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Source file src/go/types/lookup.go

Documentation: go/types

     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 various field and method lookup functions.
     6  
     7  package types
     8  
     9  // Internal use of LookupFieldOrMethod: If the obj result is a method
    10  // associated with a concrete (non-interface) type, the method's signature
    11  // may not be fully set up. Call Checker.objDecl(obj, nil) before accessing
    12  // the method's type.
    13  
    14  // LookupFieldOrMethod looks up a field or method with given package and name
    15  // in T and returns the corresponding *Var or *Func, an index sequence, and a
    16  // bool indicating if there were any pointer indirections on the path to the
    17  // field or method. If addressable is set, T is the type of an addressable
    18  // variable (only matters for method lookups).
    19  //
    20  // The last index entry is the field or method index in the (possibly embedded)
    21  // type where the entry was found, either:
    22  //
    23  //	1) the list of declared methods of a named type; or
    24  //	2) the list of all methods (method set) of an interface type; or
    25  //	3) the list of fields of a struct type.
    26  //
    27  // The earlier index entries are the indices of the embedded struct fields
    28  // traversed to get to the found entry, starting at depth 0.
    29  //
    30  // If no entry is found, a nil object is returned. In this case, the returned
    31  // index and indirect values have the following meaning:
    32  //
    33  //	- If index != nil, the index sequence points to an ambiguous entry
    34  //	(the same name appeared more than once at the same embedding level).
    35  //
    36  //	- If indirect is set, a method with a pointer receiver type was found
    37  //      but there was no pointer on the path from the actual receiver type to
    38  //	the method's formal receiver base type, nor was the receiver addressable.
    39  //
    40  func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
    41  	// Methods cannot be associated to a named pointer type
    42  	// (spec: "The type denoted by T is called the receiver base type;
    43  	// it must not be a pointer or interface type and it must be declared
    44  	// in the same package as the method.").
    45  	// Thus, if we have a named pointer type, proceed with the underlying
    46  	// pointer type but discard the result if it is a method since we would
    47  	// not have found it for T (see also issue 8590).
    48  	if t, _ := T.(*Named); t != nil {
    49  		if p, _ := t.underlying.(*Pointer); p != nil {
    50  			obj, index, indirect = lookupFieldOrMethod(p, false, pkg, name)
    51  			if _, ok := obj.(*Func); ok {
    52  				return nil, nil, false
    53  			}
    54  			return
    55  		}
    56  	}
    57  
    58  	return lookupFieldOrMethod(T, addressable, pkg, name)
    59  }
    60  
    61  // TODO(gri) The named type consolidation and seen maps below must be
    62  //           indexed by unique keys for a given type. Verify that named
    63  //           types always have only one representation (even when imported
    64  //           indirectly via different packages.)
    65  
    66  func lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
    67  	// WARNING: The code in this function is extremely subtle - do not modify casually!
    68  	//          This function and NewMethodSet should be kept in sync.
    69  
    70  	if name == "_" {
    71  		return // blank fields/methods are never found
    72  	}
    73  
    74  	typ, isPtr := deref(T)
    75  
    76  	// *typ where typ is an interface has no methods.
    77  	if isPtr && IsInterface(typ) {
    78  		return
    79  	}
    80  
    81  	// Start with typ as single entry at shallowest depth.
    82  	current := []embeddedType{{typ, nil, isPtr, false}}
    83  
    84  	// Named types that we have seen already, allocated lazily.
    85  	// Used to avoid endless searches in case of recursive types.
    86  	// Since only Named types can be used for recursive types, we
    87  	// only need to track those.
    88  	// (If we ever allow type aliases to construct recursive types,
    89  	// we must use type identity rather than pointer equality for
    90  	// the map key comparison, as we do in consolidateMultiples.)
    91  	var seen map[*Named]bool
    92  
    93  	// search current depth
    94  	for len(current) > 0 {
    95  		var next []embeddedType // embedded types found at current depth
    96  
    97  		// look for (pkg, name) in all types at current depth
    98  		for _, e := range current {
    99  			typ := e.typ
   100  
   101  			// If we have a named type, we may have associated methods.
   102  			// Look for those first.
   103  			if named, _ := typ.(*Named); named != nil {
   104  				if seen[named] {
   105  					// We have seen this type before, at a more shallow depth
   106  					// (note that multiples of this type at the current depth
   107  					// were consolidated before). The type at that depth shadows
   108  					// this same type at the current depth, so we can ignore
   109  					// this one.
