {
   109  		if cause != "" {
   110  			check.errorf(x, code, "cannot use %s as %s value in %s: %s", x, T, context, cause)
   111  		} else {
   112  			check.errorf(x, code, "cannot use %s as %s value in %s", x, T, context)
   113  		}
   114  		x.mode = invalid
   115  	}
   116  }
   117  
   118  func (check *Checker) initConst(lhs *Const, x *operand) {
   119  	if x.mode == invalid || !isValid(x.typ) || !isValid(lhs.typ) {
   120  		if lhs.typ == nil {
   121  			lhs.typ = Typ[Invalid]
   122  		}
   123  		return
   124  	}
   125  
   126  	// rhs must be a constant
   127  	if x.mode != constant_ {
   128  		check.errorf(x, InvalidConstInit, "%s is not constant", x)
   129  		if lhs.typ == nil {
   130  			lhs.typ = Typ[Invalid]
   131  		}
   132  		return
   133  	}
   134  	assert(isConstType(x.typ))
   135  
   136  	// If the lhs doesn't have a type yet, use the type of x.
   137  	if lhs.typ == nil {
   138  		lhs.typ = x.typ
   139  	}
   140  
   141  	check.assignment(x, lhs.typ, "constant declaration")
   142  	if x.mode == invalid {
   143  		return
   144  	}
   145  
   146  	lhs.val = x.val
   147  }
   148  
   149  // initVar checks the initialization lhs = x in a variable declaration.
   150  // If lhs doesn't have a type yet, it is given the type of x,
   151  // or Typ[Invalid] in case of an error.
   152  // If the initialization check fails, x.mode is set to invalid.
   153  func (check *Checker) initVar(lhs *Var, x *operand, context string) {
   154  	if x.mode == invalid || !isValid(x.typ) || !isValid(lhs.typ) {
   155  		if lhs.typ == nil {
   156  			lhs.typ = Typ[Invalid]
   157  		}
   158  		x.mode = invalid
   159  		return
   160  	}
   161  
   162  	// If lhs doesn't have a type yet, use the type of x.
   163  	if lhs.typ == nil {
   164  		typ := x.typ
   165  		if isUntyped(typ) {
   166  			// convert untyped types to default types
   167  			if typ == Typ[UntypedNil] {
   168  				check.errorf(x, UntypedNilUse, "use of untyped nil in %s", context)
   169  				lhs.typ = Typ[Invalid]
   170  				x.mode = invalid
   171  				return
   172  			}
   173  			typ = Default(typ)
   174  		}
   175  		lhs.typ = typ
   176  	}
   177  
   178  	check.assignment(x, lhs.typ, context)
   179  }
   180  
   181  // lhsVar checks a lhs variable in an assignment and returns its type.
   182  // lhsVar takes care of not counting a lhs identifier as a "use" of
   183  // that identifier. The result is nil if it is the blank identifier,
   184  // and Typ[Invalid] if it is an invalid lhs expression.
   185  func (check *Checker) lhsVar(lhs syntax.Expr) Type {
   186  	// Determine if the lhs is a (possibly parenthesized) identifier.
   187  	ident, _ := syntax.Unparen(lhs).(*syntax.Name)
   188  
   189  	// Don't evaluate lhs if it is the blank identifier.
   190  	if ident != nil && ident.Value == "_" {
   191  		check.recordDef(ident, nil)
   192  		return nil
   193  	}
   194  
   195  	// If the lhs is an identifier denoting a variable v, this reference
   196  	// is not a 'use' of v. Remember current value of v.used and restore
   197  	// after evaluating the lhs via check.expr.
   198  	var v *Var
   199  	var v_used bool
   200  	if ident != nil {
   201  		if obj := check.lookup(ident.Value); obj != nil {
   202  			// It's ok to mark non-local variables, but ignore variables
   203  			// from other packages to avoid potential race conditions with
   204  			// dot-imported variables.
   205  			if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
   206  				v = w
   207  				v_used = v.used
   208  			}
   209  		}
   210  	}
   211  
   212  	var x operand
   213  	check.expr(nil, &x, lhs)
   214  
   215  	if v != nil {
   216  		v.used = v_used // restore v.used
   217  	}
   218  
   219  	if x.mode == invalid || !isValid(x.typ) {
   220  		return Typ[Invalid]
   221  	}
   222  
   223  	// spec: "Each left-hand side operand must be addressable, a map index
   224  	// expression, or the blank identifier. Operands may be parenthesized."
