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Source file src/go/types/lookup.go

Documentation: go/types

  // Copyright 2013 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // This file implements various field and method lookup functions.
  
  package types
  
  // LookupFieldOrMethod looks up a field or method with given package and name
  // in T and returns the corresponding *Var or *Func, an index sequence, and a
  // bool indicating if there were any pointer indirections on the path to the
  // field or method. If addressable is set, T is the type of an addressable
  // variable (only matters for method lookups).
  //
  // The last index entry is the field or method index in the (possibly embedded)
  // type where the entry was found, either:
  //
  //	1) the list of declared methods of a named type; or
  //	2) the list of all methods (method set) of an interface type; or
  //	3) the list of fields of a struct type.
  //
  // The earlier index entries are the indices of the anonymous struct fields
  // traversed to get to the found entry, starting at depth 0.
  //
  // If no entry is found, a nil object is returned. In this case, the returned
  // index and indirect values have the following meaning:
  //
  //	- If index != nil, the index sequence points to an ambiguous entry
  //	(the same name appeared more than once at the same embedding level).
  //
  //	- If indirect is set, a method with a pointer receiver type was found
  //      but there was no pointer on the path from the actual receiver type to
  //	the method's formal receiver base type, nor was the receiver addressable.
  //
  func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
  	// Methods cannot be associated to a named pointer type
  	// (spec: "The type denoted by T is called the receiver base type;
  	// it must not be a pointer or interface type and it must be declared
  	// in the same package as the method.").
  	// Thus, if we have a named pointer type, proceed with the underlying
  	// pointer type but discard the result if it is a method since we would
  	// not have found it for T (see also issue 8590).
  	if t, _ := T.(*Named); t != nil {
  		if p, _ := t.underlying.(*Pointer); p != nil {
  			obj, index, indirect = lookupFieldOrMethod(p, false, pkg, name)
  			if _, ok := obj.(*Func); ok {
  				return nil, nil, false
  			}
  			return
  		}
  	}
  
  	return lookupFieldOrMethod(T, addressable, pkg, name)
  }
  
  // TODO(gri) The named type consolidation and seen maps below must be
  //           indexed by unique keys for a given type. Verify that named
  //           types always have only one representation (even when imported
  //           indirectly via different packages.)
  
  func lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
  	// WARNING: The code in this function is extremely subtle - do not modify casually!
  	//          This function and NewMethodSet should be kept in sync.
  
  	if name == "_" {
  		return // blank fields/methods are never found
  	}
  
  	typ, isPtr := deref(T)
  
  	// *typ where typ is an interface has no methods.
  	if isPtr && IsInterface(typ) {
  		return
  	}
  
  	// Start with typ as single entry at shallowest depth.
  	current := []embeddedType{{typ, nil, isPtr, false}}
  
  	// Named types that we have seen already, allocated lazily.
  	// Used to avoid endless searches in case of recursive types.
  	// Since only Named types can be used for recursive types, we
  	// only need to track those.
  	// (If we ever allow type aliases to construct recursive types,
  	// we must use type identity rather than pointer equality for
  	// the map key comparison, as we do in consolidateMultiples.)
  	var seen map[*Named]bool
  
  	// search current depth
  	for len(current) > 0 {
  		var next []embeddedType // embedded types found at current depth
  
  		// look for (pkg, name) in all types at current depth
  		for _, e := range current {
  			typ := e.typ
  
  			// If we have a named type, we may have associated methods.
  			// Look for those first.
  			if named, _ := typ.(*Named); named != nil {
  				if seen[named] {
  					// We have seen this type before, at a more shallow depth
  					// (note that multiples of this type at the current depth
  					// were consolidated before). The type at that depth shadows
  					// this same type at the current depth, so we can ignore
  					// this one.
  					continue
  				}
  				if seen == nil {
  					seen = make(map[*Named]bool)
  				}
  				seen[named] = true
  
  				// look for a matching attached method
  				if i, m := lookupMethod(named.methods, pkg, name); m != nil {
  					// potential match
  					assert(m.typ != nil)
  					index = concat(e.index, i)
  					if obj != nil || e.multiples {
  						return nil, index, false // collision
  					}
  					obj = m
  					indirect = e.indirect
  					continue // we can't have a matching field or interface method
  				}
  
  				// continue with underlying type
  				typ = named.underlying
  			}
  
  			switch t := typ.(type) {
  			case *Struct:
  				// look for a matching field and collect embedded types
  				for i, f := range t.fields {
  					if f.sameId(pkg, name) {
  						assert(f.typ != nil)
  						index = concat(e.index, i)
  						if obj != nil || e.multiples {
  							return nil, index, false // collision
  						}
  						obj = f
  						indirect = e.indirect
  						continue // we can't have a matching interface method
  					}
  					// Collect embedded struct fields for searching the next
  					// lower depth, but only if we have not seen a match yet
  					// (if we have a match it is either the desired field or
  					// we have a name collision on the same depth; in either
  					// case we don't need to look further).
  					// Embedded fields are always of the form T or *T where
  					// T is a type name. If e.typ appeared multiple times at
  					// this depth, f.typ appears multiple times at the next
  					// depth.
  					if obj == nil && f.anonymous {
  						typ, isPtr := deref(f.typ)
  						// TODO(gri) optimization: ignore types that can't
  						// have fields or methods (only Named, Struct, and
  						// Interface types need to be considered).
  						next = append(next, embeddedType{typ, concat(e.index, i), e.indirect || isPtr, e.multiples})
  					}
  				}
  
  			case *Interface:
  				// look for a matching method
  				// TODO(gri) t.allMethods is sorted - use binary search
  				if i, m := lookupMethod(t.allMethods, pkg, name); m != nil {
  					assert(m.typ != nil)
  					index = concat(e.index, i)
  					if obj != nil || e.multiples {
  						return nil, index, false // collision
  					}
  					obj = m
  					indirect = e.indirect
  				}
  			}
  		}
  
