wire/internal/wire: use on-disk GOPATH in generate tests (google/go-cloud#616)

The primary motivation is to permit a move to using go/packages instead
of go/loader. go/packages runs exclusively by shelling out to the go
tool, which precludes use of the in-memory "magic" GOPATH being used
up to this point.

This has a secondary effect of removing a lot of code to support "magic"
GOPATH from the test infrastructure. This is on the whole good, but
necessitated a change in the error scrubbing: since the filenames are
no longer fixed, error scrubbing also must remove the leading
$GOPATH/src lines.

Another related change: since all callers of Generate needed to know the
package path in order to write out wire_gen.go (necessitating a
find-only import search) and Generate already has this information,
Generate now returns this information to the caller. This should further
reduce callers' coupling to Wire's load internals. It also eliminates
code duplication.

This should hopefully shake out any difference in path separators for
running on Windows, but I have not tested that yet.

Updates google/go-cloud#78
Updates google/go-cloud#323

cmd/wire/main.go

@@ -18,14 +18,12 @@
 package main
 
 import (
+	"context"
 	"errors"
 	"fmt"
-	"go/build"
 	"go/token"
 	"go/types"
-	"io/ioutil"
 	"os"
-	"path/filepath"
 	"reflect"
 	"sort"
 	"strconv"
@@ -74,25 +72,17 @@ func generate(pkg string) error {
 	if err != nil {
 		return err
 	}
-	pkgInfo, err := build.Default.Import(pkg, wd, build.FindOnly)
-	if err != nil {
-		return err
-	}
-	out, errs := wire.Generate(&build.Default, wd, pkg)
+	out, errs := wire.Generate(context.Background(), wd, os.Environ(), pkg)
 	if len(errs) > 0 {
 		logErrors(errs)
 		return errors.New("generate failed")
 	}
-	if len(out) == 0 {
+	if len(out.Content) == 0 {
 		// No Wire directives, don't write anything.
 		fmt.Fprintln(os.Stderr, "wire: no injector found for", pkg)
 		return nil
 	}
-	p := filepath.Join(pkgInfo.Dir, "wire_gen.go")
-	if err := ioutil.WriteFile(p, out, 0666); err != nil {
-		return err
-	}
-	return nil
+	return out.Commit()
 }
 
 // show runs the show subcommand.
@@ -106,7 +96,7 @@ func show(pkgs ...string) error {
 	if err != nil {
 		return err
 	}
-	info, errs := wire.Load(&build.Default, wd, pkgs)
+	info, errs := wire.Load(context.Background(), wd, os.Environ(), pkgs)
 	if info != nil {
 		keys := make([]wire.ProviderSetID, 0, len(info.Sets))
 		for k := range info.Sets {
@@ -185,7 +175,7 @@ func check(pkgs ...string) error {
 	if err != nil {
 		return err
 	}
-	_, errs := wire.Load(&build.Default, wd, pkgs)
+	_, errs := wire.Load(context.Background(), wd, os.Environ(), pkgs)
 	if len(errs) > 0 {
 		logErrors(errs)
 		return errors.New("error loading packages")

internal/wire/parse.go

@@ -15,6 +15,7 @@
 package wire
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"go/ast"
@@ -190,11 +191,19 @@ type Value struct {
 	info *types.Info
 }
 
-// Load finds all the provider sets in the given packages, as well as
-// the provider sets' transitive dependencies. It may return both errors
-// and Info.
-func Load(bctx *build.Context, wd string, pkgs []string) (*Info, []error) {
-	prog, errs := load(bctx, wd, pkgs)
+// Load finds all the provider sets in the packages that match the given
+// patterns, as well as the provider sets' transitive dependencies. It
+// may return both errors and Info. The patterns are defined by the
+// underlying build system. For the go tool, this is described at
+// https://golang.org/cmd/go/#hdr-Package_lists_and_patterns
+//
+// wd is the working directory and env is the set of environment
+// variables to use when loading the packages specified by patterns. If
+// env is nil or empty, it is interpreted as an empty set of variables.
+// In case of duplicate environment variables, the last one in the list
+// takes precedence.
+func Load(ctx context.Context, wd string, env []string, patterns []string) (*Info, []error) {
+	prog, errs := load(ctx, wd, env, patterns)
 	if len(errs) > 0 {
 		return nil, errs
 	}
@@ -275,12 +284,22 @@ func Load(bctx *build.Context, wd string, pkgs []string) (*Info, []error) {
 	return info, ec.errors
 }
 
-// load typechecks the packages, including function body type checking
-// for the packages directly named.
-func load(bctx *build.Context, wd string, pkgs []string) (*loader.Program, []error) {
+// load typechecks the packages that match the given patterns, including
+// function body type checking for the packages that directly match. The
+// patterns are defined by the underlying build system. For the go tool,
+// this is described at
+// https://golang.org/cmd/go/#hdr-Package_lists_and_patterns
+//
+// wd is the working directory and env is the set of environment
+// variables to use when loading the packages specified by patterns. If
+// env is nil or empty, it is interpreted as an empty set of variables.
+// In case of duplicate environment variables, the last one in the list
+// takes precedence.
+func load(ctx context.Context, wd string, env []string, patterns []string) (*loader.Program, []error) {
+	bctx := buildContextFromEnv(env)
 	var foundPkgs []*build.Package
 	ec := new(errorCollector)
-	for _, name := range pkgs {
+	for _, name := range patterns {
 		p, err := bctx.Import(name, wd, build.FindOnly)
 		if err != nil {
 			ec.add(err)
@@ -320,7 +339,7 @@ func load(bctx *build.Context, wd string, pkgs []string) (*loader.Program, []err
 			return pkg, err
 		},
 	}
-	for _, name := range pkgs {
+	for _, name := range patterns {
 		conf.Import(name)
 	}
 