   225  	switch x.mode {
   226  	case invalid:
   227  		return Typ[Invalid]
   228  	case variable, mapindex:
   229  		// ok
   230  	default:
   231  		if sel, ok := x.expr.(*syntax.SelectorExpr); ok {
   232  			var op operand
   233  			check.expr(nil, &op, sel.X)
   234  			if op.mode == mapindex {
   235  				check.errorf(&x, UnaddressableFieldAssign, "cannot assign to struct field %s in map", ExprString(x.expr))
   236  				return Typ[Invalid]
   237  			}
   238  		}
   239  		check.errorf(&x, UnassignableOperand, "cannot assign to %s (neither addressable nor a map index expression)", x.expr)
   240  		return Typ[Invalid]
   241  	}
   242  
   243  	return x.typ
   244  }
   245  
   246  // assignVar checks the assignment lhs = rhs (if x == nil), or lhs = x (if x != nil).
   247  // If x != nil, it must be the evaluation of rhs (and rhs will be ignored).
   248  // If the assignment check fails and x != nil, x.mode is set to invalid.
   249  func (check *Checker) assignVar(lhs, rhs syntax.Expr, x *operand, context string) {
   250  	T := check.lhsVar(lhs) // nil if lhs is _
   251  	if !isValid(T) {
   252  		if x != nil {
   253  			x.mode = invalid
   254  		} else {
   255  			check.use(rhs)
   256  		}
   257  		return
   258  	}
   259  
   260  	if x == nil {
   261  		var target *target
   262  		// avoid calling ExprString if not needed
   263  		if T != nil {
   264  			if _, ok := under(T).(*Signature); ok {
   265  				target = newTarget(T, ExprString(lhs))
   266  			}
   267  		}
   268  		x = new(operand)
   269  		check.expr(target, x, rhs)
   270  	}
   271  
   272  	if T == nil && context == "assignment" {
   273  		context = "assignment to _ identifier"
   274  	}
   275  	check.assignment(x, T, context)
   276  }
   277  
   278  // operandTypes returns the list of types for the given operands.
   279  func operandTypes(list []*operand) (res []Type) {
   280  	for _, x := range list {
   281  		res = append(res, x.typ)
   282  	}
   283  	return res
   284  }
   285  
   286  // varTypes returns the list of types for the given variables.
   287  func varTypes(list []*Var) (res []Type) {
   288  	for _, x := range list {
   289  		res = append(res, x.typ)
   290  	}
   291  	return res
   292  }
   293  
   294  // typesSummary returns a string of the form "(t1, t2, ...)" where the
   295  // ti's are user-friendly string representations for the given types.
   296  // If variadic is set and the last type is a slice, its string is of
   297  // the form "...E" where E is the slice's element type.
   298  func (check *Checker) typesSummary(list []Type, variadic bool) string {
   299  	var res []string
   300  	for i, t := range list {
   301  		var s string
   302  		switch {
   303  		case t == nil:
   304  			fallthrough // should not happen but be cautious
   305  		case !isValid(t):
   306  			s = "unknown type"
   307  		case isUntyped(t):
   308  			if isNumeric(t) {
   309  				// Do not imply a specific type requirement:
   310  				// "have number, want float64" is better than
   311  				// "have untyped int, want float64" or
   312  				// "have int, want float64".
   313  				s = "number"
   314  			} else {
   315  				// If we don't have a number, omit the "untyped" qualifier
   316  				// for compactness.
   317  				s = strings.Replace(t.(*Basic).name, "untyped ", "", -1)
   318  			}
   319  		case variadic && i == len(list)-1:
   320  			s = check.sprintf("...%s", t.(*Slice).elem)
   321  		}
   322  		if s == "" {
   323  			s = check.sprintf("%s", t)
   324  		}
   325  		res = append(res, s)
   326  	}
   327  	return "(" + strings.Join(res, ", ") + ")"
   328  }
   329  
   330  func measure(x int, unit string) string {
   331  	if x != 1 {
   332  		unit += "s"
   333  	}
   334  	return fmt.Sprintf("%d %s", x, unit)
   335  }
   336  
   337  func (check *Checker) assignError(rhs []syntax.Expr, l, r int) {
   338  	vars := measure(l, "variable")
   339  	vals := measure(r, "value")
   340  	rhs0 := rhs[0]
   341  
   342  	if len(rhs) == 1 {
   343  		if call, _ := syntax.Unparen(rhs0).(*syntax.CallExpr); call != nil {
   344  			check.errorf(rhs0, WrongAssignCount, "assignment mismatch: %s but %s returns %s", vars, call.Fun, vals)
   345  			return
   346  		}
   347  	}
   348  	check.errorf(rhs0, WrongAssignCount, "assignment mismatch: %s but %s", vars, vals)
   349  }
   350  
   351  func (check *Checker) returnError(at poser, lhs []*Var, rhs []*operand) {
   352  	l, r := len(lhs), len(rhs)
   353  	qualifier := "not enough"
   354  	if r > l {
   355  		at = rhs[l] // report at first extra value
   356  		qualifier = "too many"
   357  	} else if r > 0 {
   358  		at = rhs[r-1] // report at last value
   359  	}
   360  	err := check.newError(WrongResultCount)
   361  	err.addf(at, "%s return values", qualifier)
   362  	err.addf(nopos, "have %s", check.typesSummary(operandTypes(rhs), false))
   363  	err.addf(nopos, "want %s", check.typesSummary(varTypes(lhs), false))
   364  	err.report()
   365  }
   366  
   367  // initVars type-checks assignments of initialization expressions orig_rhs
   368  // to variables lhs.
