selection.go

     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 Selections.
     6  
     7  package types2
     8  
     9  import (
    10  	"bytes"
    11  	"fmt"
    12  )
    13  
    14  // SelectionKind describes the kind of a selector expression x.f
    15  // (excluding qualified identifiers).
    16  //
    17  // If x is a struct or *struct, a selector expression x.f may denote a
    18  // sequence of selection operations x.a.b.c.f. The SelectionKind
    19  // describes the kind of the final (explicit) operation; all the
    20  // previous (implicit) operations are always field selections.
    21  // Each element of Indices specifies an implicit field (a, b, c)
    22  // by its index in the struct type of the field selection operand.
    23  //
    24  // For a FieldVal operation, the final selection refers to the field
    25  // specified by Selection.Obj.
    26  //
    27  // For a MethodVal operation, the final selection refers to a method.
    28  // If the "pointerness" of the method's declared receiver does not
    29  // match that of the effective receiver after implicit field
    30  // selection, then an & or * operation is implicitly applied to the
    31  // receiver variable or value.
    32  // So, x.f denotes (&x.a.b.c).f when f requires a pointer receiver but
    33  // x.a.b.c is a non-pointer variable; and it denotes (*x.a.b.c).f when
    34  // f requires a non-pointer receiver but x.a.b.c is a pointer value.
    35  //
    36  // All pointer indirections, whether due to implicit or explicit field
    37  // selections or * operations inserted for "pointerness", panic if
    38  // applied to a nil pointer, so a method call x.f() may panic even
    39  // before the function call.
    40  //
    41  // By contrast, a MethodExpr operation T.f is essentially equivalent
    42  // to a function literal of the form:
    43  //
    44  //	func(x T, args) (results) { return x.f(args) }
    45  //
    46  // Consequently, any implicit field selections and * operations
    47  // inserted for "pointerness" are not evaluated until the function is
    48  // called, so a T.f or (*T).f expression never panics.
    49  type SelectionKind int
    50  
    51  const (
    52  	FieldVal   SelectionKind = iota // x.f is a struct field selector
    53  	MethodVal                       // x.f is a method selector
    54  	MethodExpr                      // x.f is a method expression
    55  )
    56  
    57  // A Selection describes a selector expression x.f.
    58  // For the declarations:
    59  //
    60  //	type T struct{ x int; E }
    61  //	type E struct{}
    62  //	func (e E) m() {}
    63  //	var p *T
    64  //
    65  // the following relations exist:
    66  //
    67  //	Selector    Kind          Recv    Obj    Type       Index     Indirect
    68  //
    69  //	p.x         FieldVal      T       x      int        {0}       true
    70  //	p.m         MethodVal     *T      m      func()     {1, 0}    true
    71  //	T.m         MethodExpr    T       m      func(T)    {1, 0}    false
    72  type Selection struct {
    73  	kind     SelectionKind
    74  	recv     Type   // type of x
    75  	obj      Object // object denoted by x.f
    76  	index    []int  // path from x to x.f
    77  	indirect bool   // set if there was any pointer indirection on the path
    78  }
    79  
    80  // Kind returns the selection kind.
    81  func (s *Selection) Kind() SelectionKind { return s.kind }
    82  
    83  // Recv returns the type of x in x.f.
    84  func (s *Selection) Recv() Type { return s.recv }
    85  
    86  // Obj returns the object denoted by x.f; a *Var for
    87  // a field selection, and a *Func in all other cases.
    88  func (s *Selection) Obj() Object { return s.obj }
    89  
    90  // Type returns the type of x.f, which may be different from the type of f.
    91  // See Selection for more information.
    92  func (s *Selection) Type() Type {
    93  	switch s.kind {
    94  	case MethodVal:
    95  		// The type of x.f is a method with its receiver type set
    96  		// to the type of x.
    97  		sig := *s.obj.(*Func).typ.(*Signature)
    98  		recv := *sig.recv
    99  		recv.typ = s.recv
   100  		sig.recv = &recv
   101  		return &sig
   102  
   103  	case MethodExpr:
   104  		// The type of x.f is a function (without receiver)
   105  		// and an additional first argument with the same type as x.
   106  		// TODO(gri) Similar code is already in call.go - factor!
   107  		// TODO(gri) Compute this eagerly to avoid allocations.
   108  		sig := *s.obj.(*Func).typ.(*Signature)
   109  		arg0 := *sig.recv
   110  		sig.recv = nil
   111  		arg0.typ = s.recv
   112  		var params []*Var
   113  		if sig.params != nil {
   114  			params = sig.params.vars
   115  		}
   116  		sig.params = NewTuple(append([]*Var{&arg0}, params...)...)
   117  		return &sig
   118  	}
   119  
   120  	// In all other cases, the type of x.f is the type of x.
   121  	return s.obj.Type()
   122  }
   123  
   124  // Index describes the path from x to f in x.f.
   125  // The last index entry is the field or method index of the type declaring f;
   126  // either:
   127  //
   128  //  1. the list of declared methods of a named type; or
   129  //  2. the list of methods of an interface type; or
   130  //  3. the list of fields of a struct type.
   131  //
   132  // The earlier index entries are the indices of the embedded fields implicitly
   133  // traversed to get from (the type of) x to f, starting at embedding depth 0.
   134  func (s *Selection) Index() []int { return s.index }
   135  
   136  // Indirect reports whether any pointer indirection was required to get from
   137  // x to f in x.f.
   138  //
   139  // Beware: Indirect spuriously returns true (Go issue #8353) for a
   140  // MethodVal selection in which the receiver argument and parameter
   141  // both have type *T so there is no indirection.
   142  // Unfortunately, a fix is too risky.
   143  func (s *Selection) Indirect() bool { return s.indirect }
   144  
   145  func (s *Selection) String() string { return SelectionString(s, nil) }
   146  
   147  // SelectionString returns the string form of s.
   148  // The Qualifier controls the printing of
   149  // package-level objects, and may be nil.
   150  //
   151  // Examples:
   152  //
   153  //	"field (T) f int"
   154  //	"method (T) f(X) Y"
   155  //	"method expr (T) f(X) Y"
   156  func SelectionString(s *Selection, qf Qualifier) string {
   157  	var k string
   158  	switch s.kind {
   159  	case FieldVal:
   160  		k = "field "
   161  	case MethodVal:
   162  		k = "method "
   163  	case MethodExpr:
   164  		k = "method expr "
   165  	default:
   166  		panic("unreachable")
   167  	}
   168  	var buf bytes.Buffer
   169  	buf.WriteString(k)
   170  	buf.WriteByte('(')
   171  	WriteType(&buf, s.Recv(), qf)
   172  	fmt.Fprintf(&buf, ") %s", s.obj.Name())
   173  	if T := s.Type(); s.kind == FieldVal {
   174  		buf.WriteByte(' ')
   175  		WriteType(&buf, T, qf)
   176  	} else {
   177  		WriteSignature(&buf, T.(*Signature), qf)
   178  	}
   179  	return buf.String()
   180  }
   181
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