@@ -334,6 +353,35 @@ func load(bctx *build.Context, wd string, pkgs []string) (*loader.Program, []err
 	return prog, nil
 }
 
+func buildContextFromEnv(env []string) *build.Context {
+	// TODO(#78): Remove this function in favor of using go/packages,
+	// which does not need a *build.Context.
+
+	getenv := func(name string) string {
+		for i := len(env) - 1; i >= 0; i-- {
+			if strings.HasPrefix(env[i], name+"=") {
+				return env[i][len(name)+1:]
+			}
+		}
+		return ""
+	}
+	bctx := new(build.Context)
+	*bctx = build.Default
+	if v := getenv("GOARCH"); v != "" {
+		bctx.GOARCH = v
+	}
+	if v := getenv("GOOS"); v != "" {
+		bctx.GOOS = v
+	}
+	if v := getenv("GOROOT"); v != "" {
+		bctx.GOROOT = v
+	}
+	if v := getenv("GOPATH"); v != "" {
+		bctx.GOPATH = v
+	}
+	return bctx
+}
+
 func importPathInPkgList(pkgs []*build.Package, path string) bool {
 	for _, p := range pkgs {
 		if path == p.ImportPath {

internal/wire/testdata/BuildTagsRelativePkg/pkg

@@ -1 +1 @@
-./example.com/foo
+./foo

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

@@ -1,4 +1,4 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: cycle for example.com/foo.Bar:
+example.com/foo/wire.go:x:y: cycle for example.com/foo.Bar:
 example.com/foo.Bar (example.com/foo.provideBar) ->
 example.com/foo.Foo (example.com/foo.provideFoo) ->
 example.com/foo.Baz (example.com/foo.provideBaz) ->

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: var example.com/foo.myFakeSet struct{} is not a provider or a provider set
+example.com/foo/wire.go:x:y: var example.com/foo.myFakeSet struct{} is not a provider or a provider set

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: inject injectBar: input of example.com/foo.Foo conflicts with provider provideFoo at /wire_gopath/src/example.com/foo/foo.go:x:y
+example.com/foo/wire.go:x:y: inject injectBar: input of example.com/foo.Foo conflicts with provider provideFoo at example.com/foo/foo.go:x:y

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: inject injectFoo: provider for example.com/foo.Foo returns cleanup but injection does not return cleanup function
+example.com/foo/wire.go:x:y: inject injectFoo: provider for example.com/foo.Foo returns cleanup but injection does not return cleanup function

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: inject injectFoo: provider for example.com/foo.Foo returns error but injection not allowed to fail
+example.com/foo/wire.go:x:y: inject injectFoo: provider for example.com/foo.Foo returns error but injection not allowed to fail

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: string does not implement example.com/foo.Fooer
+example.com/foo/wire.go:x:y: string does not implement example.com/foo.Fooer

internal/wire/testdata/InterfaceBindingInvalidArg0/want/wire_errs.txt

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: first argument to Bind must be a pointer to an interface type; found string
+example.com/foo/wire.go:x:y: first argument to Bind must be a pointer to an interface type; found string

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: too few arguments in call to wire.Bind
+example.com/foo/wire.go:x:y: too few arguments in call to wire.Bind

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: string does not implement io.Reader
+example.com/foo/wire.go:x:y: string does not implement io.Reader

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: first argument to InterfaceValue must be a pointer to an interface type; found string
+example.com/foo/wire.go:x:y: first argument to InterfaceValue must be a pointer to an interface type; found string

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: too few arguments in call to wire.InterfaceValue
+example.com/foo/wire.go:x:y: too few arguments in call to wire.InterfaceValue

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

@@ -1,41 +1,41 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: wire.Build has multiple bindings for example.com/foo.Foo
+example.com/foo/wire.go:x:y: wire.Build has multiple bindings for example.com/foo.Foo
 current:
-<- provider "provideFooAgain" (/wire_gopath/src/example.com/foo/foo.go:x:y)
+<- provider "provideFooAgain" (example.com/foo/foo.go:x:y)
 previous:
-<- provider "provideFoo" (/wire_gopath/src/example.com/foo/foo.go:x:y)
+<- provider "provideFoo" (example.com/foo/foo.go:x:y)
 
-/wire_gopath/src/example.com/foo/wire.go:x:y: wire.Build has multiple bindings for example.com/foo.Foo
+example.com/foo/wire.go:x:y: wire.Build has multiple bindings for example.com/foo.Foo
 current:
-<- provider "provideFoo" (/wire_gopath/src/example.com/foo/foo.go:x:y)
+<- provider "provideFoo" (example.com/foo/foo.go:x:y)
 previous:
-<- provider "provideFoo" (/wire_gopath/src/example.com/foo/foo.go:x:y)
-<- provider set "Set" (/wire_gopath/src/example.com/foo/foo.go:x:y)
+<- provider "provideFoo" (example.com/foo/foo.go:x:y)
+<- provider set "Set" (example.com/foo/foo.go:x:y)
 
