16 files changed, 2705 insertions, 0 deletions

diff --git a/src/cmd/cover/cover.go b/src/cmd/cover/cover.go
new file mode 100644
index 0000000..7ee0008
--- /dev/null
+++ b/src/cmd/cover/cover.go
@@ -0,0 +1,729 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"io"
+	"log"
+	"os"
+	"sort"
+
+	"cmd/internal/edit"
+	"cmd/internal/objabi"
+)
+
+const usageMessage = "" +
+	`Usage of 'go tool cover':
+Given a coverage profile produced by 'go test':
+	go test -coverprofile=c.out
+
+Open a web browser displaying annotated source code:
+	go tool cover -html=c.out
+
+Write out an HTML file instead of launching a web browser:
+	go tool cover -html=c.out -o coverage.html
+
+Display coverage percentages to stdout for each function:
+	go tool cover -func=c.out
+
+Finally, to generate modified source code with coverage annotations
+(what go test -cover does):
+	go tool cover -mode=set -var=CoverageVariableName program.go
+`
+
+func usage() {
+	fmt.Fprintln(os.Stderr, usageMessage)
+	fmt.Fprintln(os.Stderr, "Flags:")
+	flag.PrintDefaults()
+	fmt.Fprintln(os.Stderr, "\n  Only one of -html, -func, or -mode may be set.")
+	os.Exit(2)
+}
+
+var (
+	mode    = flag.String("mode", "", "coverage mode: set, count, atomic")
+	varVar  = flag.String("var", "GoCover", "name of coverage variable to generate")
+	output  = flag.String("o", "", "file for output; default: stdout")
+	htmlOut = flag.String("html", "", "generate HTML representation of coverage profile")
+	funcOut = flag.String("func", "", "output coverage profile information for each function")
+)
+
+var profile string // The profile to read; the value of -html or -func
+
+var counterStmt func(*File, string) string
+
+const (
+	atomicPackagePath = "sync/atomic"
+	atomicPackageName = "_cover_atomic_"
+)
+
+func main() {
+	objabi.AddVersionFlag()
+	flag.Usage = usage
+	flag.Parse()
+
+	// Usage information when no arguments.
+	if flag.NFlag() == 0 && flag.NArg() == 0 {
+		flag.Usage()
+	}
+
+	err := parseFlags()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		fmt.Fprintln(os.Stderr, `For usage information, run "go tool cover -help"`)
+		os.Exit(2)
+	}
+
+	// Generate coverage-annotated source.
+	if *mode != "" {
+		annotate(flag.Arg(0))
+		return
+	}
+
+	// Output HTML or function coverage information.
+	if *htmlOut != "" {
+		err = htmlOutput(profile, *output)
+	} else {
+		err = funcOutput(profile, *output)
+	}
+
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "cover: %v\n", err)
+		os.Exit(2)
+	}
+}
+
+// parseFlags sets the profile and counterStmt globals and performs validations.
+func parseFlags() error {
+	profile = *htmlOut
+	if *funcOut != "" {
+		if profile != "" {
+			return fmt.Errorf("too many options")
+		}
+		profile = *funcOut
+	}
+
+	// Must either display a profile or rewrite Go source.
+	if (profile == "") == (*mode == "") {
+		return fmt.Errorf("too many options")
+	}
+
+	if *varVar != "" && !token.IsIdentifier(*varVar) {
+		return fmt.Errorf("-var: %q is not a valid identifier", *varVar)
+	}
+
+	if *mode != "" {
+		switch *mode {
+		case "set":
+			counterStmt = setCounterStmt
+		case "count":
+			counterStmt = incCounterStmt
+		case "atomic":
+			counterStmt = atomicCounterStmt
+		default:
+			return fmt.Errorf("unknown -mode %v", *mode)
+		}
+
+		if flag.NArg() == 0 {
+			return fmt.Errorf("missing source file")
+		} else if flag.NArg() == 1 {
+			return nil
+		}
+	} else if flag.NArg() == 0 {
+		return nil
+	}
+	return fmt.Errorf("too many arguments")
+}
+
+// Block represents the information about a basic block to be recorded in the analysis.
