Source file src/cmd/compile/internal/types2/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 types2 8 9 import ( 10 "bytes" 11 "strings" 12 ) 13 14 // LookupSelection selects the field or method whose ID is Id(pkg, 15 // name), on a value of type T. If addressable is set, T is the type 16 // of an addressable variable (this matters only for method lookups). 17 // T must not be nil. 18 // 19 // If the selection is valid: 20 // 21 // - [Selection.Obj] returns the field ([Var]) or method ([Func]); 22 // - [Selection.Indirect] reports whether there were any pointer 23 // indirections on the path to the field or method. 24 // - [Selection.Index] returns the index sequence, defined below. 25 // 26 // The last index entry is the field or method index in the (possibly 27 // embedded) type where the entry was found, either: 28 // 29 // 1. the list of declared methods of a named type; or 30 // 2. the list of all methods (method set) of an interface type; or 31 // 3. the list of fields of a struct type. 32 // 33 // The earlier index entries are the indices of the embedded struct 34 // fields traversed to get to the found entry, starting at depth 0. 35 // 36 // See also [LookupFieldOrMethod], which returns the components separately. 37 func LookupSelection(T Type, addressable bool, pkg *Package, name string) (Selection, bool) { 38 obj, index, indirect := LookupFieldOrMethod(T, addressable, pkg, name) 39 var kind SelectionKind 40 switch obj.(type) { 41 case nil: 42 return Selection{}, false 43 case *Func: 44 kind = MethodVal 45 case *Var: 46 kind = FieldVal 47 default: 48 panic(obj) // can't happen 49 } 50 return Selection{kind, T, obj, index, indirect}, true 51 } 52 53 // Internal use of LookupFieldOrMethod: If the obj result is a method 54 // associated with a concrete (non-interface) type, the method's signature 55 // may not be fully set up. Call Checker.objDecl(obj, nil) before accessing 56 // the method's type. 57 58 // LookupFieldOrMethod looks up a field or method with given package and name 59 // in T and returns the corresponding *Var or *Func, an index sequence, and a 60 // bool indicating if there were any pointer indirections on the path to the 61 // field or method. If addressable is set, T is the type of an addressable 62 // variable (only matters for method lookups). T must not be nil. 63 // 64 // The last index entry is the field or method index in the (possibly embedded) 65 // type where the entry was found, either: 66 // 67 // 1. the list of declared methods of a named type; or 68 // 2. the list of all methods (method set) of an interface type; or 69 // 3. the list of fields of a struct type. 70 // 71 // The earlier index entries are the indices of the embedded struct fields 72 // traversed to get to the found entry, starting at depth 0. 73 // 74 // If no entry is found, a nil object is returned. In this case, the returned 75 // index and indirect values have the following meaning: 76 // 77 // - If index != nil, the index sequence points to an ambiguous entry 78 // (the same name appeared more than once at the same embedding level). 79 // 80 // - If indirect is set, a method with a pointer receiver type was found 81 // but there was no pointer on the path from the actual receiver type to 82 // the method's formal receiver base type, nor was the receiver addressable. 83 // 84 // See also [LookupSelection], which returns the result as a [Selection]. 85 func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) { 86 if T == nil { 87 panic("LookupFieldOrMethod on nil type") 88 } 89 return lookupFieldOrMethod(T, addressable, pkg, name, false) 90 } 91 92 // lookupFieldOrMethod is like LookupFieldOrMethod but with the additional foldCase parameter 93 // (see Object.sameId for the meaning of foldCase). 94 func lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string, foldCase bool) (obj Object, index []int, indirect bool) { 95 // Methods cannot be associated to a named pointer type. 96 // (spec: "The type denoted by T is called the receiver base type; 97 // it must not be a pointer or interface type and it must be declared 98 // in the same package as the method."). 