size: fix compile error for a fixed-size convert shim (#446)

_generated/convert.go

@@ -81,3 +81,22 @@ type ConvertErrVal string
 type ConvertErr struct {
 	Err ConvertErrVal
 }
+
+//msgp:shim ConvertIntVal as:int64 using:fromConvertIntVal/toConvertIntVal mode:convert
+//msgp:ignore ConvertIntVal
+
+func fromConvertIntVal(v ConvertIntVal) (int64, error) {
+	return int64(v), nil
+}
+
+func toConvertIntVal(i int64) (ConvertIntVal, error) {
+	return ConvertIntVal(i), nil
+}
+
+type ConvertIntVal int64
+
+// ConvertInt exercises a fixed-size (int64) convert shim, whose generated
+// Msgsize must not declare an unused temporary (#446).
+type ConvertInt struct {
+	Int ConvertIntVal
+}

_generated/convert_test.go

@@ -58,3 +58,23 @@ func TestConvertToMarshalError(t *testing.T) {
 		t.Fatalf("expected conversion error, found %v", err.Error())
 	}
 }
+
+func TestConvertInt(t *testing.T) {
+	// #446: a fixed-size convert shim must generate a Msgsize that is an
+	// accurate constant and compiles (no unassigned temporary).
+	in := ConvertInt{Int: 42}
+	b, err := in.MarshalMsg(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if in.Msgsize() < len(b) {
+		t.Fatalf("Msgsize %d under-reports marshaled size %d", in.Msgsize(), len(b))
+	}
+	var out ConvertInt
+	if _, err = out.UnmarshalMsg(b); err != nil {
+		t.Fatal(err)
+	}
+	if out != in {
+		t.Fatalf("round-trip mismatch: %v != %v", out, in)
+	}
+}

gen/size.go

@@ -228,16 +228,23 @@ func (s *sizeGen) gBase(b *BaseElem) {
 		return
 	}
 	if b.Convert && b.ShimMode == Convert {
-		s.state = add
-		vname := randIdent()
-		s.p.printf("\nvar %s %s", vname, b.BaseType())
-
-		// ensure we don't get "unused variable" warnings from outer slice iterations
-		s.p.printf("\n_ = %s", b.Varname())
-
-		s.p.printf("\ns += %s", basesizeExpr(b.Value, vname, b.BaseName()))
-		s.state = expr
-
+		if fixedSize(b.Value) {
+			// A fixed-size base has a constant wire size, so there is no need
+			// for a temporary holding the converted value. Emitting an
+			// (unassigned) temporary left it unused, producing a "declared and
+			// not used" compile error in the generated Msgsize (#446).
+			s.addConstant(basesizeExpr(b.Value, "", b.BaseName()))
+		} else {
+			s.state = add
+			vname := randIdent()
+			s.p.printf("\nvar %s %s", vname, b.BaseType())
+
+			// ensure we don't get "unused variable" warnings from outer slice iterations
+			s.p.printf("\n_ = %s", b.Varname())
+
+			s.p.printf("\ns += %s", basesizeExpr(b.Value, vname, b.BaseName()))
+			s.state = expr
+		}
 	} else {
 		vname := b.Varname()
 		if b.Convert {