   369  // If returnStmt is non-nil, initVars type-checks the implicit assignment
   370  // of result expressions orig_rhs to function result parameters lhs.
   371  func (check *Checker) initVars(lhs []*Var, orig_rhs []syntax.Expr, returnStmt syntax.Stmt) {
   372  	context := "assignment"
   373  	if returnStmt != nil {
   374  		context = "return statement"
   375  	}
   376  
   377  	l, r := len(lhs), len(orig_rhs)
   378  
   379  	// If l == 1 and the rhs is a single call, for a better
   380  	// error message don't handle it as n:n mapping below.
   381  	isCall := false
   382  	if r == 1 {
   383  		_, isCall = syntax.Unparen(orig_rhs[0]).(*syntax.CallExpr)
   384  	}
   385  
   386  	// If we have a n:n mapping from lhs variable to rhs expression,
   387  	// each value can be assigned to its corresponding variable.
   388  	if l == r && !isCall {
   389  		var x operand
   390  		for i, lhs := range lhs {
   391  			desc := lhs.name
   392  			if returnStmt != nil && desc == "" {
   393  				desc = "result variable"
   394  			}
   395  			check.expr(newTarget(lhs.typ, desc), &x, orig_rhs[i])
   396  			check.initVar(lhs, &x, context)
   397  		}
   398  		return
   399  	}
   400  
   401  	// If we don't have an n:n mapping, the rhs must be a single expression
   402  	// resulting in 2 or more values; otherwise we have an assignment mismatch.
   403  	if r != 1 {
   404  		// Only report a mismatch error if there are no other errors on the rhs.
   405  		if check.use(orig_rhs...) {
   406  			if returnStmt != nil {
   407  				rhs := check.exprList(orig_rhs)
   408  				check.returnError(returnStmt, lhs, rhs)
   409  			} else {
   410  				check.assignError(orig_rhs, l, r)
   411  			}
   412  		}
   413  		// ensure that LHS variables have a type
   414  		for _, v := range lhs {
   415  			if v.typ == nil {
   416  				v.typ = Typ[Invalid]
   417  			}
   418  		}
   419  		return
   420  	}
   421  
   422  	rhs, commaOk := check.multiExpr(orig_rhs[0], l == 2 && returnStmt == nil)
   423  	r = len(rhs)
   424  	if l == r {
   425  		for i, lhs := range lhs {
   426  			check.initVar(lhs, rhs[i], context)
   427  		}
   428  		// Only record comma-ok expression if both initializations succeeded
   429  		// (go.dev/issue/59371).
   430  		if commaOk && rhs[0].mode != invalid && rhs[1].mode != invalid {
   431  			check.recordCommaOkTypes(orig_rhs[0], rhs)
   432  		}
   433  		return
   434  	}
   435  
   436  	// In all other cases we have an assignment mismatch.
   437  	// Only report a mismatch error if there are no other errors on the rhs.
   438  	if rhs[0].mode != invalid {
   439  		if returnStmt != nil {
   440  			check.returnError(returnStmt, lhs, rhs)
   441  		} else {
   442  			check.assignError(orig_rhs, l, r)
   443  		}
   444  	}
   445  	// ensure that LHS variables have a type
   446  	for _, v := range lhs {
   447  		if v.typ == nil {
   448  			v.typ = Typ[Invalid]
   449  		}
   450  	}
   451  	// orig_rhs[0] was already evaluated
   452  }
   453  
   454  // assignVars type-checks assignments of expressions orig_rhs to variables lhs.
   455  func (check *Checker) assignVars(lhs, orig_rhs []syntax.Expr) {
   456  	l, r := len(lhs), len(orig_rhs)
   457  
   458  	// If l == 1 and the rhs is a single call, for a better
   459  	// error message don't handle it as n:n mapping below.
   460  	isCall := false
   461  	if r == 1 {
   462  		_, isCall = syntax.Unparen(orig_rhs[0]).(*syntax.CallExpr)
   463  	}
   464  
   465  	// If we have a n:n mapping from lhs variable to rhs expression,
   466  	// each value can be assigned to its corresponding variable.
