cmd/compile: factor out access to thisT

isifacemethod accessed thisT without checking if it was initialized,
opening the possibility for a bug during type checking. Give better
name, move it to package types, and provide accessor instead.

Change-Id: I29ffc408252a4ba4ef1de218fa154397786c9be6
Reviewed-on: https://go-review.googlesource.com/41673Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/bimport.go

@@ -631,7 +631,7 @@ func (p *importer) method() *types.Field {
 	f := types.NewField()
 	f.Sym = sym
 	f.Nname = asTypesNode(newname(sym))
-	f.Type = functypefield(fakethisfield(), params, result)
+	f.Type = functypefield(fakeRecvField(), params, result)
 	return f
 }
 

src/cmd/compile/internal/gc/dcl.go

@@ -783,30 +783,21 @@ func embedded(s *types.Sym, pkg *types.Pkg) *Node {
 	return n
 }
 
-// thisT is the singleton type used for interface method receivers.
-var thisT *types.Type
-
-func fakethis() *Node {
-	if thisT == nil {
-		thisT = types.NewPtr(types.New(TSTRUCT))
-	}
-	return anonfield(thisT)
+func fakeRecv() *Node {
+	return anonfield(types.FakeRecvType())
 }
 
-func fakethisfield() *types.Field {
-	if thisT == nil {
-		thisT = types.NewPtr(types.New(TSTRUCT))
-	}
+func fakeRecvField() *types.Field {
 	f := types.NewField()
-	f.Type = thisT
+	f.Type = types.FakeRecvType()
 	return f
 }
 
-// Is this field a method on an interface?
-// Those methods have thisT as the receiver.
-// (See fakethis above.)
+// isifacemethod reports whether (field) m is
+// an interface method. Such methods have the
+// special receiver type types.FakeRecvType().
 func isifacemethod(f *types.Type) bool {
-	return f.Recv().Type == thisT
+	return f.Recv().Type == types.FakeRecvType()
 }
 
 // turn a parsed function declaration into a type

src/cmd/compile/internal/gc/noder.go

@@ -594,7 +594,7 @@ func (p *noder) interfaceType(expr *syntax.InterfaceType) *Node {
 		} else {
 			mname := p.newname(method.Name)
 			sig := p.typeExpr(method.Type)
-			sig.Left = fakethis()
+			sig.Left = fakeRecv()
 			n = p.nod(method, ODCLFIELD, mname, sig)
 			ifacedcl(n)
 		}

src/cmd/compile/internal/gc/universe.go

@@ -369,7 +369,7 @@ func typeinit() {
 func makeErrorInterface() *types.Type {
 	field := types.NewField()
 	field.Type = types.Types[TSTRING]
-	f := functypefield(fakethisfield(), nil, []*types.Field{field})
+	f := functypefield(fakeRecvField(), nil, []*types.Field{field})
 
 	field = types.NewField()
 	field.Sym = lookup("Error")

src/cmd/compile/internal/types/type.go

@@ -1322,3 +1322,13 @@ func (t *Type) Tie() byte {
 	}
 	return 'T'
 }
+
+var recvType *Type
+
+// FakeRecvType returns the singleton type used for interface method receivers.
+func FakeRecvType() *Type {
+	if recvType == nil {
+		recvType = NewPtr(New(TSTRUCT))
+	}
+	return recvType
+}