-/wire_gopath/src/example.com/foo/wire.go:x:y: wire.Build has multiple bindings for example.com/foo.Foo
+example.com/foo/wire.go:x:y: wire.Build has multiple bindings for example.com/foo.Foo
 current:
-<- provider "provideFoo" (/wire_gopath/src/example.com/foo/foo.go:x:y)
+<- provider "provideFoo" (example.com/foo/foo.go:x:y)
 previous:
-<- provider "provideFoo" (/wire_gopath/src/example.com/foo/foo.go:x:y)
-<- provider set "Set" (/wire_gopath/src/example.com/foo/foo.go:x:y)
-<- provider set "SuperSet" (/wire_gopath/src/example.com/foo/foo.go:x:y)
+<- provider "provideFoo" (example.com/foo/foo.go:x:y)
+<- provider set "Set" (example.com/foo/foo.go:x:y)
+<- provider set "SuperSet" (example.com/foo/foo.go:x:y)
 
-/wire_gopath/src/example.com/foo/foo.go:x:y: SetWithDuplicateBindings has multiple bindings for example.com/foo.Foo
+example.com/foo/foo.go:x:y: SetWithDuplicateBindings has multiple bindings for example.com/foo.Foo
 current:
-<- provider "provideFoo" (/wire_gopath/src/example.com/foo/foo.go:x:y)
-<- provider set "Set" (/wire_gopath/src/example.com/foo/foo.go:x:y)
-<- provider set "SuperSet" (/wire_gopath/src/example.com/foo/foo.go:x:y)
+<- provider "provideFoo" (example.com/foo/foo.go:x:y)
+<- provider set "Set" (example.com/foo/foo.go:x:y)
+<- provider set "SuperSet" (example.com/foo/foo.go:x:y)
 previous:
-<- provider "provideFoo" (/wire_gopath/src/example.com/foo/foo.go:x:y)
-<- provider set "Set" (/wire_gopath/src/example.com/foo/foo.go:x:y)
+<- provider "provideFoo" (example.com/foo/foo.go:x:y)
+<- provider set "Set" (example.com/foo/foo.go:x:y)
 
-/wire_gopath/src/example.com/foo/wire.go:x:y: wire.Build has multiple bindings for example.com/foo.Foo
+example.com/foo/wire.go:x:y: wire.Build has multiple bindings for example.com/foo.Foo
 current:
-<- wire.Value (/wire_gopath/src/example.com/foo/wire.go:x:y)
+<- wire.Value (example.com/foo/wire.go:x:y)
 previous:
-<- provider "provideFoo" (/wire_gopath/src/example.com/foo/foo.go:x:y)
+<- provider "provideFoo" (example.com/foo/foo.go:x:y)
 
-/wire_gopath/src/example.com/foo/wire.go:x:y: wire.Build has multiple bindings for example.com/foo.Bar
+example.com/foo/wire.go:x:y: wire.Build has multiple bindings for example.com/foo.Bar
 current:
-<- wire.Bind (/wire_gopath/src/example.com/foo/wire.go:x:y)
+<- wire.Bind (example.com/foo/wire.go:x:y)
 previous:
-<- provider "provideBar" (/wire_gopath/src/example.com/foo/foo.go:x:y)
+<- provider "provideBar" (example.com/foo/foo.go:x:y)

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

@@ -1,12 +1,12 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: inject injectMissingOutputType: no provider found for example.com/foo.Foo, output of injector
+example.com/foo/wire.go:x:y: inject injectMissingOutputType: no provider found for example.com/foo.Foo, output of injector
 
-/wire_gopath/src/example.com/foo/wire.go:x:y: 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:x:y)
+example.com/foo/wire.go:x:y: inject injectMultipleMissingTypes: no provider found for example.com/foo.Foo
+needed by example.com/foo.Baz in provider "provideBaz" (example.com/foo/foo.go:x:y)
 
-/wire_gopath/src/example.com/foo/wire.go:x:y: 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:x:y)
+example.com/foo/wire.go:x:y: inject injectMultipleMissingTypes: no provider found for example.com/foo.Bar
+needed by example.com/foo.Baz in provider "provideBaz" (example.com/foo/foo.go:x:y)
 
-/wire_gopath/src/example.com/foo/wire.go:x:y: 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:x:y)
-needed by example.com/foo.Zap in provider "provideZap" (/wire_gopath/src/example.com/foo/foo.go:x:y)
-needed by example.com/foo.Zop in provider "provideZop" (/wire_gopath/src/example.com/foo/foo.go:x:y)
+example.com/foo/wire.go:x:y: inject injectMissingRecursiveType: no provider found for example.com/foo.Foo
+needed by example.com/foo.Zip in provider "provideZip" (example.com/foo/foo.go:x:y)
+needed by example.com/foo.Zap in provider "provideZap" (example.com/foo/foo.go:x:y)
+needed by example.com/foo.Zop in provider "provideZop" (example.com/foo/foo.go:x:y)

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: inject injectFooer: no provider found for example.com/foo.Fooer, output of injector
+example.com/foo/wire.go:x:y: inject injectFooer: no provider found for example.com/foo.Fooer, output of injector

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: foo not exported by package bar
+example.com/foo/wire.go:x:y: foo not exported by package bar