  		if obj != nil {
  			// found a potential match
  			// spec: "A method call x.m() is valid if the method set of (the type of) x
  			//        contains m and the argument list can be assigned to the parameter
  			//        list of m. If x is addressable and &x's method set contains m, x.m()
  			//        is shorthand for (&x).m()".
  			if f, _ := obj.(*Func); f != nil && ptrRecv(f) && !indirect && !addressable {
  				return nil, nil, true // pointer/addressable receiver required
  			}
  			return
  		}
  
  		current = consolidateMultiples(next)
  	}
  
  	return nil, nil, false // not found
  }
  
  // embeddedType represents an embedded type
  type embeddedType struct {
  	typ       Type
  	index     []int // embedded field indices, starting with index at depth 0
  	indirect  bool  // if set, there was a pointer indirection on the path to this field
  	multiples bool  // if set, typ appears multiple times at this depth
  }
  
  // consolidateMultiples collects multiple list entries with the same type
  // into a single entry marked as containing multiples. The result is the
  // consolidated list.
  func consolidateMultiples(list []embeddedType) []embeddedType {
  	if len(list) <= 1 {
  		return list // at most one entry - nothing to do
  	}
  
  	n := 0                     // number of entries w/ unique type
  	prev := make(map[Type]int) // index at which type was previously seen
  	for _, e := range list {
  		if i, found := lookupType(prev, e.typ); found {
  			list[i].multiples = true
  			// ignore this entry
  		} else {
  			prev[e.typ] = n
  			list[n] = e
  			n++
  		}
  	}
  	return list[:n]
  }
  
  func lookupType(m map[Type]int, typ Type) (int, bool) {
  	// fast path: maybe the types are equal
  	if i, found := m[typ]; found {
  		return i, true
  	}
  
  	for t, i := range m {
  		if Identical(t, typ) {
  			return i, true
  		}
  	}
  
  	return 0, false
  }
  
  // MissingMethod returns (nil, false) if V implements T, otherwise it
  // returns a missing method required by T and whether it is missing or
  // just has the wrong type.
  //
  // For non-interface types V, or if static is set, V implements T if all
  // methods of T are present in V. Otherwise (V is an interface and static
  // is not set), MissingMethod only checks that methods of T which are also
  // present in V have matching types (e.g., for a type assertion x.(T) where
  // x is of interface type V).
  //
  func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) {
  	// fast path for common case
  	if T.Empty() {
  		return
  	}
  
  	// TODO(gri) Consider using method sets here. Might be more efficient.
  
  	if ityp, _ := V.Underlying().(*Interface); ityp != nil {
  		// TODO(gri) allMethods is sorted - can do this more efficiently
  		for _, m := range T.allMethods {
  			_, obj := lookupMethod(ityp.allMethods, m.pkg, m.name)
  			switch {
  			case obj == nil:
  				if static {
  					return m, false
  				}
  			case !Identical(obj.Type(), m.typ):
  				return m, true
  			}
  		}
  		return
  	}
  
  	// A concrete type implements T if it implements all methods of T.
  	for _, m := range T.allMethods {
  		obj, _, _ := lookupFieldOrMethod(V, false, m.pkg, m.name)
  
  		f, _ := obj.(*Func)
  		if f == nil {
  			return m, false
  		}
  
  		if !Identical(f.typ, m.typ) {
  			return m, true
  		}
  	}
  
  	return
  }
  
  // assertableTo reports whether a value of type V can be asserted to have type T.
  // It returns (nil, false) as affirmative answer. Otherwise it returns a missing
  // method required by V and whether it is missing or just has the wrong type.
  func assertableTo(V *Interface, T Type) (method *Func, wrongType bool) {
  	// no static check is required if T is an interface
  	// spec: "If T is an interface type, x.(T) asserts that the
  	//        dynamic type of x implements the interface T."
  	if _, ok := T.Underlying().(*Interface); ok && !strict {
  		return
  	}
  	return MissingMethod(T, V, false)
  }
  
  // deref dereferences typ if it is a *Pointer and returns its base and true.
  // Otherwise it returns (typ, false).
  func deref(typ Type) (Type, bool) {
  	if p, _ := typ.(*Pointer); p != nil {
  		return p.base, true
  	}
  	return typ, false
  }
  
  // derefStructPtr dereferences typ if it is a (named or unnamed) pointer to a
  // (named or unnamed) struct and returns its base. Otherwise it returns typ.
  func derefStructPtr(typ Type) Type {
  	if p, _ := typ.Underlying().(*Pointer); p != nil {
  		if _, ok := p.base.Underlying().(*Struct); ok {
  			return p.base
  		}
  	}
  	return typ
  }
  
  // concat returns the result of concatenating list and i.
  // The result does not share its underlying array with list.
  func concat(list []int, i int) []int {
  	var t []int
  	t = append(t, list...)
  	return append(t, i)
  }
  
  // fieldIndex returns the index for the field with matching package and name, or a value < 0.
  func fieldIndex(fields []*Var, pkg *Package, name string) int {
  	if name != "_" {
  		for i, f := range fields {
  			if f.sameId(pkg, name) {
  				return i
  			}
  		}
  	}
  	return -1
  }
  
  // lookupMethod returns the index of and method with matching package and name, or (-1, nil).
  func lookupMethod(methods []*Func, pkg *Package, name string) (int, *Func) {
  	if name != "_" {
  		for i, m := range methods {
  			if m.sameId(pkg, name) {
  				return i, m
  			}
  		}
  	}
  	return -1, nil
  }
  

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