   110  					continue
   111  				}
   112  				if seen == nil {
   113  					seen = make(map[*Named]bool)
   114  				}
   115  				seen[named] = true
   116  
   117  				// look for a matching attached method
   118  				if i, m := lookupMethod(named.methods, pkg, name); m != nil {
   119  					// potential match
   120  					// caution: method may not have a proper signature yet
   121  					index = concat(e.index, i)
   122  					if obj != nil || e.multiples {
   123  						return nil, index, false // collision
   124  					}
   125  					obj = m
   126  					indirect = e.indirect
   127  					continue // we can't have a matching field or interface method
   128  				}
   129  
   130  				// continue with underlying type
   131  				typ = named.underlying
   132  			}
   133  
   134  			switch t := typ.(type) {
   135  			case *Struct:
   136  				// look for a matching field and collect embedded types
   137  				for i, f := range t.fields {
   138  					if f.sameId(pkg, name) {
   139  						assert(f.typ != nil)
   140  						index = concat(e.index, i)
   141  						if obj != nil || e.multiples {
   142  							return nil, index, false // collision
   143  						}
   144  						obj = f
   145  						indirect = e.indirect
   146  						continue // we can't have a matching interface method
   147  					}
   148  					// Collect embedded struct fields for searching the next
   149  					// lower depth, but only if we have not seen a match yet
   150  					// (if we have a match it is either the desired field or
   151  					// we have a name collision on the same depth; in either
   152  					// case we don't need to look further).
   153  					// Embedded fields are always of the form T or *T where
   154  					// T is a type name. If e.typ appeared multiple times at
   155  					// this depth, f.typ appears multiple times at the next
   156  					// depth.
   157  					if obj == nil && f.embedded {
   158  						typ, isPtr := deref(f.typ)
   159  						// TODO(gri) optimization: ignore types that can't
   160  						// have fields or methods (only Named, Struct, and
   161  						// Interface types need to be considered).
   162  						next = append(next, embeddedType{typ, concat(e.index, i), e.indirect || isPtr, e.multiples})
   163  					}
   164  				}
   165  
   166  			case *Interface:
   167  				// look for a matching method
   168  				// TODO(gri) t.allMethods is sorted - use binary search
   169  				if i, m := lookupMethod(t.allMethods, pkg, name); m != nil {
   170  					assert(m.typ != nil)
   171  					index = concat(e.index, i)
   172  					if obj != nil || e.multiples {
   173  						return nil, index, false // collision
   174  					}
   175  					obj = m
   176  					indirect = e.indirect
   177  				}
   178  			}
   179  		}
   180  
   181  		if obj != nil {
   182  			// found a potential match
   183  			// spec: "A method call x.m() is valid if the method set of (the type of) x
   184  			//        contains m and the argument list can be assigned to the parameter
   185  			//        list of m. If x is addressable and &x's method set contains m, x.m()
   186  			//        is shorthand for (&x).m()".
   187  			if f, _ := obj.(*Func); f != nil && ptrRecv(f) && !indirect && !addressable {
   188  				return nil, nil, true // pointer/addressable receiver required
   189  			}
   190  			return
   191  		}
   192  
   193  		current = consolidateMultiples(next)
   194  	}
   195  
   196  	return nil, nil, false // not found
   197  }
   198  
   199  // embeddedType represents an embedded type
   200  type embeddedType struct {
   201  	typ       Type
   202  	index     []int // embedded field indices, starting with index at depth 0
   203  	indirect  bool  // if set, there was a pointer indirection on the path to this field
   204  	multiples bool  // if set, typ appears multiple times at this depth
   205  }
   206  
   207  // consolidateMultiples collects multiple list entries with the same type
   208  // into a single entry marked as containing multiples. The result is the
   209  // consolidated list.
   210  func consolidateMultiples(list []embeddedType) []embeddedType {
   211  	if len(list) <= 1 {
   212  		return list // at most one entry - nothing to do
   213  	}
   214  
   215  	n := 0                     // number of entries w/ unique type
   216  	prev := make(map[Type]int) // index at which type was previously seen
   217  	for _, e := range list {
   218  		if i, found := lookupType(prev, e.typ); found {
   219  			list[i].multiples = true
   220  			// ignore this entry
   221  		} else {
   222  			prev[e.typ] = n
   223  			list[n] = e
   224  			n++
   225  		}
   226  	}
   227  	return list[:n]
   228  }
   229  
   230  func lookupType(m map[Type]int, typ Type) (int, bool) {
   231  	// fast path: maybe the types are equal
   232  	if i, found := m[typ]; found {
   233  		return i, true
   234  	}
   235  
   236  	for t, i := range m {
   237  		if Identical(t, typ) {
   238  			return i, true
   239  		}
   240  	}
   241  
   242  	return 0, false
   243  }
   244  
   245  // MissingMethod returns (nil, false) if V implements T, otherwise it
   246  // returns a missing method required by T and whether it is missing or
   247  // just has the wrong type.