99 // Thus, if we have a named pointer type, proceed with the underlying 100 // pointer type but discard the result if it is a method since we would 101 // not have found it for T (see also go.dev/issue/8590). 102 if t := asNamed(T); t != nil { 103 if p, _ := t.Underlying().(*Pointer); p != nil { 104 obj, index, indirect = lookupFieldOrMethodImpl(p, false, pkg, name, foldCase) 105 if _, ok := obj.(*Func); ok { 106 return nil, nil, false 107 } 108 return 109 } 110 } 111 112 obj, index, indirect = lookupFieldOrMethodImpl(T, addressable, pkg, name, foldCase) 113 114 // If we didn't find anything and if we have a type parameter with a common underlying 115 // type, see if there is a matching field (but not a method, those need to be declared 116 // explicitly in the constraint). If the constraint is a named pointer type (see above), 117 // we are ok here because only fields are accepted as results. 118 const enableTParamFieldLookup = false // see go.dev/issue/51576 119 if enableTParamFieldLookup && obj == nil && isTypeParam(T) { 120 if t, _ := commonUnder(T, nil); t != nil { 121 obj, index, indirect = lookupFieldOrMethodImpl(t, addressable, pkg, name, foldCase) 122 if _, ok := obj.(*Var); !ok { 123 obj, index, indirect = nil, nil, false // accept fields (variables) only 124 } 125 } 126 } 127 return 128 } 129 130 // lookupFieldOrMethodImpl is the implementation of lookupFieldOrMethod. 131 // Notably, in contrast to lookupFieldOrMethod, it won't find struct fields 132 // in base types of defined (*Named) pointer types T. For instance, given 133 // the declaration: 134 // 135 // type T *struct{f int} 136 // 137 // lookupFieldOrMethodImpl won't find the field f in the defined (*Named) type T 138 // (methods on T are not permitted in the first place). 139 // 140 // Thus, lookupFieldOrMethodImpl should only be called by lookupFieldOrMethod 141 // and missingMethod (the latter doesn't care about struct fields). 142 // 143 // The resulting object may not be fully type-checked. 144 func lookupFieldOrMethodImpl(T Type, addressable bool, pkg *Package, name string, foldCase bool) (obj Object, index []int, indirect bool) { 145 // WARNING: The code in this function is extremely subtle - do not modify casually! 146 147 if name == "_" { 148 return // blank fields/methods are never found 149 } 150 151 // Importantly, we must not call Underlying before the call to deref below (nor 152 // does deref call Underlying), as doing so could incorrectly result in finding 153 // methods of the pointer base type when T is a (*Named) pointer type. 154 typ, isPtr := deref(T) 155 156 // *typ where typ is an interface (incl. a type parameter) has no methods. 157 if isPtr { 158 if _, ok := typ.Underlying().(*Interface); ok { 159 return 160 } 161 } 162 163 // Start with typ as single entry at shallowest depth. 164 current := []embeddedType{{typ, nil, isPtr, false}} 165 166 // seen tracks named types that we have seen already, allocated lazily. 167 // Used to avoid endless searches in case of recursive types. 168 // 169 // We must use a lookup on identity rather than a simple map[*Named]bool as 170 // instantiated types may be identical but not equal. 171 var seen instanceLookup 172 173 // search current depth 174 for len(current) > 0 { 175 var next []embeddedType // embedded types found at current depth 176 177 // look for (pkg, name) in all types at current depth 178 for _, e := range current { 179 typ := e.typ 180 181 // If we have a named type, we may have associated methods. 182 // Look for those first. 183 if named := asNamed(typ); named != nil { 184 if alt := seen.lookup(named); alt != nil { 185 // We have seen this type before, at a more shallow depth 186 // (note that multiples of this type at the current depth 187 // were consolidated before). The type at that depth shadows 188 // this same type at the current depth, so we can ignore 189 // this one. 190 continue 191 } 192 seen.add(named) 193 194 // look for a matching attached method 195 if i, m := named.lookupMethod(pkg, name, foldCase); m != nil { 196 // potential match 197 // caution: method may not have a proper signature yet 198 index = concat(e.index, i) 199 if obj != nil || e.multiples { 200 return nil, index, false // collision 201 } 202 obj = m 203 indirect = e.indirect 204 continue // we can't have a matching field or interface method 205 } 206 } 207 208 switch t := typ.Underlying().