index.go

     1  // Copyright 2021 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 typechecking of index/slice expressions.
     6  
     7  package types2
     8  
     9  import (
    10  	"cmd/compile/internal/syntax"
    11  	"go/constant"
    12  	. "internal/types/errors"
    13  )
    14  
    15  // If e is a valid function instantiation, indexExpr returns true.
    16  // In that case x represents the uninstantiated function value and
    17  // it is the caller's responsibility to instantiate the function.
    18  func (check *Checker) indexExpr(x *operand, e *syntax.IndexExpr) (isFuncInst bool) {
    19  	check.exprOrType(x, e.X, true)
    20  	// x may be generic
    21  
    22  	switch x.mode {
    23  	case invalid:
    24  		check.use(e.Index)
    25  		return false
    26  
    27  	case typexpr:
    28  		// type instantiation
    29  		x.mode = invalid
    30  		// TODO(gri) here we re-evaluate e.X - try to avoid this
    31  		x.typ = check.varType(e)
    32  		if isValid(x.typ) {
    33  			x.mode = typexpr
    34  		}
    35  		return false
    36  
    37  	case value:
    38  		if sig, _ := under(x.typ).(*Signature); sig != nil && sig.TypeParams().Len() > 0 {
    39  			// function instantiation
    40  			return true
    41  		}
    42  	}
    43  
    44  	// x should not be generic at this point, but be safe and check
    45  	check.nonGeneric(nil, x)
    46  	if x.mode == invalid {
    47  		return false
    48  	}
    49  
    50  	// ordinary index expression
    51  	valid := false
    52  	length := int64(-1) // valid if >= 0
    53  	switch typ := under(x.typ).(type) {
    54  	case *Basic:
    55  		if isString(typ) {
    56  			valid = true
    57  			if x.mode == constant_ {
    58  				length = int64(len(constant.StringVal(x.val)))
    59  			}
    60  			// an indexed string always yields a byte value
    61  			// (not a constant) even if the string and the
    62  			// index are constant
    63  			x.mode = value
    64  			x.typ = universeByte // use 'byte' name
    65  		}
    66  
    67  	case *Array:
    68  		valid = true
    69  		length = typ.len
    70  		if x.mode != variable {
    71  			x.mode = value
    72  		}
    73  		x.typ = typ.elem
    74  
    75  	case *Pointer:
    76  		if typ, _ := under(typ.base).(*Array); typ != nil {
    77  			valid = true
    78  			length = typ.len
    79  			x.mode = variable
    80  			x.typ = typ.elem
    81  		}
    82  
    83  	case *Slice:
    84  		valid = true
    85  		x.mode = variable
    86  		x.typ = typ.elem
    87  
    88  	case *Map:
    89  		index := check.singleIndex(e)
    90  		if index == nil {
    91  			x.mode = invalid
    92  			return false
    93  		}
    94  		var key operand
    95  		check.expr(nil, &key, index)
    96  		check.assignment(&key, typ.key, "map index")
    97  		// ok to continue even if indexing failed - map element type is known
    98  		x.mode = mapindex
    99  		x.typ = typ.elem
   100  		x.expr = e
   101  		return false
   102  
   103  	case *Interface:
   104  		if !isTypeParam(x.typ) {
   105  			break
   106  		}
   107  		// TODO(gri) report detailed failure cause for better error messages
   108  		var key, elem Type // key != nil: we must have all maps
   109  		mode := variable   // non-maps result mode
   110  		// TODO(gri) factor out closure and use it for non-typeparam cases as well
   111  		if underIs(x.typ, func(u Type) bool {
   112  			l := int64(-1) // valid if >= 0
   113  			var k, e Type  // k is only set for maps
   114  			switch t := u.(type) {
   115  			case *Basic:
   116  				if isString(t) {
   117  					e = universeByte
   118  					mode = value
   119  				}
   120  			case *Array:
   121  				l = t.len
   122  				e = t.elem
   123  				if x.mode != variable {
   124  					mode = value
   125  				}
   126  			case *Pointer:
   127  				if t, _ := under(t.base).(*Array); t != nil {
   128  					l = t.len
   129  					e = t.elem
   130  				}
   131  			case *Slice:
   132  				e = t.elem
   133  			case *Map:
   134  				k = t.key
   135  				e = t.elem
   136  			}
   137  			if e == nil {
   138  				return false
   139  			}
   140  			if elem == nil {
   141  				// first type
   142  				length = l
   143  				key, elem = k, e
   144  				return true
   145  			}
   146  			// all map keys must be identical (incl. all nil)
   147  			// (that is, we cannot mix maps with other types)
   148  			if !Identical(key, k) {
   149  				return false
   150  			}
   151  			// all element types must be identical
   152  			if !Identical(elem, e) {
   153  				return false
   154  			}
   155  			// track the minimal length for arrays, if any
   156  			if l >= 0 && l < length {
   157  				length = l
   158  			}
   159  			return true
   160  		}) {
   161  			// For maps, the index expression must be assignable to the map key type.
