internal/lsp/source: support inverse "implementations"

Now "implementations" supports finding interfaces implemented by the
specified concrete type. This is the inverse of what "implementations"
normally does. There isn't currently a better place in LSP to put this
functionality. The reverse lookup isn't an important feature for most
languages because types often must explicitly declare what interfaces
they implement.

An argument can be made that this functionality fits better into find-
references, but it is still a stretch. Plus, that would require
find-references to search all packages instead of just transitive
dependents.

Updates golang/go#35550.

Change-Id: I72dc78ef52b5bf501da2f39692e99cd304b5406e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/219678
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/source/implementation.go

@@ -46,8 +46,10 @@ func Implementation(ctx context.Context, s Snapshot, f FileHandle, pp protocol.P
 	return locations, nil
 }
 
-var ErrNotAnInterface = errors.New("not an interface or interface method")
+var ErrNotAType = errors.New("not a type name or method")
 
+// implementations returns the concrete implementations of the specified
+// interface, or the interfaces implemented by the specified concrete type.
 func implementations(ctx context.Context, s Snapshot, f FileHandle, pp protocol.Position) ([]qualifiedObject, error) {
 	var (
 		impls []qualifiedObject
@@ -62,25 +64,25 @@ func implementations(ctx context.Context, s Snapshot, f FileHandle, pp protocol.
 
 	for _, qo := range qos {
 		var (
-			T      *types.Interface
-			method *types.Func
+			queryType   types.Type
+			queryMethod *types.Func
 		)
 
 		switch obj := qo.obj.(type) {
 		case *types.Func:
-			method = obj
+			queryMethod = obj
 			if recv := obj.Type().(*types.Signature).Recv(); recv != nil {
-				T, _ = recv.Type().Underlying().(*types.Interface)
+				queryType = ensurePointer(recv.Type())
 			}
 		case *types.TypeName:
-			T, _ = obj.Type().Underlying().(*types.Interface)
+			queryType = ensurePointer(obj.Type())
 		}
 
-		if T == nil {
-			return nil, ErrNotAnInterface
+		if queryType == nil {
+			return nil, ErrNotAType
 		}
 
-		if T.NumMethods() == 0 {
+		if types.NewMethodSet(queryType).Len() == 0 {
 			return nil, nil
 		}
 
@@ -108,42 +110,48 @@ func implementations(ctx context.Context, s Snapshot, f FileHandle, pp protocol.
 				if !ok || obj.IsAlias() {
 					continue
 				}
-				named, ok := obj.Type().(*types.Named)
-				// We skip interface types since we only want concrete
-				// implementations.
-				if !ok || isInterface(named) {
-					continue
+				if named, ok := obj.Type().(*types.Named); ok {
+					allNamed = append(allNamed, named)
 				}
-				allNamed = append(allNamed, named)
 			}
 		}
 
-		// Find all the named types that implement our interface.
-		for _, U := range allNamed {
-			var concrete types.Type = U
-			if !types.AssignableTo(concrete, T) {
-				// We also accept T if *T implements our interface.
-				concrete = types.NewPointer(concrete)
-				if !types.AssignableTo(concrete, T) {
+		// Find all the named types that match our query.
+		for _, named := range allNamed {
+			var (
+				candObj  types.Object = named.Obj()
+				candType              = ensurePointer(named)
+			)
+
+			if !concreteImplementsIntf(candType, queryType) {
+				continue
+			}
+
+			ms := types.NewMethodSet(candType)
+			if ms.Len() == 0 {
+				// Skip empty interfaces.
+				continue
+			}
+
+			// If client queried a method, look up corresponding candType method.
+			if queryMethod != nil {
+				sel := ms.Lookup(queryMethod.Pkg(), queryMethod.Name())
+				if sel == nil {
 					continue
 				}
+				candObj = sel.Obj()
 			}
 
-			var obj types.Object = U.Obj()
-			if method != nil {
-				obj = types.NewMethodSet(concrete).Lookup(method.Pkg(), method.Name()).Obj()
-			}
-
-			pos := fset.Position(obj.Pos())
-			if obj == method || seen[pos] {
+			pos := fset.Position(candObj.Pos())
+			if candObj == queryMethod || seen[pos] {
 				continue
 			}
 
 			seen[pos] = true
 
 			impls = append(impls, qualifiedObject{
-				obj: obj,
-				pkg: pkgs[obj.Pkg()],
+				obj: candObj,
+				pkg: pkgs[candObj.Pkg()],
 			})
 		}
 	}
@@ -151,6 +159,35 @@ func implementations(ctx context.Context, s Snapshot, f FileHandle, pp protocol.
 	return impls, nil
 }
 