(type) { 209 case *Struct: 210 // look for a matching field and collect embedded types 211 for i, f := range t.fields { 212 if f.sameId(pkg, name, foldCase) { 213 assert(f.typ != nil) 214 index = concat(e.index, i) 215 if obj != nil || e.multiples { 216 return nil, index, false // collision 217 } 218 obj = f 219 indirect = e.indirect 220 continue // we can't have a matching interface method 221 } 222 // Collect embedded struct fields for searching the next 223 // lower depth, but only if we have not seen a match yet 224 // (if we have a match it is either the desired field or 225 // we have a name collision on the same depth; in either 226 // case we don't need to look further). 227 // Embedded fields are always of the form T or *T where 228 // T is a type name. If e.typ appeared multiple times at 229 // this depth, f.typ appears multiple times at the next 230 // depth. 231 if obj == nil && f.embedded { 232 typ, isPtr := deref(f.typ) 233 // TODO(gri) optimization: ignore types that can't 234 // have fields or methods (only Named, Struct, and 235 // Interface types need to be considered). 236 next = append(next, embeddedType{typ, concat(e.index, i), e.indirect || isPtr, e.multiples}) 237 } 238 } 239 240 case *Interface: 241 // look for a matching method (interface may be a type parameter) 242 if i, m := t.typeSet().LookupMethod(pkg, name, foldCase); m != nil { 243 assert(m.typ != nil) 244 index = concat(e.index, i) 245 if obj != nil || e.multiples { 246 return nil, index, false // collision 247 } 248 obj = m 249 indirect = e.indirect 250 } 251 } 252 } 253 254 if obj != nil { 255 // found a potential match 256 // spec: "A method call x.m() is valid if the method set of (the type of) x 257 // contains m and the argument list can be assigned to the parameter 258 // list of m. If x is addressable and &x's method set contains m, x.m() 259 // is shorthand for (&x).m()". 260 if f, _ := obj.(*Func); f != nil { 261 // determine if method has a pointer receiver 262 if f.hasPtrRecv() && !indirect && !addressable { 263 return nil, nil, true // pointer/addressable receiver required 264 } 265 } 266 return 267 } 268 269 current = consolidateMultiples(next) 270 } 271 272 return nil, nil, false // not found 273 } 274 275 // embeddedType represents an embedded type 276 type embeddedType struct { 277 typ Type 278 index []int // embedded field indices, starting with index at depth 0 279 indirect bool // if set, there was a pointer indirection on the path to this field 280 multiples bool // if set, typ appears multiple times at this depth 281 } 282 283 // consolidateMultiples collects multiple list entries with the same type 284 // into a single entry marked as containing multiples. The result is the 285 // consolidated list. 286 func consolidateMultiples(list []embeddedType) []embeddedType { 287 if len(list) <= 1 { 288 return list // at most one entry - nothing to do 289 } 290 291 n := 0 // number of entries w/ unique type 292 prev := make(map[Type]int) // index at which type was previously seen 293 for _, e := range list { 294 if i, found := lookupType(prev, e.typ); found { 295 list[i].multiples = true 296 // ignore this entry 297 } else { 298 prev[e.typ] = n 299 list[n] = e 300 n++ 301 } 302 } 303 return list[:n] 304 } 305 306 func lookupType(m map[Type]int, typ Type) (int, bool) { 307 // fast path: maybe the types are equal 308 if i, found := m[typ]; found { 309 return i, true 310 } 311 312 for t, i := range m { 313 if Identical(t, typ) { 314 return i, true 315 } 316 } 317 318 return 0, false 319 } 320 321 type instanceLookup struct { 322 // buf is used to avoid allocating the map m in the common case of a small 323 // number of instances. 