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: inject injectedMessage: value string can't be used: uses unexported identifier privateMsg
+example.com/foo/wire.go:x:y: inject injectedMessage: value string can't be used: uses unexported identifier privateMsg

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

@@ -1,7 +1,7 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: inject injectBar: unused provider set "unusedSet"
+example.com/foo/wire.go:x:y: inject injectBar: unused provider set "unusedSet"
 
-/wire_gopath/src/example.com/foo/wire.go:x:y: inject injectBar: unused provider "provideUnused"
+example.com/foo/wire.go:x:y: inject injectBar: unused provider "provideUnused"
 
-/wire_gopath/src/example.com/foo/wire.go:x:y: inject injectBar: unused value of type string
+example.com/foo/wire.go:x:y: inject injectBar: unused value of type string
 
-/wire_gopath/src/example.com/foo/wire.go:x:y: inject injectBar: unused interface binding to type example.com/foo.Fooer
+example.com/foo/wire.go:x:y: inject injectBar: unused interface binding to type example.com/foo.Fooer

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: inject injectBar: value int can't be used: f is not declared in package scope
+example.com/foo/wire.go:x:y: inject injectBar: value int can't be used: f is not declared in package scope

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

@@ -1 +1 @@
-/wire_gopath/src/example.com/foo/wire.go:x:y: argument to Value may not be an interface value (found io.Reader); use InterfaceValue instead
+example.com/foo/wire.go:x:y: argument to Value may not be an interface value (found io.Reader); use InterfaceValue instead

internal/wire/wire.go

@@ -18,13 +18,15 @@ package wire
 
 import (
 	"bytes"
+	"context"
+	"errors"
 	"fmt"
 	"go/ast"
-	"go/build"
 	"go/format"
 	"go/printer"
 	"go/token"
 	"go/types"
+	"io/ioutil"
 	"path/filepath"
 	"sort"
 	"strconv"
@@ -36,22 +38,50 @@ import (
 	"golang.org/x/tools/go/loader"
 )
 
+// GeneratedFile stores the content of a call to Generate and the
+// desired on-disk location of the file.
+type GeneratedFile struct {
+	Path    string
+	Content []byte
+}
+
+// Commit writes the generated file to disk.
+func (gen GeneratedFile) Commit() error {
+	if len(gen.Content) == 0 {
+		return nil
+	}
+	return ioutil.WriteFile(gen.Path, gen.Content, 0666)
+}
+
 // Generate performs dependency injection for a single package,
-// returning the gofmt'd Go source code.
-func Generate(bctx *build.Context, wd string, pkg string) ([]byte, []error) {
-	prog, errs := load(bctx, wd, []string{pkg})
+// returning the gofmt'd Go source code. The package pattern is defined
+// by the underlying build system. For the go tool, this is described at
+// https://golang.org/cmd/go/#hdr-Package_lists_and_patterns
+//
+// wd is the working directory and env is the set of environment
+// variables to use when loading the package specified by pkgPattern. If
+// env is nil or empty, it is interpreted as an empty set of variables.
+// In case of duplicate environment variables, the last one in the list
+// takes precedence.
+func Generate(ctx context.Context, wd string, env []string, pkgPattern string) (GeneratedFile, []error) {
+	prog, errs := load(ctx, wd, env, []string{pkgPattern})
 	if len(errs) > 0 {
-		return nil, errs
+		return GeneratedFile{}, errs
 	}
 	if len(prog.InitialPackages()) != 1 {
 		// This is more of a violated precondition than anything else.
-		return nil, []error{fmt.Errorf("load: got %d packages", len(prog.InitialPackages()))}
+		return GeneratedFile{}, []error{fmt.Errorf("load: got %d packages", len(prog.InitialPackages()))}
 	}
 	pkgInfo := prog.InitialPackages()[0]
+	outDir, err := detectOutputDir(prog.Fset, pkgInfo.Files)
+	if err != nil {
+		return GeneratedFile{}, []error{fmt.Errorf("load: %v", err)}
+	}
+	outFname := filepath.Join(outDir, "wire_gen.go")
 	g := newGen(prog, pkgInfo.Pkg.Path())
 	injectorFiles, errs := generateInjectors(g, pkgInfo)
 	if len(errs) > 0 {
-		return nil, errs
+		return GeneratedFile{}, errs
 	}
 	copyNonInjectorDecls(g, injectorFiles, &pkgInfo.Info)
 	goSrc := g.frame()
@@ -59,9 +89,22 @@ func Generate(bctx *build.Context, wd string, pkg string) ([]byte, []error) {
 	if err != nil {
 		// This is likely a bug from a poorly generated source file.
 		// Return an error and the unformatted source.
-		return goSrc, []error{err}
+		return GeneratedFile{Path: outFname, Content: goSrc}, []error{err}
 	}
-	return fmtSrc, nil
+	return GeneratedFile{Path: outFname, Content: fmtSrc}, nil
+}
+
+func detectOutputDir(fset *token.FileSet, files []*ast.File) (string, error) {
+	if len(files) == 0 {
+		return "", errors.New("no files to derive output directory from")
+	}
+	dir := filepath.Dir(fset.File(files[0].Package).Name())
+	for _, f := range files[1:] {
+		if dir2 := filepath.Dir(fset.File(f.Package).Name()); dir2 != dir {
+			return "", fmt.Errorf("found conflicting directories %q and %q", dir, dir2)
+		}
+	}
+	return dir, nil
 }
 