+// concreteImplementsIntf returns true if a is an interface type implemented by
+// concrete type b, or vice versa.
+func concreteImplementsIntf(a, b types.Type) bool {
+	aIsIntf, bIsIntf := isInterface(a), isInterface(b)
+
+	// Make sure exactly one is an interface type.
+	if aIsIntf == bIsIntf {
+		return false
+	}
+
+	// Rearrange if needed so "a" is the concrete type.
+	if aIsIntf {
+		a, b = b, a
+	}
+
+	return types.AssignableTo(a, b)
+}
+
+// ensurePointer wraps T in a *types.Pointer if T is a named, non-interface
+// type. This is useful to make sure you consider a named type's full method
+// set.
+func ensurePointer(T types.Type) types.Type {
+	if _, ok := T.(*types.Named); ok && !isInterface(T) {
+		return types.NewPointer(T)
+	}
+
+	return T
+}
+
 type qualifiedObject struct {
 	obj types.Object
 

internal/lsp/testdata/lsp/primarymod/implementation/implementation.go

@@ -2,32 +2,30 @@ package implementation
 
 import "golang.org/x/tools/internal/lsp/implementation/other"
 
-type ImpP struct{} //@ImpP
+type ImpP struct{} //@ImpP,implementations("ImpP", Laugher, OtherLaugher)
 
-func (*ImpP) Laugh() { //@mark(LaughP, "Laugh")
+func (*ImpP) Laugh() { //@mark(LaughP, "Laugh"),implementations("Laugh", Laugh, OtherLaugh)
 }
 
-type ImpS struct{} //@ImpS
+type ImpS struct{} //@ImpS,implementations("ImpS", Laugher, OtherLaugher)
 
-func (ImpS) Laugh() { //@mark(LaughS, "Laugh")
+func (ImpS) Laugh() { //@mark(LaughS, "Laugh"),implementations("Laugh", Laugh, OtherLaugh)
 }
 
-type ImpI interface {
-	Laugh() //@implementations("Laugh", LaughP, OtherLaughP, LaughS, OtherLaughS)
+type Laugher interface { //@Laugher,implementations("Laugher", ImpP, OtherImpP, ImpS, OtherImpS)
+	Laugh() //@Laugh,implementations("Laugh", LaughP, OtherLaughP, LaughS, OtherLaughS)
 }
 
-type Laugher interface { //@implementations("Laugher", ImpP, OtherImpP, ImpS, OtherImpS)
-	Laugh() //@implementations("Laugh", LaughP, OtherLaughP, LaughS, OtherLaughS)
-}
-
-type Foo struct {
+type Foo struct { //@implementations("Foo", Joker)
 	other.Foo
 }
 
-type U interface {
-	U() //@implementations("U", ImpU)
+type Joker interface { //@Joker
+	Joke() //@Joke,implementations("Joke", ImpJoker)
 }
 
-type cryer int
+type cryer int //@implementations("cryer", Cryer)
 
-func (cryer) Cry(other.CryType) {} //@mark(CryImpl, "Cry")
+func (cryer) Cry(other.CryType) {} //@mark(CryImpl, "Cry"),implementations("Cry", Cry)
+
+type Empty interface{} //@implementations("Empty")

internal/lsp/testdata/lsp/primarymod/implementation/other/other.go

@@ -10,20 +10,18 @@ type ImpS struct{} //@mark(OtherImpS, "ImpS")
 func (ImpS) Laugh() { //@mark(OtherLaughS, "Laugh")
 }
 
-type ImpI interface {
-	Laugh()
+type ImpI interface { //@mark(OtherLaugher, "ImpI")
+	Laugh() //@mark(OtherLaugh, "Laugh")
 }
 
-type Foo struct {
+type Foo struct { //@implementations("Foo", Joker)
 }
 
-func (Foo) U() { //@mark(ImpU, "U")
+func (Foo) Joke() { //@mark(ImpJoker, "Joke"),implementations("Joke", Joke)
 }
 
 type CryType int
 
-const Sob CryType = 1
-
-type Cryer interface {
-	Cry(CryType) //@implementations("Cry", CryImpl)
+type Cryer interface { //@Cryer
+	Cry(CryType) //@Cry,implementations("Cry", CryImpl)
 }

internal/lsp/testdata/lsp/summary.txt.golden

@@ -24,5 +24,5 @@ FuzzyWorkspaceSymbolsCount = 3
 CaseSensitiveWorkspaceSymbolsCount = 2
 SignaturesCount = 23
 LinksCount = 8
-ImplementationsCount = 5
+ImplementationsCount = 14