cmd/go,testdeps: move import of internal/coverage/cfile to testmain

Instead of having testing/internal/testdeps import the
internal/coverage/cfile package directly, have the code in testmain
pass in pointers to cfile functions during setup in the case that
we're running a "go test -cover" binary. This reduces the size of
regular non-coverage test binaries back to what they were before CL
585820.

Updates #67401.
Fixes #67588.

Change-Id: Iaf1a613bc7d3c9df9943189065d0161ca9120d34
Reviewed-on: https://go-review.googlesource.com/c/go/+/587795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/load/test.go

@@ -298,6 +298,9 @@ func TestPackagesAndErrors(ctx context.Context, done func(), opts PackageOpts, p
 	// Also the linker introduces implicit dependencies reported by LinkerDeps.
 	stk.Push("testmain")
 	deps := TestMainDeps // cap==len, so safe for append
+	if cover != nil && cfg.Experiment.CoverageRedesign {
+		deps = append(deps, "internal/coverage/cfile")
+	}
 	ldDeps, err := LinkerDeps(p)
 	if err != nil && pmain.Error == nil {
 		pmain.Error = &PackageError{Err: err}
@@ -907,6 +910,9 @@ import (
 {{end}}
 	"testing"
 	"testing/internal/testdeps"
+{{if .Cover}}
+	"internal/coverage/cfile"
+{{end}}
 
 {{if .ImportTest}}
 	{{if .NeedTest}}_test{{else}}_{{end}} {{.Package.ImportPath | printf "%q"}}
@@ -944,6 +950,10 @@ func init() {
 {{if .Cover}}
 	testdeps.CoverMode = {{printf "%q" .Cover.Mode}}
 	testdeps.Covered = {{printf "%q" .Covered}}
+	testdeps.CoverSnapshotFunc = cfile.Snapshot
+	testdeps.CoverProcessTestDirFunc = cfile.ProcessCoverTestDir
+	testdeps.CoverMarkProfileEmittedFunc = cfile.MarkProfileEmitted
+
 {{end}}
 	testdeps.ImportPath = {{.ImportPath | printf "%q"}}
 }

src/testing/internal/testdeps/deps.go

@@ -13,7 +13,6 @@ package testdeps
 import (
 	"bufio"
 	"context"
-	"internal/coverage/cfile"
 	"internal/fuzz"
 	"internal/testlog"
 	"io"
@@ -205,11 +204,21 @@ func (TestDeps) SnapshotCoverage() {
 var CoverMode string
 var Covered string
 
+// These variables below are set at runtime (via code in testmain) to point
+// to the equivalent functions in package internal/coverage/cfile; doing
+// things this way allows us to have tests import internal/coverage/cfile
+// only when -cover is in effect (as opposed to importing for all tests).
+var (
+	CoverSnapshotFunc           func() float64
+	CoverProcessTestDirFunc     func(dir string, cfile string, cm string, cpkg string, w io.Writer) error
+	CoverMarkProfileEmittedFunc func(val bool)
+)
+
 func (TestDeps) InitRuntimeCoverage() (mode string, tearDown func(string, string) (string, error), snapcov func() float64) {
 	if CoverMode == "" {
 		return
 	}
-	return CoverMode, coverTearDown, cfile.Snapshot
+	return CoverMode, coverTearDown, CoverSnapshotFunc
 }
 
 func coverTearDown(coverprofile string, gocoverdir string) (string, error) {
@@ -221,9 +230,9 @@ func coverTearDown(coverprofile string, gocoverdir string) (string, error) {
 		}
 		defer os.RemoveAll(gocoverdir)
 	}
-	cfile.MarkProfileEmitted(true)
+	CoverMarkProfileEmittedFunc(true)
 	cmode := CoverMode
-	if err := cfile.ProcessCoverTestDir(gocoverdir, coverprofile, cmode, Covered, os.Stdout); err != nil {
+	if err := CoverProcessTestDirFunc(gocoverdir, coverprofile, cmode, Covered, os.Stdout); err != nil {
 		return "error generating coverage report", err
 	}
 	return "", nil