324 buf [3]*Named 325 m map[*Named][]*Named 326 } 327 328 func (l *instanceLookup) lookup(inst *Named) *Named { 329 for _, t := range l.buf { 330 if t != nil && Identical(inst, t) { 331 return t 332 } 333 } 334 for _, t := range l.m[inst.Origin()] { 335 if Identical(inst, t) { 336 return t 337 } 338 } 339 return nil 340 } 341 342 func (l *instanceLookup) add(inst *Named) { 343 for i, t := range l.buf { 344 if t == nil { 345 l.buf[i] = inst 346 return 347 } 348 } 349 if l.m == nil { 350 l.m = make(map[*Named][]*Named) 351 } 352 insts := l.m[inst.Origin()] 353 l.m[inst.Origin()] = append(insts, inst) 354 } 355 356 // MissingMethod returns (nil, false) if V implements T, otherwise it 357 // returns a missing method required by T and whether it is missing or 358 // just has the wrong type: either a pointer receiver or wrong signature. 359 // 360 // For non-interface types V, or if static is set, V implements T if all 361 // methods of T are present in V. Otherwise (V is an interface and static 362 // is not set), MissingMethod only checks that methods of T which are also 363 // present in V have matching types (e.g., for a type assertion x.(T) where 364 // x is of interface type V). 365 func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) { 366 return (*Checker)(nil).missingMethod(V, T, static, Identical, nil) 367 } 368 369 // missingMethod is like MissingMethod but accepts a *Checker as receiver, 370 // a comparator equivalent for type comparison, and a *string for error causes. 371 // The receiver may be nil if missingMethod is invoked through an exported 372 // API call (such as MissingMethod), i.e., when all methods have been type- 373 // checked. 374 // The underlying type of T must be an interface; T (rather than its under- 375 // lying type) is used for better error messages (reported through *cause). 376 // The comparator is used to compare signatures. 377 // If a method is missing and cause is not nil, *cause describes the error. 378 func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y Type) bool, cause *string) (method *Func, wrongType bool) { 379 methods := T.Underlying().(*Interface).typeSet().methods // T must be an interface 380 if len(methods) == 0 { 381 return nil, false 382 } 383 384 const ( 385 ok = iota 386 notFound 387 wrongName 388 unexported 389 wrongSig 390 ambigSel 391 ptrRecv 392 field 393 nointerface 394 ) 395 396 state := ok 397 var m *Func // method on T we're trying to implement 398 var f *Func // method on V, if found (state is one of ok, wrongName, wrongSig) 399 400 if u, _ := V.Underlying().(*Interface); u != nil { 401 tset := u.typeSet() 402 for _, m = range methods { 403 _, f = tset.LookupMethod(m.pkg, m.name, false) 404 405 if f == nil { 406 if !static { 407 continue 408 } 409 state = notFound 410 break 411 } 412 413 if !equivalent(f.typ, m.typ) { 414 state = wrongSig 415 break 416 } 417 } 418 } else { 419 for _, m = range methods { 420 obj, index, indirect := lookupFieldOrMethodImpl(V, false, m.pkg, m.name, false) 421 422 // check if m is ambiguous, on *V, or on V with case-folding 423 if obj == nil { 424 switch { 425 case index != nil: 426 state = ambigSel 427 case indirect: 428 state = ptrRecv 429 default: 430 state = notFound 431 obj, _, _ = lookupFieldOrMethodImpl(V, false, m.pkg, m.name, true /* fold case */) 432 f, _ = obj.(*Func) 433 if f != nil { 434 state = wrongName 435 if f.name == m.name { 436 // If the names are equal, f must be unexported 437 // (otherwise the package wouldn't matter). 438 state = unexported 439 } 440 } 441 } 442 break 443 } 444 445 // we must have a method (not a struct field) 446 f, _ = obj.(*Func) 447 if f == nil { 448 state = field 449 break 450 } 451 452 // methods may not have a fully set up signature yet 453 if check != nil { 454 check.objDecl(f) 455 } 456 457 if f.nointerface { 458 state = nointerface 459 break 460 } 461 462 if !equivalent(f.typ, m.typ) { 463 state = wrongSig 464 break 465 } 466 } 467 } 468 469 if state == ok { 470 return nil, false 471 } 472 473 if cause != nil { 474 if f != nil { 475 // This method may be formatted in funcString below, so must have a fully 476 // set up signature. 