+// Note: Our definition of basic block is based on control structures; we don't break
+// apart && and ||. We could but it doesn't seem important enough to bother.
+type Block struct {
+	startByte token.Pos
+	endByte   token.Pos
+	numStmt   int
+}
+
+// File is a wrapper for the state of a file used in the parser.
+// The basic parse tree walker is a method of this type.
+type File struct {
+	fset    *token.FileSet
+	name    string // Name of file.
+	astFile *ast.File
+	blocks  []Block
+	content []byte
+	edit    *edit.Buffer
+}
+
+// findText finds text in the original source, starting at pos.
+// It correctly skips over comments and assumes it need not
+// handle quoted strings.
+// It returns a byte offset within f.src.
+func (f *File) findText(pos token.Pos, text string) int {
+	b := []byte(text)
+	start := f.offset(pos)
+	i := start
+	s := f.content
+	for i < len(s) {
+		if bytes.HasPrefix(s[i:], b) {
+			return i
+		}
+		if i+2 <= len(s) && s[i] == '/' && s[i+1] == '/' {
+			for i < len(s) && s[i] != '\n' {
+				i++
+			}
+			continue
+		}
+		if i+2 <= len(s) && s[i] == '/' && s[i+1] == '*' {
+			for i += 2; ; i++ {
+				if i+2 > len(s) {
+					return 0
+				}
+				if s[i] == '*' && s[i+1] == '/' {
+					i += 2
+					break
+				}
+			}
+			continue
+		}
+		i++
+	}
+	return -1
+}
+
+// Visit implements the ast.Visitor interface.
+func (f *File) Visit(node ast.Node) ast.Visitor {
+	switch n := node.(type) {
+	case *ast.BlockStmt:
+		// If it's a switch or select, the body is a list of case clauses; don't tag the block itself.
+		if len(n.List) > 0 {
+			switch n.List[0].(type) {
+			case *ast.CaseClause: // switch
+				for _, n := range n.List {
+					clause := n.(*ast.CaseClause)
+					f.addCounters(clause.Colon+1, clause.Colon+1, clause.End(), clause.Body, false)
+				}
+				return f
+			case *ast.CommClause: // select
+				for _, n := range n.List {
+					clause := n.(*ast.CommClause)
+					f.addCounters(clause.Colon+1, clause.Colon+1, clause.End(), clause.Body, false)
+				}
+				return f
+			}
+		}
+		f.addCounters(n.Lbrace, n.Lbrace+1, n.Rbrace+1, n.List, true) // +1 to step past closing brace.
+	case *ast.IfStmt:
+		if n.Init != nil {
+			ast.Walk(f, n.Init)
+		}
+		ast.Walk(f, n.Cond)
+		ast.Walk(f, n.Body)
+		if n.Else == nil {
+			return nil
+		}
+		// The elses are special, because if we have
+		//	if x {
+		//	} else if y {
+		//	}
+		// we want to cover the "if y". To do this, we need a place to drop the counter,
+		// so we add a hidden block:
+		//	if x {
+		//	} else {
+		//		if y {
+		//		}
+		//	}
+		elseOffset := f.findText(n.Body.End(), "else")
+		if elseOffset < 0 {
+			panic("lost else")
+		}
+		f.edit.Insert(elseOffset+4, "{")
+		f.edit.Insert(f.offset(n.Else.End()), "}")
+
+		// We just created a block, now walk it.
+		// Adjust the position of the new block to start after
+		// the "else". That will cause it to follow the "{"
+		// we inserted above.
+		pos := f.fset.File(n.Body.End()).Pos(elseOffset + 4)
+		switch stmt := n.Else.(type) {
+		case *ast.IfStmt:
+			block := &ast.BlockStmt{
+				Lbrace: pos,
+				List:   []ast.Stmt{stmt},
+				Rbrace: stmt.End(),
+			}
+			n.Else = block
+		case *ast.BlockStmt:
+			stmt.Lbrace = pos
+		default:
+			panic("unexpected node type in if")
+		}
+		ast.Walk(f, n.Else)
+		return nil
+	case *ast.SelectStmt:
+		// Don't annotate an empty select - creates a syntax error.