   162  			if key != nil {
   163  				index := check.singleIndex(e)
   164  				if index == nil {
   165  					x.mode = invalid
   166  					return false
   167  				}
   168  				var k operand
   169  				check.expr(nil, &k, index)
   170  				check.assignment(&k, key, "map index")
   171  				// ok to continue even if indexing failed - map element type is known
   172  				x.mode = mapindex
   173  				x.typ = elem
   174  				x.expr = e
   175  				return false
   176  			}
   177  
   178  			// no maps
   179  			valid = true
   180  			x.mode = mode
   181  			x.typ = elem
   182  		}
   183  	}
   184  
   185  	if !valid {
   186  		check.errorf(e.Pos(), NonSliceableOperand, "cannot index %s", x)
   187  		check.use(e.Index)
   188  		x.mode = invalid
   189  		return false
   190  	}
   191  
   192  	index := check.singleIndex(e)
   193  	if index == nil {
   194  		x.mode = invalid
   195  		return false
   196  	}
   197  
   198  	// In pathological (invalid) cases (e.g.: type T1 [][[]T1{}[0][0]]T0)
   199  	// the element type may be accessed before it's set. Make sure we have
   200  	// a valid type.
   201  	if x.typ == nil {
   202  		x.typ = Typ[Invalid]
   203  	}
   204  
   205  	check.index(index, length)
   206  	return false
   207  }
   208  
   209  func (check *Checker) sliceExpr(x *operand, e *syntax.SliceExpr) {
   210  	check.expr(nil, x, e.X)
   211  	if x.mode == invalid {
   212  		check.use(e.Index[:]...)
   213  		return
   214  	}
   215  
   216  	// determine common underlying type cu
   217  	var ct, cu Type // type and respective common underlying type
   218  	var hasString bool
   219  	typeset(x.typ, func(t, u Type) bool {
   220  		if u == nil {
   221  			check.errorf(x, NonSliceableOperand, "cannot slice %s: no specific type in %s", x, x.typ)
   222  			cu = nil
   223  			return false
   224  		}
   225  
   226  		// Treat strings like byte slices but remember that we saw a string.
   227  		if isString(u) {
   228  			u = NewSlice(universeByte)
   229  			hasString = true
   230  		}
   231  
   232  		// If this is the first type we're seeing, we're done.
   233  		if cu == nil {
   234  			ct, cu = t, u
   235  			return true
   236  		}
   237  
   238  		// Otherwise, the current type must have the same underlying type as all previous types.
   239  		if !Identical(cu, u) {
   240  			check.errorf(x, NonSliceableOperand, "cannot slice %s: %s and %s have different underlying types", x, ct, t)
   241  			cu = nil
   242  			return false
   243  		}
   244  
   245  		return true
   246  	})
   247  	if hasString {
   248  		// If we saw a string, proceed with string type,
   249  		// but don't go from untyped string to string.
