cmd/{cover,covdata}: minor code cleanups

Delete some unused code, and fix a few warnings from staticcheck.

Change-Id: I3d3a6f13dccffda060449948769c305d93a0389c
Reviewed-on: https://go-review.googlesource.com/c/go/+/441936
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/covdata/subtractintersect.go

@@ -37,7 +37,6 @@ func makeSubtractIntersectOp(mode string) covOperation {
 // away most of the grubby details of reading coverage data files.
 type sstate struct {
 	mm    *metaMerge
-	indir string // current input directory
 	inidx int
 	mode  string
 	// Used only for intersection; keyed by pkg/fn ID, it keeps track of

src/cmd/covdata/tool_test.go

@@ -11,7 +11,6 @@ import (
 	"internal/coverage/pods"
 	"internal/goexperiment"
 	"internal/testenv"
-	"io/ioutil"
 	"log"
 	"os"
 	"os/exec"
@@ -104,11 +103,11 @@ func gobuild(t *testing.T, indir string, bargs []string) {
 }
 
 func emitFile(t *testing.T, dst, src string) {
-	payload, err := ioutil.ReadFile(src)
+	payload, err := os.ReadFile(src)
 	if err != nil {
 		t.Fatalf("error reading %q: %v", src, err)
 	}
-	if err := ioutil.WriteFile(dst, payload, 0666); err != nil {
+	if err := os.WriteFile(dst, payload, 0666); err != nil {
 		t.Fatalf("writing %q: %v", dst, err)
 	}
 }
@@ -134,8 +133,8 @@ func buildProg(t *testing.T, prog string, dir string, tag string, flags []string
 
 	// Emit go.mod.
 	mod := filepath.Join(subdir, "go.mod")
-	modsrc := fmt.Sprintf("\nmodule prog\n\ngo 1.19\n")
+	modsrc := "\nmodule prog\n\ngo 1.19\n"
-	if err := ioutil.WriteFile(mod, []byte(modsrc), 0666); err != nil {
+	if err := os.WriteFile(mod, []byte(modsrc), 0666); err != nil {
 		t.Fatal(err)
 	}
 	exepath := filepath.Join(subdir, prog+".exe")
@@ -418,7 +417,7 @@ func testTextfmt(t *testing.T, s state) {
 	}
 
 	// Open and read the first few bits of the file.
-	payload, err := ioutil.ReadFile(outf)
+	payload, err := os.ReadFile(outf)
 	if err != nil {
 		t.Errorf("opening %s: %v\n", outf, err)
 	}
@@ -616,7 +615,6 @@ func testMergeSelect(t *testing.T, s state, indir1, indir2 string, tag string) {
 			}
 			want[line] = 1
 			continue
-		} else {
 		}
 		// no other functions or packages expected.
 		if strings.HasPrefix(line, "Func:") || strings.HasPrefix(line, "Package path:") {

src/cmd/cover/cfg_test.go

@@ -167,7 +167,7 @@ func TestCoverWithCfg(t *testing.T) {
 
 	// Expect err if config file contains unknown stuff.
 	t.Logf("mangling in config")
-	writeFile(t, incfg, []byte(fmt.Sprintf("blah=foo\n")))
+	writeFile(t, incfg, []byte("blah=foo\n"))
 	_, _, errmsg = runPkgCover(t, instdira, tag, incfg, mode,
 		pfiles("a"), errExpected)
 	want = "error reading pkgconfig file"
@@ -177,7 +177,7 @@ func TestCoverWithCfg(t *testing.T) {
 
 	// Expect error on empty config file.
 	t.Logf("writing empty config")
-	writeFile(t, incfg, []byte(fmt.Sprintf("\n")))
+	writeFile(t, incfg, []byte("\n"))
 	_, _, errmsg = runPkgCover(t, instdira, tag, incfg, mode,
 		pfiles("a"), errExpected)
 	if !strings.Contains(errmsg, want) {

src/cmd/cover/cover.go

@@ -16,7 +16,6 @@ import (
 	"internal/coverage/encodemeta"
 	"internal/coverage/slicewriter"
 	"io"
-	"io/ioutil"
 	"log"
 	"os"
 	"path/filepath"
@@ -190,7 +189,7 @@ func parseFlags() error {
 }
 
 func readOutFileList(path string) ([]string, error) {
-	data, err := ioutil.ReadFile(path)
+	data, err := os.ReadFile(path)
 	if err != nil {
 		return nil, fmt.Errorf("error reading -outfilelist file %q: %v", path, err)
 	}
@@ -198,7 +197,7 @@ func readOutFileList(path string) ([]string, error) {
 }
 
 func readPackageConfig(path string) error {
-	data, err := ioutil.ReadFile(path)
+	data, err := os.ReadFile(path)
 	if err != nil {
 		return fmt.Errorf("error reading pkgconfig file %q: %v", path, err)
 	}
@@ -1008,35 +1007,6 @@ func dedup(p1, p2 token.Position) (r1, r2 token.Position) {
 	return key.p1, key.p2
 }
 
-type sliceWriteSeeker struct {
-	payload []byte
-	off     int64
-}
-
-func (d *sliceWriteSeeker) Write(p []byte) (n int, err error) {
-	amt := len(p)
-	towrite := d.payload[d.off:]
-	if len(towrite) < amt {
-		d.payload = append(d.payload, make([]byte, amt-len(towrite))...)
-		towrite = d.payload[d.off:]
-	}
-	copy(towrite, p)
-	d.off += int64(amt)
-	return amt, nil
-}
-
-func (d *sliceWriteSeeker) Seek(offset int64, whence int) (int64, error) {
-	if whence == io.SeekStart {
-		d.off = offset
-		return offset, nil
-	} else if whence == io.SeekCurrent {
-		d.off += offset
-		return d.off, nil
-	}
-	// other modes not supported
-	panic("bad")
-}
-
 func (p *Package) emitMetaData(w io.Writer) {
 	if *pkgcfg == "" {
 		return