   467  	if l == r && !isCall {
   468  		for i, lhs := range lhs {
   469  			check.assignVar(lhs, orig_rhs[i], nil, "assignment")
   470  		}
   471  		return
   472  	}
   473  
   474  	// If we don't have an n:n mapping, the rhs must be a single expression
   475  	// resulting in 2 or more values; otherwise we have an assignment mismatch.
   476  	if r != 1 {
   477  		// Only report a mismatch error if there are no other errors on the lhs or rhs.
   478  		okLHS := check.useLHS(lhs...)
   479  		okRHS := check.use(orig_rhs...)
   480  		if okLHS && okRHS {
   481  			check.assignError(orig_rhs, l, r)
   482  		}
   483  		return
   484  	}
   485  
   486  	rhs, commaOk := check.multiExpr(orig_rhs[0], l == 2)
   487  	r = len(rhs)
   488  	if l == r {
   489  		for i, lhs := range lhs {
   490  			check.assignVar(lhs, nil, rhs[i], "assignment")
   491  		}
   492  		// Only record comma-ok expression if both assignments succeeded
   493  		// (go.dev/issue/59371).
   494  		if commaOk && rhs[0].mode != invalid && rhs[1].mode != invalid {
   495  			check.recordCommaOkTypes(orig_rhs[0], rhs)
   496  		}
   497  		return
   498  	}
   499  
   500  	// In all other cases we have an assignment mismatch.
   501  	// Only report a mismatch error if there are no other errors on the rhs.
   502  	if rhs[0].mode != invalid {
   503  		check.assignError(orig_rhs, l, r)
   504  	}
   505  	check.useLHS(lhs...)
   506  	// orig_rhs[0] was already evaluated
   507  }
   508  
   509  func (check *Checker) shortVarDecl(pos poser, lhs, rhs []syntax.Expr) {
   510  	top := len(check.delayed)
   511  	scope := check.scope
   512  
   513  	// collect lhs variables
   514  	seen := make(map[string]bool, len(lhs))
   515  	lhsVars := make([]*Var, len(lhs))
   516  	newVars := make([]*Var, 0, len(lhs))
   517  	hasErr := false
   518  	for i, lhs := range lhs {
   519  		ident, _ := lhs.(*syntax.Name)
   520  		if ident == nil {
   521  			check.useLHS(lhs)
   522  			// TODO(gri) This is redundant with a go/parser error. Consider omitting in go/types?
   523  			check.errorf(lhs, BadDecl, "non-name %s on left side of :=", lhs)
   524  			hasErr = true
   525  			continue
   526  		}
   527  
   528  		name := ident.Value
   529  		if name != "_" {
   530  			if seen[name] {
   531  				check.errorf(lhs, RepeatedDecl, "%s repeated on left side of :=", lhs)
   532  				hasErr = true
   533  				continue
   534  			}
   535  			seen[name] = true
   536  		}
   537  
   538  		// Use the correct obj if the ident is redeclared. The
   539  		// variable's scope starts after the declaration; so we
   540  		// must use Scope.Lookup here and call Scope.Insert
   541  		// (via check.declare) later.
   542  		if alt := scope.Lookup(name); alt != nil {
   543  			check.recordUse(ident, alt)
   544  			// redeclared object must be a variable
   545  			if obj, _ := alt.(*Var); obj != nil {
   546  				lhsVars[i] = obj
   547  			} else {
   548  				check.errorf(lhs, UnassignableOperand, "cannot assign to %s", lhs)
   549  				hasErr = true
   550  			}
   551  			continue
   552  		}
   553  
   554  		// declare new variable
   555  		obj := NewVar(ident.Pos(), check.pkg, name, nil)
   556  		lhsVars[i] = obj
   557  		if name != "_" {
   558  			newVars = append(newVars, obj)
   559  		}
   560  		check.recordDef(ident, obj)
   561  	}
   562  
   563  	// create dummy variables where the lhs is invalid
   564  	for i, obj := range lhsVars {
   565  		if obj == nil {
   566  			lhsVars[i] = NewVar(lhs[i].Pos(), check.pkg, "_", nil)
   567  		}
   568  	}
   569  
   570  	check.initVars(lhsVars, rhs, nil)
   571  
   572  	// process function literals in rhs expressions before scope changes
   573  	check.processDelayed(top)
   574  
   575  	if len(newVars) == 0 && !hasErr {
   576  		check.softErrorf(pos, NoNewVar, "no new variables on left side of :=")
   577  		return
   578  	}
   579  
   580  	// declare new variables
   581  	// spec: "The scope of a constant or variable identifier declared inside
   582  	// a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl
   583  	// for short variable declarations) and ends at the end of the innermost
   584  	// containing block."
   585  	scopePos := endPos(rhs[len(rhs)-1])
   586  	for _, obj := range newVars {
   587  		check.declare(scope, nil, obj, scopePos) // id = nil: recordDef already called
   588  	}
   589  }
   590
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