   248  //
   249  // For non-interface types V, or if static is set, V implements T if all
   250  // methods of T are present in V. Otherwise (V is an interface and static
   251  // is not set), MissingMethod only checks that methods of T which are also
   252  // present in V have matching types (e.g., for a type assertion x.(T) where
   253  // x is of interface type V).
   254  //
   255  func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) {
   256  	return (*Checker)(nil).missingMethod(V, T, static)
   257  }
   258  
   259  // missingMethod is like MissingMethod but accepts a receiver.
   260  // The receiver may be nil if missingMethod is invoked through
   261  // an exported API call (such as MissingMethod), i.e., when all
   262  // methods have been type-checked.
   263  func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) {
   264  	// fast path for common case
   265  	if T.Empty() {
   266  		return
   267  	}
   268  
   269  	// TODO(gri) Consider using method sets here. Might be more efficient.
   270  
   271  	if ityp, _ := V.Underlying().(*Interface); ityp != nil {
   272  		// TODO(gri) allMethods is sorted - can do this more efficiently
   273  		for _, m := range T.allMethods {
   274  			_, obj := lookupMethod(ityp.allMethods, m.pkg, m.name)
   275  			switch {
   276  			case obj == nil:
   277  				if static {
   278  					return m, false
   279  				}
   280  			case !Identical(obj.Type(), m.typ):
   281  				return m, true
   282  			}
   283  		}
   284  		return
   285  	}
   286  
   287  	// A concrete type implements T if it implements all methods of T.
   288  	for _, m := range T.allMethods {
   289  		obj, _, _ := lookupFieldOrMethod(V, false, m.pkg, m.name)
   290  
   291  		// we must have a method (not a field of matching function type)
   292  		f, _ := obj.(*Func)
   293  		if f == nil {
   294  			return m, false
   295  		}
   296  
   297  		// methods may not have a fully set up signature yet
   298  		if check != nil {
   299  			check.objDecl(f, nil)
   300  		}
   301  
   302  		if !Identical(f.typ, m.typ) {
   303  			return m, true
   304  		}
   305  	}
   306  
   307  	return
   308  }
   309  
   310  // assertableTo reports whether a value of type V can be asserted to have type T.
   311  // It returns (nil, false) as affirmative answer. Otherwise it returns a missing
   312  // method required by V and whether it is missing or just has the wrong type.
   313  // The receiver may be nil if assertableTo is invoked through an exported API call
   314  // (such as AssertableTo), i.e., when all methods have been type-checked.
   315  func (check *Checker) assertableTo(V *Interface, T Type) (method *Func, wrongType bool) {
   316  	// no static check is required if T is an interface
   317  	// spec: "If T is an interface type, x.(T) asserts that the
   318  	//        dynamic type of x implements the interface T."
   319  	if _, ok := T.Underlying().(*Interface); ok && !strict {
   320  		return
   321  	}
   322  	return check.missingMethod(T, V, false)
   323  }
   324  
   325  // deref dereferences typ if it is a *Pointer and returns its base and true.
   326  // Otherwise it returns (typ, false).
   327  func deref(typ Type) (Type, bool) {
   328  	if p, _ := typ.(*Pointer); p != nil {
   329  		return p.base, true
   330  	}
   331  	return typ, false
   332  }
   333  
   334  // derefStructPtr dereferences typ if it is a (named or unnamed) pointer to a
   335  // (named or unnamed) struct and returns its base. Otherwise it returns typ.
   336  func derefStructPtr(typ Type) Type {
   337  	if p, _ := typ.Underlying().(*Pointer); p != nil {
   338  		if _, ok := p.base.Underlying().(*Struct); ok {
   339  			return p.base
   340  		}
   341  	}
   342  	return typ
   343  }
   344  
   345  // concat returns the result of concatenating list and i.
   346  // The result does not share its underlying array with list.
   347  func concat(list []int, i int) []int {
   348  	var t []int
   349  	t = append(t, list...)
   350  	return append(t, i)
   351  }
   352  
   353  // fieldIndex returns the index for the field with matching package and name, or a value < 0.
   354  func fieldIndex(fields []*Var, pkg *Package, name string) int {
   355  	if name != "_" {
   356  		for i, f := range fields {
   357  			if f.sameId(pkg, name) {
   358  				return i
   359  			}
   360  		}
   361  	}
   362  	return -1
   363  }
   364  
   365  // lookupMethod returns the index of and method with matching package and name, or (-1, nil).
   366  func lookupMethod(methods []*Func, pkg *Package, name string) (int, *Func) {
   367  	if name != "_" {
   368  		for i, m := range methods {
   369  			if m.sameId(pkg, name) {
   370  				return i, m
   371  			}
   372  		}
   373  	}
   374  	return -1, nil
   375  }
   376  

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