+		if n.Body == nil || len(n.Body.List) == 0 {
+			return nil
+		}
+	case *ast.SwitchStmt:
+		// Don't annotate an empty switch - creates a syntax error.
+		if n.Body == nil || len(n.Body.List) == 0 {
+			if n.Init != nil {
+				ast.Walk(f, n.Init)
+			}
+			if n.Tag != nil {
+				ast.Walk(f, n.Tag)
+			}
+			return nil
+		}
+	case *ast.TypeSwitchStmt:
+		// Don't annotate an empty type switch - creates a syntax error.
+		if n.Body == nil || len(n.Body.List) == 0 {
+			if n.Init != nil {
+				ast.Walk(f, n.Init)
+			}
+			ast.Walk(f, n.Assign)
+			return nil
+		}
+	case *ast.FuncDecl:
+		// Don't annotate functions with blank names - they cannot be executed.
+		if n.Name.Name == "_" {
+			return nil
+		}
+	}
+	return f
+}
+
+func annotate(name string) {
+	fset := token.NewFileSet()
+	content, err := os.ReadFile(name)
+	if err != nil {
+		log.Fatalf("cover: %s: %s", name, err)
+	}
+	parsedFile, err := parser.ParseFile(fset, name, content, parser.ParseComments)
+	if err != nil {
+		log.Fatalf("cover: %s: %s", name, err)
+	}
+
+	file := &File{
+		fset:    fset,
+		name:    name,
+		content: content,
+		edit:    edit.NewBuffer(content),
+		astFile: parsedFile,
+	}
+	if *mode == "atomic" {
+		// Add import of sync/atomic immediately after package clause.
+		// We do this even if there is an existing import, because the
+		// existing import may be shadowed at any given place we want
+		// to refer to it, and our name (_cover_atomic_) is less likely to
+		// be shadowed.
+		file.edit.Insert(file.offset(file.astFile.Name.End()),
+			fmt.Sprintf("; import %s %q", atomicPackageName, atomicPackagePath))
+	}
+
+	ast.Walk(file, file.astFile)
+	newContent := file.edit.Bytes()
+
+	fd := os.Stdout
+	if *output != "" {
+		var err error
+		fd, err = os.Create(*output)
+		if err != nil {
+			log.Fatalf("cover: %s", err)
+		}
+	}
+
+	fmt.Fprintf(fd, "//line %s:1\n", name)
+	fd.Write(newContent)
+
+	// After printing the source tree, add some declarations for the counters etc.
+	// We could do this by adding to the tree, but it's easier just to print the text.
+	file.addVariables(fd)
+}
+
+// setCounterStmt returns the expression: __count[23] = 1.
+func setCounterStmt(f *File, counter string) string {
+	return fmt.Sprintf("%s = 1", counter)
+}
+
+// incCounterStmt returns the expression: __count[23]++.
+func incCounterStmt(f *File, counter string) string {
+	return fmt.Sprintf("%s++", counter)
+}
+
+// atomicCounterStmt returns the expression: atomic.AddUint32(&__count[23], 1)
+func atomicCounterStmt(f *File, counter string) string {
+	return fmt.Sprintf("%s.AddUint32(&%s, 1)", atomicPackageName, counter)
+}
+
+// newCounter creates a new counter expression of the appropriate form.
+func (f *File) newCounter(start, end token.Pos, numStmt int) string {
+	stmt := counterStmt(f, fmt.Sprintf("%s.Count[%d]", *varVar, len(f.blocks)))
+	f.blocks = append(f.blocks, Block{start, end, numStmt})
+	return stmt
+}
+
+// addCounters takes a list of statements and adds counters to the beginning of
+// each basic block at the top level of that list. For instance, given
+//
+//	S1
+//	if cond {
+//		S2
+// 	}
+//	S3
+//
+// counters will be added before S1 and before S3. The block containing S2
+// will be visited in a separate call.
