gotypes,build: pointer dereference (#61)

Provides a method on `gotypes.Component` to allow pointer dereference. This will enable `gotypes` helpers to be used with struct pointer arguments, for example.

Updates #53 
Fixes #54

gotypes/components_test.go

@@ -2,7 +2,12 @@ package gotypes
 
 import (
 	"go/types"
+	"strings"
 	"testing"
+
+	"github.com/mmcloughlin/avo/reg"
+
+	"github.com/mmcloughlin/avo/operand"
 )
 
 func TestBasicKindsArePrimitive(t *testing.T) {
@@ -33,8 +38,61 @@ func TestPointersArePrimitive(t *testing.T) {
 }
 
 func AssertPrimitive(t *testing.T, typ types.Type) {
-	c := NewComponent("primitive", typ, 0)
+	c := NewComponent(typ, operand.NewParamAddr("primitive", 0))
 	if _, err := c.Resolve(); err != nil {
 		t.Errorf("expected type %s to be primitive: got error '%s'", typ, err)
 	}
 }
+
+func TestComponentErrors(t *testing.T) {
+	comp := NewComponent(types.Typ[types.Uint32], operand.Mem{})
+	cases := []struct {
+		Component      Component
+		ErrorSubstring string
+	}{
+		{comp.Base(), "only slices and strings"},
+		{comp.Len(), "only slices and strings"},
+		{comp.Cap(), "only slices have"},
+		{comp.Real(), "only complex"},
+		{comp.Imag(), "only complex"},
+		{comp.Index(42), "not array type"},
+		{comp.Field("a"), "not struct type"},
+		{comp.Dereference(reg.R12), "not pointer type"},
+	}
+	for _, c := range cases {
+		_, err := c.Component.Resolve()
+		if err == nil {
+			t.Fatal("expected error")
+		}
+		if !strings.Contains(err.Error(), c.ErrorSubstring) {
+			t.Fatalf("error message %q; expected substring %q", err.Error(), c.ErrorSubstring)
+		}
+	}
+}
+
+func TestComponentErrorChaining(t *testing.T) {
+	// Build a component with an error.
+	comp := NewComponent(types.Typ[types.Uint32], operand.Mem{}).Index(3)
+	_, expect := comp.Resolve()
+	if expect == nil {
+		t.Fatal("expected error")
+	}
+
+	// Confirm that the error is preserved through chaining operations.
+	cases := []Component{
+		comp.Dereference(reg.R13),
+		comp.Base(),
+		comp.Len(),
+		comp.Cap(),
+		comp.Real(),
+		comp.Imag(),
+		comp.Index(42),
+		comp.Field("field"),
+	}
+	for _, c := range cases {
+		_, err := c.Resolve()
+		if err != expect {
+			t.Fatal("chaining should preserve error")
+		}
+	}
+}