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

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