 // generateInjectors generates the injectors for a given package.

internal/wire/wire_test.go

@@ -16,21 +16,16 @@ package wire
 
 import (
 	"bytes"
-	"errors"
+	"context"
 	"fmt"
 	"go/build"
 	"go/types"
-	"io"
 	"io/ioutil"
 	"os"
 	"os/exec"
 	"path/filepath"
-	"regexp"
-	"runtime"
-	"sort"
 	"strings"
 	"testing"
-	"time"
 	"unicode"
 	"unicode/utf8"
 
@@ -63,29 +58,39 @@ func TestWire(t *testing.T) {
 		}
 		tests = append(tests, test)
 	}
-	wd := filepath.Join(magicGOPATH(), "src")
 
+	var goToolPath string
 	if *setup.Record {
-		if _, err := os.Stat(filepath.Join(build.Default.GOROOT, "bin", "go")); err != nil {
+		goToolPath = filepath.Join(build.Default.GOROOT, "bin", "go")
+		if _, err := os.Stat(goToolPath); err != nil {
 			t.Fatal("go toolchain not available:", err)
 		}
 	}
+	ctx := context.Background()
 	for _, test := range tests {
 		test := test
 		t.Run(test.name, func(t *testing.T) {
 			t.Parallel()
 
-			// Run Wire from a fake build context.
-			bctx := test.buildContext()
-			gen, errs := Generate(bctx, wd, test.pkg)
-			if len(gen) > 0 {
-				defer t.Logf("wire_gen.go:\n%s", gen)
+			// Materialize a temporary GOPATH directory.
+			gopath, err := ioutil.TempDir("", "wire_test")
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer os.RemoveAll(gopath)
+			if err := test.materialize(gopath); err != nil {
+				t.Fatal(err)
+			}
+			wd := filepath.Join(gopath, "src", "example.com")
+			gen, errs := Generate(ctx, wd, append(os.Environ(), "GOPATH="+gopath), test.pkg)
+			if len(gen.Content) > 0 {
+				defer t.Logf("wire_gen.go:\n%s", gen.Content)
 			}
 			if len(errs) > 0 {
 				gotErrStrings := make([]string, len(errs))
 				for i, e := range errs {
-					gotErrStrings[i] = scrubError(e.Error())
-					t.Log(gotErrStrings[i])
+					t.Log(e.Error())
+					gotErrStrings[i] = scrubError(gopath, e.Error())
 				}
 				if !test.wantWireError {
 					t.Fatal("Did not expect errors. To -record an error, create want/wire_errs.txt.")
@@ -105,26 +110,37 @@ func TestWire(t *testing.T) {
 			if test.wantWireError {
 				t.Fatal("wire succeeded; want error")
 			}
+			outPathSane := true
+			if prefix := gopath + string(os.PathSeparator) + "src" + string(os.PathSeparator); !strings.HasPrefix(gen.Path, prefix) {
+				outPathSane = false
+				t.Errorf("suggested output path = %q; want to start with %q", gen.Path, prefix)
+			}
 
 			if *setup.Record {
 				// Record ==> Build the generated Wire code,
 				// check that the program's output matches the
 				// expected output, save wire output on
 				// success.
-				if err := goBuildCheck(test, wd, bctx, gen); err != nil {
+				if !outPathSane {
+					return
+				}
+				if err := gen.Commit(); err != nil {
+					t.Fatalf("failed to write wire_gen.go to test GOPATH: %v", err)
+				}
+				if err := goBuildCheck(goToolPath, gopath, test); err != nil {
 					t.Fatalf("go build check failed: %v", err)
 				}
-				wireGenFile := filepath.Join(testRoot, test.name, "want", "wire_gen.go")
-				if err := ioutil.WriteFile(wireGenFile, gen, 0666); err != nil {
-					t.Fatalf("failed to write wire_gen.go file: %v", err)
+				testdataWireGenPath := filepath.Join(testRoot, test.name, "want", "wire_gen.go")
+				if err := ioutil.WriteFile(testdataWireGenPath, gen.Content, 0666); err != nil {
+					t.Fatalf("failed to record wire_gen.go to testdata: %v", err)
 				}
 			} else {
 				// Replay ==> Load golden file and compare to
 				// generated result. This check is meant to
 				// detect non-deterministic behavior in the
 				// Generate function.
-				if !bytes.Equal(gen, test.wantWireOutput) {
-					gotS, wantS := string(gen), string(test.wantWireOutput)
+				if !bytes.Equal(gen.Content, test.wantWireOutput) {
+					gotS, wantS := string(gen.Content), string(test.wantWireOutput)
 					diff := cmp.Diff(strings.Split(gotS, "\n"), strings.Split(wantS, "\n"))
 					t.Fatalf("wire output differs from golden file. If this change is expected, run with -record to update the wire_gen.go file.\n*** got:\n%s\n\n*** want:\n%s\n\n*** diff:\n%s", gotS, wantS, diff)
 				}
@@ -133,49 +149,27 @@ func TestWire(t *testing.T) {
 	}
 }
 