477 if check != nil { 478 check.objDecl(f) 479 } 480 } 481 switch state { 482 case notFound: 483 switch { 484 case isInterfacePtr(V): 485 *cause = "(" + check.interfacePtrError(V) + ")" 486 case isInterfacePtr(T): 487 *cause = "(" + check.interfacePtrError(T) + ")" 488 default: 489 *cause = check.sprintf("(missing method %s)", m.Name()) 490 } 491 case wrongName: 492 fs, ms := check.funcString(f, false), check.funcString(m, false) 493 *cause = check.sprintf("(missing method %s)\n\t\thave %s\n\t\twant %s", m.Name(), fs, ms) 494 case unexported: 495 *cause = check.sprintf("(unexported method %s)", m.Name()) 496 case wrongSig: 497 fs, ms := check.funcString(f, false), check.funcString(m, false) 498 if fs == ms { 499 // Don't report "want Foo, have Foo". 500 // Add package information to disambiguate (go.dev/issue/54258). 501 fs, ms = check.funcString(f, true), check.funcString(m, true) 502 } 503 if fs == ms { 504 // We still have "want Foo, have Foo". 505 // This is most likely due to different type parameters with 506 // the same name appearing in the instantiated signatures 507 // (go.dev/issue/61685). 508 // Rather than reporting this misleading error cause, for now 509 // just point out that the method signature is incorrect. 510 // TODO(gri) should find a good way to report the root cause 511 *cause = check.sprintf("(wrong type for method %s)", m.Name()) 512 break 513 } 514 *cause = check.sprintf("(wrong type for method %s)\n\t\thave %s\n\t\twant %s", m.Name(), fs, ms) 515 case ambigSel: 516 *cause = check.sprintf("(ambiguous selector %s.%s)", V, m.Name()) 517 case ptrRecv: 518 *cause = check.sprintf("(method %s has pointer receiver)", m.Name()) 519 case field: 520 *cause = check.sprintf("(%s.%s is a field, not a method)", V, m.Name()) 521 case nointerface: 522 *cause = check.sprintf("(%s method is marked 'nointerface')", m.Name()) 523 default: 524 panic("unreachable") 525 } 526 } 527 528 return m, state == wrongSig || state == ptrRecv 529 } 530 531 // hasAllMethods is similar to checkMissingMethod but instead reports whether all methods are present. 532 // If V is not a valid type, or if it is a struct containing embedded fields with invalid types, the 533 // result is true because it is not possible to say with certainty whether a method is missing or not 534 // (an embedded field may have the method in question). 535 // If the result is false and cause is not nil, *cause describes the error. 536 // Use hasAllMethods to avoid follow-on errors due to incorrect types. 537 func (check *Checker) hasAllMethods(V, T Type, static bool, equivalent func(x, y Type) bool, cause *string) bool { 538 if !isValid(V) { 539 return true // we don't know anything about V, assume it implements T 540 } 541 m, _ := check.missingMethod(V, T, static, equivalent, cause) 542 return m == nil || hasInvalidEmbeddedFields(V, nil) 543 } 544 545 // hasInvalidEmbeddedFields reports whether T is a struct (or a pointer to a struct) that contains 546 // (directly or indirectly) embedded fields with invalid types. 547 func hasInvalidEmbeddedFields(T Type, seen map[*Struct]bool) bool { 548 if S, _ := derefStructPtr(T).Underlying().(*Struct); S != nil && !seen[S] { 549 if seen == nil { 550 seen = make(map[*Struct]bool) 551 } 552 seen[S] = true 553 for _, f := range S.fields { 554 if f.embedded && (!isValid(f.typ) || hasInvalidEmbeddedFields(f.typ, seen)) { 555 return true 556 } 557 } 558 } 559 return false 560 } 561 562 func isInterfacePtr(T Type) bool { 563 p, _ := T.Underlying().(*Pointer) 564 return p != nil && IsInterface(p.base) 565 } 566 567 // check may be nil. 568 func (check *Checker) interfacePtrError(T Type) string { 569 assert(isInterfacePtr(T)) 570 if p, _ := T.Underlying().(*Pointer); isTypeParam(p.base) { 571 return check.sprintf("type %s is pointer to type parameter, not type parameter", T) 572 } 573 return check.sprintf("type %s is pointer to interface, not interface", T) 574 } 575 576 // funcString returns a string of the form name + signature for f. 577 // check may be nil. 578 func (check *Checker) funcString(f *Func, pkgInfo bool) string { 579 buf := bytes.NewBufferString(f.name) 580 var qf Qualifier 581 if check != nil && !pkgInfo { 582 qf = check.qualifier 583 } 584 w := newTypeWriter(buf, qf) 585 w.pkgInfo = pkgInfo 586 w.paramNames = false 587 w.signature(f.typ.