wire: add info from the dependency graph when a type is not provided

internal/wire/testdata/MultipleMissingInputs/foo/foo.go

@@ -17,7 +17,9 @@ package main
 import "fmt"
 
 func main() {
-	fmt.Println(injectBaz())
+	fmt.Println(injectMissingOutputType())
+	fmt.Println(injectMultipleMissingTypes())
+	fmt.Println(injectMissingRecursiveType())
 }
 
 type Foo int
@@ -27,3 +29,19 @@ type Baz int
 func provideBaz(foo Foo, bar Bar) Baz {
 	return 0
 }
+
+type Zip int
+type Zap int
+type Zop int
+
+func provideZip(foo Foo) Zip {
+	return 0
+}
+
+func provideZap(zip Zip) Zap {
+	return 0
+}
+
+func provideZop(zap Zap) Zop {
+	return 0
+}

internal/wire/testdata/MultipleMissingInputs/foo/wire.go

@@ -20,7 +20,23 @@ import (
 	"github.com/google/go-cloud/wire"
 )
 
-func injectBaz() Baz {
+func injectMissingOutputType() Foo {
+	// Error: no provider for Foo.
+	wire.Build()
+	return Foo(0)
+}
+
+func injectMultipleMissingTypes() Baz {
+	// Error: provideBaz needs Foo and Bar, both missing.
 	wire.Build(provideBaz)
 	return Baz(0)
 }
+
+func injectMissingRecursiveType() Zop {
+	// Error:
+	// Zop  -> Zap -> Zip -> Foo
+	// provideZop needs Zap, provideZap needs Zip, provideZip needs Foo,
+	// which is missing.
+	wire.Build(provideZop, provideZap, provideZip)
+	return Zop(0)
+}

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

@@ -1,2 +1,4 @@
-no provider found for example.com/foo.Foo
-no provider found for example.com/foo.Bar
+inject injectMissingOutputType: no provider found for example.com/foo.Foo, output of injector
+inject injectMultipleMissingTypes: no provider found for example.com/foo.Foo, needed by example.com/foo.Baz in provider "provideBaz" (/wire_gopath/src/example.com/foo/foo.go:29:6)
+inject injectMultipleMissingTypes: no provider found for example.com/foo.Bar, needed by example.com/foo.Baz in provider "provideBaz" (/wire_gopath/src/example.com/foo/foo.go:29:6)
+inject injectMissingRecursiveType: no provider found for example.com/foo.Foo, needed by example.com/foo.Zip in provider "provideZip" (/wire_gopath/src/example.com/foo/foo.go:37:6), needed by example.com/foo.Zap in provider "provideZap" (/wire_gopath/src/example.com/foo/foo.go:41:6), needed by example.com/foo.Zop in provider "provideZop" (/wire_gopath/src/example.com/foo/foo.go:45:6)