-func goBuildCheck(test *testCase, wd string, bctx *build.Context, gen []byte) error {
-	// Find the absolute import path, since test.pkg may be a relative
-	// import path.
-	genPkg, err := bctx.Import(test.pkg, wd, build.FindOnly)
-	if err != nil {
-		return err
-	}
-
-	// Run a `go build` with the generated output.
-	gopath, err := ioutil.TempDir("", "wire_test")
-	if err != nil {
-		return err
-	}
-	defer os.RemoveAll(gopath)
-	if err := test.materialize(gopath); err != nil {
-		return err
-	}
-	if len(gen) > 0 {
-		genPath := filepath.Join(gopath, "src", filepath.FromSlash(genPkg.ImportPath), "wire_gen.go")
-		if err := ioutil.WriteFile(genPath, gen, 0666); err != nil {
-			return err
-		}
-	}
+func goBuildCheck(goToolPath, gopath string, test *testCase) error {
+	// Write go.mod files for example.com and the wire package.
+	// TODO(#78): Move this to happen in materialize() once modules work.
 	if err := writeGoMod(gopath); err != nil {
 		return err
 	}
+
+	// Run `go build`.
 	testExePath := filepath.Join(gopath, "bin", "testprog")
-	realBuildCtx := &build.Context{
-		GOARCH:      bctx.GOARCH,
-		GOOS:        bctx.GOOS,
-		GOROOT:      bctx.GOROOT,
-		GOPATH:      gopath,
-		CgoEnabled:  bctx.CgoEnabled,
-		Compiler:    bctx.Compiler,
-		BuildTags:   bctx.BuildTags,
-		ReleaseTags: bctx.ReleaseTags,
-	}
-	buildDir := filepath.Join(gopath, "src", genPkg.ImportPath)
 	buildCmd := []string{"build", "-o", testExePath}
 	if test.name == "Vendor" && os.Getenv("GO111MODULE") == "on" {
 		buildCmd = append(buildCmd, "-mod=vendor")
 	}
-	if err := runGo(realBuildCtx, buildDir, buildCmd...); err != nil {
+	buildCmd = append(buildCmd, test.pkg)
+	cmd := exec.Command(goToolPath, buildCmd...)
+	cmd.Dir = filepath.Join(gopath, "src", "example.com")
+	cmd.Env = append(os.Environ(), "GOPATH="+gopath)
+	if buildOut, err := cmd.CombinedOutput(); err != nil {
+		if len(buildOut) > 0 {
+			return fmt.Errorf("build: %v; output:\n%s", err, buildOut)
+		}
 		return fmt.Errorf("build: %v", err)
 	}
 
@@ -332,24 +326,15 @@ func TestDisambiguate(t *testing.T) {
 
 func isIdent(s string) bool {
 	if len(s) == 0 {
-		if s == "foo" {
-			panic("BREAK3")
-		}
 		return false
 	}
 	r, i := utf8.DecodeRuneInString(s)
 	if !unicode.IsLetter(r) && r != '_' {
-		if s == "foo" {
-			panic("BREAK2")
-		}
 		return false
 	}
 	for i < len(s) {
 		r, sz := utf8.DecodeRuneInString(s[i:])
 		if !unicode.IsLetter(r) && !unicode.IsDigit(r) && r != '_' {
-			if s == "foo" {
-				panic("BREAK1")
-			}
 			return false
 		}
 		i += sz
@@ -357,6 +342,80 @@ func isIdent(s string) bool {
 	return true
 }
 
+// scrubError rewrites the given string to remove occurrences of GOPATH/src,
+// rewrites OS-specific path separators to slashes, and any line/column
+// information to a fixed ":x:y". For example, if the gopath parameter is
+// "C:\GOPATH" and running on Windows, the string
+// "C:\GOPATH\src\foo\bar.go:15:4" would be rewritten to "foo/bar.go:x:y".
+func scrubError(gopath string, s string) string {
+	sb := new(strings.Builder)
+	query := gopath + string(os.PathSeparator) + "src" + string(os.PathSeparator)
+	for {
+		// Find next occurrence of source root. This indicates the next path to
+		// scrub.
+		start := strings.Index(s, query)
+		if start == -1 {
+			sb.WriteString(s)
+			break
+		}
+
+		// Find end of file name (extension ".go").
+		fileStart := start + len(query)
+		fileEnd := strings.Index(s[fileStart:], ".go")
+		if fileEnd == -1 {
+			// If no ".go" occurs to end of string, further searches will fail too.
+			// Break the loop.
+			sb.WriteString(s)
+			break
+		}
+		fileEnd += fileStart + 3 // Advance to end of extension.
+
+		// Write out file name and advance scrub position.
+		file := s[fileStart:fileEnd]
+		if os.PathSeparator != '/' {
+			file = strings.Replace(file, string(os.PathSeparator), "/", -1)
+		}
+		sb.WriteString(s[:start])
+		sb.WriteString(file)
+		s = s[fileEnd:]
+
+		// Peek past to see if there is line/column info.
+		linecol, linecolLen := scrubLineColumn(s)
+		sb.WriteString(linecol)
+		s = s[linecolLen:]
+	}
+	return sb.String()
+}
+
+func scrubLineColumn(s string) (replacement string, n int) {
+	if !strings.HasPrefix(s, ":") {
+		return "", 0
+	}
+	// Skip first colon and run of digits.
+	for n++; len(s) > n && '0' <= s[n] && s[n] <= '9'; {
+		n++
+	}
+	if n == 1 {
+		// No digits followed colon.
+		return "", 0
+	}
+
+	// Start on column part.
+	if !strings.HasPrefix(s[n:], ":") {
+		return ":x", n
+	}
+	lineEnd := n
+	// Skip second colon and run of digits.
+	for n++; len(s) > n && '0' <= s[n] && s[n] <= '9'; {
+		n++
+	}
+	if n == lineEnd+1 {
+		// No digits followed second colon.
+		return ":x", lineEnd
+	}
+	return ":x:y", n
+}
+
 type testCase struct {
 	name                 string
 	pkg                  string
@@ -367,14 +426,6 @@ type testCase struct {
 	wantWireErrorStrings []string
 }
 