+// TODO: Nested simple blocks get unnecessary (but correct) counters
+func (f *File) addCounters(pos, insertPos, blockEnd token.Pos, list []ast.Stmt, extendToClosingBrace bool) {
+	// Special case: make sure we add a counter to an empty block. Can't do this below
+	// or we will add a counter to an empty statement list after, say, a return statement.
+	if len(list) == 0 {
+		f.edit.Insert(f.offset(insertPos), f.newCounter(insertPos, blockEnd, 0)+";")
+		return
+	}
+	// Make a copy of the list, as we may mutate it and should leave the
+	// existing list intact.
+	list = append([]ast.Stmt(nil), list...)
+	// We have a block (statement list), but it may have several basic blocks due to the
+	// appearance of statements that affect the flow of control.
+	for {
+		// Find first statement that affects flow of control (break, continue, if, etc.).
+		// It will be the last statement of this basic block.
+		var last int
+		end := blockEnd
+		for last = 0; last < len(list); last++ {
+			stmt := list[last]
+			end = f.statementBoundary(stmt)
+			if f.endsBasicSourceBlock(stmt) {
+				// If it is a labeled statement, we need to place a counter between
+				// the label and its statement because it may be the target of a goto
+				// and thus start a basic block. That is, given
+				//	foo: stmt
+				// we need to create
+				//	foo: ; stmt
+				// and mark the label as a block-terminating statement.
+				// The result will then be
+				//	foo: COUNTER[n]++; stmt
+				// However, we can't do this if the labeled statement is already
+				// a control statement, such as a labeled for.
+				if label, isLabel := stmt.(*ast.LabeledStmt); isLabel && !f.isControl(label.Stmt) {
+					newLabel := *label
+					newLabel.Stmt = &ast.EmptyStmt{
+						Semicolon: label.Stmt.Pos(),
+						Implicit:  true,
+					}
+					end = label.Pos() // Previous block ends before the label.
+					list[last] = &newLabel
+					// Open a gap and drop in the old statement, now without a label.
+					list = append(list, nil)
+					copy(list[last+1:], list[last:])
+					list[last+1] = label.Stmt
+				}
+				last++
+				extendToClosingBrace = false // Block is broken up now.
+				break
+			}
+		}
+		if extendToClosingBrace {
+			end = blockEnd
+		}
+		if pos != end { // Can have no source to cover if e.g. blocks abut.
+			f.edit.Insert(f.offset(insertPos), f.newCounter(pos, end, last)+";")
+		}
+		list = list[last:]
+		if len(list) == 0 {
+			break
+		}
+		pos = list[0].Pos()
+		insertPos = pos
+	}
+}
+
+// hasFuncLiteral reports the existence and position of the first func literal
+// in the node, if any. If a func literal appears, it usually marks the termination
+// of a basic block because the function body is itself a block.
+// Therefore we draw a line at the start of the body of the first function literal we find.
+// TODO: what if there's more than one? Probably doesn't matter much.
+func hasFuncLiteral(n ast.Node) (bool, token.Pos) {
+	if n == nil {
+		return false, 0
+	}
+	var literal funcLitFinder
+	ast.Walk(&literal, n)
+	return literal.found(), token.Pos(literal)
+}
+
+// statementBoundary finds the location in s that terminates the current basic
+// block in the source.
+func (f *File) statementBoundary(s ast.Stmt) token.Pos {
+	// Control flow statements are easy.
+	switch s := s.(type) {
+	case *ast.BlockStmt:
+		// Treat blocks like basic blocks to avoid overlapping counters.