(*Signature)) 588 return buf.String() 589 } 590 591 // assertableTo reports whether a value of type V can be asserted to have type T. 592 // The receiver may be nil if assertableTo is invoked through an exported API call 593 // (such as AssertableTo), i.e., when all methods have been type-checked. 594 // The underlying type of V must be an interface. 595 // If the result is false and cause is not nil, *cause describes the error. 596 // TODO(gri) replace calls to this function with calls to newAssertableTo. 597 func (check *Checker) assertableTo(V, T Type, cause *string) bool { 598 // no static check is required if T is an interface 599 // spec: "If T is an interface type, x.(T) asserts that the 600 // dynamic type of x implements the interface T." 601 if IsInterface(T) { 602 return true 603 } 604 // TODO(gri) fix this for generalized interfaces 605 return check.hasAllMethods(T, V, false, Identical, cause) 606 } 607 608 // newAssertableTo reports whether a value of type V can be asserted to have type T. 609 // It also implements behavior for interfaces that currently are only permitted 610 // in constraint position (we have not yet defined that behavior in the spec). 611 // The underlying type of V must be an interface. 612 // If the result is false and cause is not nil, *cause is set to the error cause. 613 func (check *Checker) newAssertableTo(V, T Type, cause *string) bool { 614 // no static check is required if T is an interface 615 // spec: "If T is an interface type, x.(T) asserts that the 616 // dynamic type of x implements the interface T." 617 if IsInterface(T) { 618 return true 619 } 620 return check.implements(T, V, false, cause) 621 } 622 623 // deref dereferences typ if it is a *Pointer (but not a *Named type 624 // with an underlying pointer type!) and returns its base and true. 625 // Otherwise it returns (typ, false). 626 func deref(typ Type) (Type, bool) { 627 if p, _ := Unalias(typ).(*Pointer); p != nil { 628 // p.base should never be nil, but be conservative 629 if p.base == nil { 630 if debug { 631 panic("pointer with nil base type (possibly due to an invalid cyclic declaration)") 632 } 633 return Typ[Invalid], true 634 } 635 return p.base, true 636 } 637 return typ, false 638 } 639 640 // derefStructPtr dereferences typ if it is a (named or unnamed) pointer to a 641 // (named or unnamed) struct and returns its base. Otherwise it returns typ. 642 func derefStructPtr(typ Type) Type { 643 if p, _ := typ.Underlying().(*Pointer); p != nil { 644 if _, ok := p.base.Underlying().(*Struct); ok { 645 return p.base 646 } 647 } 648 return typ 649 } 650 651 // concat returns the result of concatenating list and i. 652 // The result does not share its underlying array with list. 653 func concat(list []int, i int) []int { 654 var t []int 655 t = append(t, list...) 656 return append(t, i) 657 } 658 659 // methodIndex returns the index of and method with matching package and name, or (-1, nil). 660 // See Object.sameId for the meaning of foldCase. 661 func methodIndex(methods []*Func, pkg *Package, name string, foldCase bool) (int, *Func) { 662 if name != "_" { 663 for i, m := range methods { 664 if m.sameId(pkg, name, foldCase) { 665 return i, m 666 } 667 } 668 } 669 return -1, nil 670 } 671 672 // Given a (possibly pointer to a) struct type and field index sequence, 673 // fieldPath returns the dot-separated concatenated field names for the 674 // given index sequence (e.g. "a.b.c"). 675 // Use for error reporting etc. where speed is not important. 676 func fieldPath(typ Type, index []int) string { 677 var names []string 678 for _, i := range index { 679 u, ok := derefStructPtr(typ).Underlying().(*Struct) 680 if !ok { 681 // should not happen if index is valid for typ 682 break 683 } 684 fld := u.Field(i) 685 names = append(names, fld.name) 686 typ = fld.typ 687 } 688 return strings.Join(names, ".") 689 } 690