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.

README.md

@@ -317,6 +317,14 @@ package; references to variables will be evaluated during the injector
 package's initialization. Wire will emit an error if the expression calls
 any functions or receives from any channels.
 
+For interface values, use `InterfaceValue`:
+
+```go
+func injectReader() io.Reader {
+	wire.Build(wire.InterfaceValue(new(io.Reader), os.Stdin))
+	return Foo{}
+}
+```
 ### Cleanup functions
 
 If a provider creates a value that needs to be cleaned up (e.g. closing a file),

internal/wire/parse.go

@@ -454,6 +454,12 @@ func (oc *objectCache) processExpr(pkg *loader.PackageInfo, expr ast.Expr, varNa
 				return nil, []error{notePosition(exprPos, err)}
 			}
 			return v, nil
+		case "InterfaceValue":
+			v, err := processInterfaceValue(oc.prog.Fset, &pkg.Info, call)
+			if err != nil {
+				return nil, []error{notePosition(exprPos, err)}
+			}
+			return v, nil
 		default:
 			return nil, []error{notePosition(exprPos, errors.New("unknown pattern"))}
 		}
@@ -739,6 +745,11 @@ func processValue(fset *token.FileSet, info *types.Info, call *ast.CallExpr) (*V
 	if !ok {
 		return nil, notePosition(fset.Position(call.Pos()), errors.New("argument to Value is too complex"))
 	}
+	// Result type can't be an interface type; use wire.InterfaceValue for that.
+	argType := info.TypeOf(call.Args[0])
+	if _, isInterfaceType := argType.Underlying().(*types.Interface); isInterfaceType {
+		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("argument to Value may not be an interface value (found %s); use InterfaceValue instead", types.TypeString(argType, nil)))
+	}
 	return &Value{
 		Pos:  call.Args[0].Pos(),
 		Out:  info.TypeOf(call.Args[0]),
@@ -747,6 +758,35 @@ func processValue(fset *token.FileSet, info *types.Info, call *ast.CallExpr) (*V
 	}, nil
 }
 
+// processInterfaceValue creates a value from a wire.InterfaceValue call.
+func processInterfaceValue(fset *token.FileSet, info *types.Info, call *ast.CallExpr) (*Value, error) {
+	// Assumes that call.Fun is wire.InterfaceValue.
+
+	if len(call.Args) != 2 {
+		return nil, notePosition(fset.Position(call.Pos()), errors.New("call to InterfaceValue takes exactly two arguments"))
+	}
+	ifaceArgType := info.TypeOf(call.Args[0])
+	ifacePtr, ok := ifaceArgType.(*types.Pointer)
+	if !ok {
+		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("first argument to InterfaceValue must be a pointer to an interface type; found %s", types.TypeString(ifaceArgType, nil)))
+	}
+	iface := ifacePtr.Elem()
+	methodSet, ok := iface.Underlying().(*types.Interface)
+	if !ok {
+		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("first argument to InterfaceValue must be a pointer to an interface type; found %s", types.TypeString(ifaceArgType, nil)))
+	}
+	provided := info.TypeOf(call.Args[1])
+	if !types.Implements(provided, methodSet) {
+		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("%s does not implement %s", types.TypeString(provided, nil), types.TypeString(ifaceArgType, nil)))
+	}
+	return &Value{
+		Pos:  call.Args[1].Pos(),
+		Out:  iface,
+		expr: call.Args[1],
+		info: info,
+	}, nil
+}
+
 // isInjector checks whether a given function declaration is an
 // injector template, returning the wire.Build call. It returns nil if
 // the function is not an injector template.
@@ -845,7 +885,7 @@ func (pv ProviderOrValue) Provider() *Provider {
 	return pv.p
 }
 
-// Provider returns pv as a Value pointer. It panics if pv points to a
+// Value returns pv as a Value pointer. It panics if pv points to a
 // Provider.
 func (pv ProviderOrValue) Value() *Value {
 	if pv.p != nil {

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

wire.go

@@ -20,8 +20,8 @@ type ProviderSet struct{}
 
 // NewSet creates a new provider set that includes the providers in
 // its arguments. Each argument is either an exported function value,
-// an exported struct (zero) value, a provider set, a call to Bind, or
+// an exported struct (zero) value, a provider set, a call to Bind, a
-// a call to Value.
+// a call to Value, or a call to InterfaceValue.
 func NewSet(...interface{}) ProviderSet {
 	return ProviderSet{}
 }
@@ -72,6 +72,7 @@ func Bind(iface, to interface{}) Binding {
 type ProvidedValue struct{}
 
 // Value binds an expression to provide the type of the expression.
+// The expression may not be an interface value; use InterfaceValue for that.
 //
 // Example:
 //
@@ -79,3 +80,12 @@ type ProvidedValue struct{}
 func Value(interface{}) ProvidedValue {
 	return ProvidedValue{}
 }
+
+// InterfaceValue binds an expression to provide a specific interface type.
+//
+// Example:
+//
+//	var MySet = wire.NewSet(wire.InterfaceValue(new(io.Reader), os.Stdin))
+func InterfaceValue(typ interface{}, x interface{}) ProvidedValue {
+	return ProvidedValue{}
+}