-var scrubLineNumberAndPositionRegex = regexp.MustCompile("\\.go:[\\d]+:[\\d]+")
-var scrubLineNumberRegex = regexp.MustCompile("\\.go:[\\d]+")
-
-func scrubError(s string) string {
-	s = scrubLineNumberAndPositionRegex.ReplaceAllString(s, ".go:x:y")
-	return scrubLineNumberRegex.ReplaceAllString(s, ".go:x")
-}
-
 // loadTestCase reads a test case from a directory.
 //
 // The directory structure is:
@@ -395,7 +446,7 @@ func scrubError(s string) string {
 //					missing if no errors expected.
 //					Distinct errors are separated by a blank line,
 //					and line numbers and line positions are scrubbed
-//					(e.g., "foo.go:52:8" --> "foo.go:x:y").
+//					(e.g. "$GOPATH/src/foo.go:52:8" --> "foo.go:x:y").
 //
 //			wire_gen.go
 //					verified output of wire from a test run with
@@ -417,7 +468,7 @@ func loadTestCase(root string, wireGoSrc []byte) (*testCase, error) {
 	wantWireError := err == nil
 	var wantWireErrorStrings []string
 	if wantWireError {
-		wantWireErrorStrings = strings.Split(scrubError(string(wireErrb)), "\n\n")
+		wantWireErrorStrings = strings.Split(string(wireErrb), "\n\n")
 	} else {
 		if !*setup.Record {
 			wantWireOutput, err = ioutil.ReadFile(filepath.Join(root, "want", "wire_gen.go"))
@@ -448,7 +499,7 @@ func loadTestCase(root string, wireGoSrc []byte) (*testCase, error) {
 		if err != nil {
 			return err
 		}
-		goFiles[filepath.Join("example.com", rel)] = data
+		goFiles["example.com/"+filepath.ToSlash(rel)] = data
 		return nil
 	})
 	if err != nil {
@@ -465,187 +516,11 @@ func loadTestCase(root string, wireGoSrc []byte) (*testCase, error) {
 	}, nil
 }
 
