wire: Add wire.InterfaceValue, required instead of wire.Value if the value is an interface value (google/go-cloud#322)

* Add wire.InterfaceValue, required instead of wire.Value if the value is an interface value.

* Update guestbook sample to use InterfaceValue where appropriate.

* Remove unnecessary ok := true

* Addressing comments from code review.

internal/wire/testdata/InterfaceValue/foo/foo.go

@@ -0,0 +1,26 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+)
+
+func main() {
+	r := injectedReader()
+	buf, _ := ioutil.ReadAll(r)
+	fmt.Println(string(buf))
+}

internal/wire/testdata/InterfaceValue/foo/wire.go

@@ -0,0 +1,29 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//+build wireinject
+
+package main
+
+import (
+	"io"
+	"strings"
+
+	"github.com/google/go-cloud/wire"
+)
+
+func injectedReader() io.Reader {
+	wire.Build(wire.InterfaceValue(new(io.Reader), strings.NewReader("hello world")))
+	return nil
+}

internal/wire/testdata/InterfaceValue/pkg

@@ -0,0 +1 @@
+foo

internal/wire/testdata/InterfaceValue/want/program_out.txt

@@ -0,0 +1 @@
+hello world

internal/wire/testdata/InterfaceValue/want/wire_gen.go

@@ -0,0 +1,22 @@
+// Code generated by Wire. DO NOT EDIT.
+
+//go:generate wire
+//+build !wireinject
+
+package main
+
+import (
+	io "io"
+	strings "strings"
+)
+
+// Injectors from wire.go:
+
+func injectedReader() io.Reader {
+	reader := _wireReaderValue
+	return reader
+}
+
+var (
+	_wireReaderValue = strings.NewReader("hello world")
+)

internal/wire/testdata/InterfaceValueInvalidArg0/foo/foo.go

@@ -0,0 +1,21 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println(injectedMessage())
+}

internal/wire/testdata/InterfaceValueInvalidArg0/foo/wire.go

@@ -0,0 +1,26 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//+build wireinject
+
+package main
+
+import (
+	"github.com/google/go-cloud/wire"
+)
+
+func injectedMessage() string {
+	wire.Build(wire.InterfaceValue("foo", "bar"))
+	return ""
+}

internal/wire/testdata/InterfaceValueInvalidArg0/pkg

@@ -0,0 +1 @@
+foo

internal/wire/testdata/InterfaceValueInvalidArg0/want/wire_errs.txt

@@ -0,0 +1 @@
+first argument to InterfaceValue must be a pointer to an interface type; found string

internal/wire/testdata/InterfaceValueNotEnoughArgs/foo/foo.go

@@ -0,0 +1,21 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println(injectedMessage())
+}

internal/wire/testdata/InterfaceValueNotEnoughArgs/foo/wire.go

@@ -0,0 +1,26 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//+build wireinject
+
+package main
+
+import (
+	"github.com/google/go-cloud/wire"
+)
+
+func injectedMessage() string {
+	wire.Build(wire.InterfaceValue("foo"))
+	return ""
+}

internal/wire/testdata/InterfaceValueNotEnoughArgs/pkg

@@ -0,0 +1 @@
+foo

internal/wire/testdata/InterfaceValueNotEnoughArgs/want/wire_errs.txt

@@ -0,0 +1 @@
+too few arguments in call to wire.InterfaceValue

internal/wire/testdata/ValueIsInterfaceValue/foo/foo.go

@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"strings"
+)
+
+func main() {
+	r := injectedReader(strings.NewReader("hello world"))
+	buf, _ := ioutil.ReadAll(r)
+	fmt.Println(string(buf))
+}

internal/wire/testdata/ValueIsInterfaceValue/foo/wire.go

@@ -0,0 +1,29 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//+build wireinject
+
+package main
+
+import (
+	"io"
+	"strings"
+
+	"github.com/google/go-cloud/wire"
+)
+
+func injectedReader(r *strings.Reader) io.Reader {
+	wire.Build(wire.Value(io.Reader(r)))
+	return nil
+}

internal/wire/testdata/ValueIsInterfaceValue/pkg

@@ -0,0 +1 @@
+foo

internal/wire/testdata/ValueIsInterfaceValue/want/wire_errs.txt

@@ -0,0 +1 @@
+argument to Value may not be an interface value (found io.Reader); use InterfaceValue instead