proc: implement conversion of integers to string

Go allows converting a single integer value to string, resulting in a
string containing a single unicode rune with the same code as the value
of the integer.
Allow the same conversion to happen.

Fixes #1322

pkg/proc/eval.go

@@ -113,9 +113,15 @@ func (scope *EvalScope) evalToplevelTypeCast(t ast.Expr, cfg LoadConfig) (*Varia
 		return v, nil
 
 	case "string":
-		if argv.Kind != reflect.Slice {
-			return nil, nil
-		}
+		switch argv.Kind {
+		case reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64, reflect.Int, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uint, reflect.Uintptr:
+			b, _ := constant.Int64Val(argv.Value)
+			s := string(b)
+			v.Value = constant.MakeString(s)
+			v.Len = int64(len(s))
+			return v, nil
+
+		case reflect.Slice:
 			switch elemType := argv.RealType.(*godwarf.SliceType).ElemType.(type) {
 			case *godwarf.UintType:
 				if elemType.Name != "uint8" && elemType.Name != "byte" {
@@ -144,6 +150,10 @@ func (scope *EvalScope) evalToplevelTypeCast(t ast.Expr, cfg LoadConfig) (*Varia
 			}
 			v.Len = int64(len(constant.StringVal(v.Value)))
 			return v, nil
+
+		default:
+			return nil, nil
+		}
 	}
 
 	return nil, nil

service/test/variables_test.go

@@ -748,6 +748,8 @@ func TestEvalExpression(t *testing.T) {
 		{"uint8(i5)", false, "253", "253", "uint8", nil},
 		{"int8(i5)", false, "-3", "-3", "int8", nil},
 		{"int8(i6)", false, "12", "12", "int8", nil},
+		{"string(byteslice[0])", false, `"t"`, `"t"`, "string", nil},
+		{"string(runeslice[0])", false, `"t"`, `"t"`, "string", nil},
 
 		// misc
 		{"i1", true, "1", "1", "int", nil},