-func (test *testCase) buildContext() *build.Context {
-	return &build.Context{
-		GOARCH:      build.Default.GOARCH,
-		GOOS:        build.Default.GOOS,
-		GOROOT:      build.Default.GOROOT,
-		GOPATH:      magicGOPATH(),
-		CgoEnabled:  build.Default.CgoEnabled,
-		Compiler:    build.Default.Compiler,
-		ReleaseTags: build.Default.ReleaseTags,
-		HasSubdir:   test.hasSubdir,
-		ReadDir:     test.readDir,
-		OpenFile:    test.openFile,
-		IsDir:       test.isDir,
-	}
-}
-
-const (
-	magicGOPATHUnix    = "/wire_gopath"
-	magicGOPATHWindows = `C:\wire_gopath`
-)
-
-func magicGOPATH() string {
-	if runtime.GOOS == "windows" {
-		return magicGOPATHWindows
-	}
-
-	return magicGOPATHUnix
-}
-
-func (test *testCase) hasSubdir(root, dir string) (rel string, ok bool) {
-	// Don't consult filesystem, just lexical.
-
-	if dir == root {
-		return "", true
-	}
-	prefix := root
-	if !strings.HasSuffix(prefix, string(filepath.Separator)) {
-		prefix += string(filepath.Separator)
-	}
-	if !strings.HasPrefix(dir, prefix) {
-		return "", false
-	}
-	return filepath.ToSlash(dir[len(prefix):]), true
-}
-
-func (test *testCase) resolve(path string) (resolved string, pathType int) {
-	subpath, isMagic := test.hasSubdir(magicGOPATH(), path)
-	if !isMagic {
-		return path, systemPath
-	}
-	if subpath == "src" {
-		return "", gopathRoot
-	}
-	const srcPrefix = "src/"
-	if !strings.HasPrefix(subpath, srcPrefix) {
-		return subpath, gopathRoot
-	}
-	return subpath[len(srcPrefix):], gopathSrc
-}
-
-// Path types
-const (
-	systemPath = iota
-	gopathRoot
-	gopathSrc
-)
-
-func (test *testCase) readDir(dir string) ([]os.FileInfo, error) {
-	rpath, pathType := test.resolve(dir)
-	switch {
-	case pathType == systemPath:
-		return ioutil.ReadDir(rpath)
-	case pathType == gopathRoot && rpath == "":
-		return []os.FileInfo{dirInfo{name: "src"}}, nil
-	case pathType == gopathSrc:
-		names := make([]string, 0, len(test.goFiles))
-		prefix := rpath + string(filepath.Separator)
-		for name := range test.goFiles {
-			if strings.HasPrefix(name, prefix) {
-				names = append(names, name[len(prefix):])
-			}
-		}
-		sort.Strings(names)
-		ents := make([]os.FileInfo, 0, len(names))
-		for _, name := range names {
-			if i := strings.IndexRune(name, filepath.Separator); i != -1 {
-				// Directory
-				dirName := name[:i]
-				if len(ents) == 0 || ents[len(ents)-1].Name() != dirName {
-					ents = append(ents, dirInfo{name: dirName})
-				}
-				continue
-			}
-			ents = append(ents, fileInfo{
-				name: name,
-				size: int64(len(test.goFiles[name])),
-			})
-		}
-		return ents, nil
-	default:
-		return nil, &os.PathError{
-			Op:   "open",
-			Path: dir,
-			Err:  os.ErrNotExist,
-		}
-	}
-}
-
-func (test *testCase) isDir(path string) bool {
-	rpath, pathType := test.resolve(path)
-	switch {
-	case pathType == systemPath:
-		info, err := os.Stat(rpath)
-		return err == nil && info.IsDir()
-	case pathType == gopathRoot && rpath == "":
-		return true
-	case pathType == gopathSrc:
-		prefix := rpath + string(filepath.Separator)
-		for name := range test.goFiles {
-			if strings.HasPrefix(name, prefix) {
-				return true
-			}
-		}
-		return false
-	default:
-		return false
-	}
-}
-
-type dirInfo struct {
-	name string
-}
-
-func (d dirInfo) Name() string       { return d.name }
-func (d dirInfo) Size() int64        { return 0 }
-func (d dirInfo) Mode() os.FileMode  { return os.ModeDir | os.ModePerm }
-func (d dirInfo) ModTime() time.Time { return time.Unix(0, 0) }
-func (d dirInfo) IsDir() bool        { return true }
-func (d dirInfo) Sys() interface{}   { return nil }
-
-type fileInfo struct {
-	name string
-	size int64
-}
-
-func (f fileInfo) Name() string       { return f.name }
-func (f fileInfo) Size() int64        { return f.size }
-func (f fileInfo) Mode() os.FileMode  { return os.ModeDir | 0666 }
-func (f fileInfo) ModTime() time.Time { return time.Unix(0, 0) }
-func (f fileInfo) IsDir() bool        { return false }
-func (f fileInfo) Sys() interface{}   { return nil }
-
-func (test *testCase) openFile(path string) (io.ReadCloser, error) {
-	rpath, pathType := test.resolve(path)
-	switch {
-	case pathType == systemPath:
-		return os.Open(path)
-	case pathType == gopathSrc:
-		content, ok := test.goFiles[rpath]
-		if !ok {
-			return nil, &os.PathError{
-				Op:   "open",
-				Path: path,
-				Err:  errors.New("does not exist or is not a file"),
-			}
-		}
-		return ioutil.NopCloser(bytes.NewReader(content)), nil
-	default:
-		return nil, &os.PathError{
-			Op:   "open",
-			Path: path,
-			Err:  errors.New("does not exist or is not a file"),
-		}
-	}
-}
-
 // materialize creates a new GOPATH at the given directory, which may or
 // may not exist.
 func (test *testCase) materialize(gopath string) error {
 	for name, content := range test.goFiles {
-		dst := filepath.Join(gopath, "src", name)
+		dst := filepath.Join(gopath, "src", filepath.FromSlash(name))
 		if err := os.MkdirAll(filepath.Dir(dst), 0777); err != nil {
 			return fmt.Errorf("materialize GOPATH: %v", err)
 		}
@@ -675,11 +550,11 @@ func (test *testCase) materialize(gopath string) error {
 //
 //			... (Dependency files copied)
 func writeGoMod(gopath string) error {
-	importPath := "example.com"
-	depPath := "github.com/google/go-cloud"
+	const importPath = "example.com"
+	const depPath = "github.com/google/go-cloud"
 	depLoc := filepath.Join(gopath, "src", filepath.FromSlash(depPath))
 	example := fmt.Sprintf("module %s\n\nreplace %s => %s\n", importPath, depPath, depLoc)
-	gomod := filepath.Join(gopath, "src", importPath, "go.mod")
+	gomod := filepath.Join(gopath, "src", filepath.FromSlash(importPath), "go.mod")
 	if err := ioutil.WriteFile(gomod, []byte(example), 0666); err != nil {
 		return fmt.Errorf("generate go.mod for %s: %v", gomod, err)
 	}
@@ -688,25 +563,3 @@ func writeGoMod(gopath string) error {
 	}
 	return nil
 }
-
-// runGo runs a go command in dir.
-func runGo(bctx *build.Context, dir string, args ...string) error {
-	exe := filepath.Join(bctx.GOROOT, "bin", "go")
-	c := exec.Command(exe, args...)
-	c.Env = append(os.Environ(), "GOROOT="+bctx.GOROOT, "GOARCH="+bctx.GOARCH, "GOOS="+bctx.GOOS, "GOPATH="+bctx.GOPATH)
-	c.Dir = dir
-	if bctx.CgoEnabled {
-		c.Env = append(c.Env, "CGO_ENABLED=1")
-	} else {
-		c.Env = append(c.Env, "CGO_ENABLED=0")
-	}
-	// TODO(someday): Set -compiler flag if needed.
-	out, err := c.CombinedOutput()
-	if err != nil {
-		if len(out) > 0 {
-			return fmt.Errorf("%v; output:\n%s", err, out)
-		}
-		return err
-	}
-	return nil
-}