   250  		cu = Typ[String]
   251  		if !isTypeParam(x.typ) {
   252  			cu = under(x.typ) // untyped string remains untyped
   253  		}
   254  	}
   255  
   256  	valid := false
   257  	length := int64(-1) // valid if >= 0
   258  	switch u := cu.(type) {
   259  	case nil:
   260  		// error reported above
   261  		x.mode = invalid
   262  		return
   263  
   264  	case *Basic:
   265  		if isString(u) {
   266  			if e.Full {
   267  				at := e.Index[2]
   268  				if at == nil {
   269  					at = e // e.Index[2] should be present but be careful
   270  				}
   271  				check.error(at, InvalidSliceExpr, invalidOp+"3-index slice of string")
   272  				x.mode = invalid
   273  				return
   274  			}
   275  			valid = true
   276  			if x.mode == constant_ {
   277  				length = int64(len(constant.StringVal(x.val)))
   278  			}
   279  			// spec: "For untyped string operands the result
   280  			// is a non-constant value of type string."
   281  			if isUntyped(x.typ) {
   282  				x.typ = Typ[String]
   283  			}
   284  		}
   285  
   286  	case *Array:
   287  		valid = true
   288  		length = u.len
   289  		if x.mode != variable {
   290  			check.errorf(x, NonSliceableOperand, "cannot slice unaddressable value %s", x)
   291  			x.mode = invalid
   292  			return
   293  		}
   294  		x.typ = &Slice{elem: u.elem}
   295  
   296  	case *Pointer:
   297  		if u, _ := under(u.base).(*Array); u != nil {
   298  			valid = true
   299  			length = u.len
   300  			x.typ = &Slice{elem: u.elem}
   301  		}
   302  
   303  	case *Slice:
   304  		valid = true
   305  		// x.typ doesn't change
   306  	}
   307  
   308  	if !valid {
   309  		check.errorf(x, NonSliceableOperand, "cannot slice %s", x)
   310  		x.mode = invalid
   311  		return
   312  	}
   313  
   314  	x.mode = value
   315  
   316  	// spec: "Only the first index may be omitted; it defaults to 0."
   317  	if e.Full && (e.Index[1] == nil || e.Index[2] == nil) {
   318  		check.error(e, InvalidSyntaxTree, "2nd and 3rd index required in 3-index slice")
   319  		x.mode = invalid
   320  		return
   321  	}
   322  
   323  	// check indices
   324  	var ind [3]int64
   325  	for i, expr := range e.Index {
   326  		x := int64(-1)
   327  		switch {
   328  		case expr != nil:
   329  			// The "capacity" is only known statically for strings, arrays,
   330  			// and pointers to arrays, and it is the same as the length for
   331  			// those types.
   332  			max := int64(-1)
   333  			if length >= 0 {
   334  				max = length + 1
   335  			}
   336  			if _, v := check.index(expr, max); v >= 0 {
   337  				x = v
   338  			}
   339  		case i == 0:
   340  			// default is 0 for the first index
   341  			x = 0
   342  		case length >= 0:
   343  			// default is length (== capacity) otherwise
   344  			x = length
   345  		}
   346  		ind[i] = x
   347  	}
   348  
   349  	// constant indices must be in range
   350  	// (check.index already checks that existing indices >= 0)
   351  L:
   352  	for i, x := range ind[:len(ind)-1] {
   353  		if x > 0 {
   354  			for j, y := range ind[i+1:] {
   355  				if y >= 0 && y < x {
   356  					// The value y corresponds to the expression e.Index[i+1+j].
   357  					// Because y >= 0, it must have been set from the expression
   358  					// when checking indices and thus e.Index[i+1+j] is not nil.
   359  					check.errorf(e.Index[i+1+j], SwappedSliceIndices, "invalid slice indices: %d < %d", y, x)
   360  					break L // only report one error, ok to continue
   361  				}
   362  			}
   363  		}
   364  	}
   365  }
   366  
   367  // singleIndex returns the (single) index from the index expression e.
   368  // If the index is missing, or if there are multiple indices, an error
   369  // is reported and the result is nil.
   370  func (check *Checker) singleIndex(e *syntax.IndexExpr) syntax.Expr {
   371  	index := e.Index
   372  	if index == nil {
   373  		check.errorf(e, InvalidSyntaxTree, "missing index for %s", e.X)
   374  		return nil
   375  	}
   376  	if l, _ := index.(*syntax.ListExpr); l != nil {
   377  		if n := len(l.ElemList); n <= 1 {
   378  			check.errorf(e, InvalidSyntaxTree, "invalid use of ListExpr for index expression %v with %d indices", e, n)
   379  			return nil
   380  		}
   381  		// len(l.ElemList) > 1
   382  		check.error(l.ElemList[1], InvalidIndex, invalidOp+"more than one index")
   383  		index = l.ElemList[0] // continue with first index
   384  	}
   385  	return index
   386  }
   387  
   388  // index checks an index expression for validity.
   389  // If max >= 0, it is the upper bound for index.
   390  // If the result typ is != Typ[Invalid], index is valid and typ is its (possibly named) integer type.
   391  // If the result val >= 0, index is valid and val is its constant int value.
   392  func (check *Checker) index(index syntax.Expr, max int64) (typ Type, val int64) {
   393  	typ = Typ[Invalid]
   394  	val = -1
   395  
   396  	var x operand
   397  	check.expr(nil, &x, index)
   398  	if !check.isValidIndex(&x, InvalidIndex, "index", false) {
   399  		return
   400  	}
   401  
   402  	if x.mode != constant_ {
   403  		return x.typ, -1
   404  	}
   405  
   406  	if x.val.Kind() == constant.Unknown {
   407  		return
   408  	}
   409  
   410  	v, ok := constant.Int64Val(x.val)
   411  	assert(ok)
   412  	if max >= 0 && v >= max {
   413  		check.errorf(&x, InvalidIndex, invalidArg+"index %s out of bounds [0:%d]", x.val.String(), max)
   414  		return
   415  	}
   416  
   417  	// 0 <= v [ && v < max ]
   418  	return x.typ, v
   419  }
   420  
   421  // isValidIndex checks whether operand x satisfies the criteria for integer
   422  // index values. If allowNegative is set, a constant operand may be negative.
   423  // If the operand is not valid, an error is reported (using what as context)
   424  // and the result is false.
   425  func (check *Checker) isValidIndex(x *operand, code Code, what string, allowNegative bool) bool {
   426  	if x.mode == invalid {
   427  		return false
   428  	}
   429  
   430  	// spec: "a constant index that is untyped is given type int"
   431  	check.convertUntyped(x, Typ[Int])
   432  	if x.mode == invalid {
   433  		return false
   434  	}
   435  
   436  	// spec: "the index x must be of integer type or an untyped constant"
   437  	if !allInteger(x.typ) {
   438  		check.errorf(x, code, invalidArg+"%s %s must be integer", what, x)
   439  		return false
   440  	}
   441  
   442  	if x.mode == constant_ {
   443  		// spec: "a constant index must be non-negative ..."
   444  		if !allowNegative && constant.Sign(x.val) < 0 {
   445  			check.errorf(x, code, invalidArg+"%s %s must not be negative", what, x)
   446  			return false
   447  		}
   448  
   449  		// spec: "... and representable by a value of type int"
   450  		if !representableConst(x.val, check, Typ[Int], &x.val) {
   451  			check.errorf(x, code, invalidArg+"%s %s overflows int", what, x)
   452  			return false
   453  		}
   454  	}
   455  
   456  	return true
   457  }
   458
View as plain text