+		return s.Lbrace
+	case *ast.IfStmt:
+		found, pos := hasFuncLiteral(s.Init)
+		if found {
+			return pos
+		}
+		found, pos = hasFuncLiteral(s.Cond)
+		if found {
+			return pos
+		}
+		return s.Body.Lbrace
+	case *ast.ForStmt:
+		found, pos := hasFuncLiteral(s.Init)
+		if found {
+			return pos
+		}
+		found, pos = hasFuncLiteral(s.Cond)
+		if found {
+			return pos
+		}
+		found, pos = hasFuncLiteral(s.Post)
+		if found {
+			return pos
+		}
+		return s.Body.Lbrace
+	case *ast.LabeledStmt:
+		return f.statementBoundary(s.Stmt)
+	case *ast.RangeStmt:
+		found, pos := hasFuncLiteral(s.X)
+		if found {
+			return pos
+		}
+		return s.Body.Lbrace
+	case *ast.SwitchStmt:
+		found, pos := hasFuncLiteral(s.Init)
+		if found {
+			return pos
+		}
+		found, pos = hasFuncLiteral(s.Tag)
+		if found {
+			return pos
+		}
+		return s.Body.Lbrace
+	case *ast.SelectStmt:
+		return s.Body.Lbrace
+	case *ast.TypeSwitchStmt:
+		found, pos := hasFuncLiteral(s.Init)
+		if found {
+			return pos
+		}
+		return s.Body.Lbrace
+	}
+	// If not a control flow statement, it is a declaration, expression, call, etc. and it may have a function literal.
+	// If it does, that's tricky because we want to exclude the body of the function from this block.
+	// Draw a line at the start of the body of the first function literal we find.
+	// TODO: what if there's more than one? Probably doesn't matter much.
+	found, pos := hasFuncLiteral(s)
+	if found {
+		return pos
+	}
+	return s.End()
+}
+
+// endsBasicSourceBlock reports whether s changes the flow of control: break, if, etc.,
+// or if it's just problematic, for instance contains a function literal, which will complicate
+// accounting due to the block-within-an expression.
+func (f *File) endsBasicSourceBlock(s ast.Stmt) bool {
+	switch s := s.(type) {
+	case *ast.BlockStmt:
+		// Treat blocks like basic blocks to avoid overlapping counters.
+		return true
+	case *ast.BranchStmt:
+		return true
+	case *ast.ForStmt:
+		return true
+	case *ast.IfStmt:
+		return true
+	case *ast.LabeledStmt:
+		return true // A goto may branch here, starting a new basic block.
+	case *ast.RangeStmt:
+		return true
+	case *ast.SwitchStmt:
+		return true
+	case *ast.SelectStmt:
+		return true
+	case *ast.TypeSwitchStmt:
+		return true
+	case *ast.ExprStmt:
+		// Calls to panic change the flow.
+		// We really should verify that "panic" is the predefined function,
+		// but without type checking we can't and the likelihood of it being
+		// an actual problem is vanishingly small.
+		if call, ok := s.X.(*ast.CallExpr); ok {
+			if ident, ok := call.Fun.(*ast.Ident); ok && ident.Name == "panic" && len(call.Args) == 1 {
+				return true
+			}
+		}
+	}
+	found, _ := hasFuncLiteral(s)
+	return found
+}
+
+// isControl reports whether s is a control statement that, if labeled, cannot be
+// separated from its label.
+func (f *File) isControl(s ast.Stmt) bool {
+	switch s.(type) {
+	case *ast.ForStmt, *ast.RangeStmt, *ast.SwitchStmt, *ast.SelectStmt, *ast.TypeSwitchStmt:
+		return true
+	}
+	return false
+}
+
+// funcLitFinder implements the ast.Visitor pattern to find the location of any
+// function literal in a subtree.
+type funcLitFinder token.Pos
+
+func (f *funcLitFinder) Visit(node ast.Node) (w ast.Visitor) {
+	if f.found() {
+		return nil // Prune search.
+	}
+	switch n := node.(type) {
+	case *ast.FuncLit:
+		*f = funcLitFinder(n.Body.Lbrace)
+		return nil // Prune search.
+	}
+	return f
+}
+
+func (f *funcLitFinder) found() bool {
+	return token.Pos(*f) != token.NoPos
+}
+
+// Sort interface for []block1; used for self-check in addVariables.
+
+type block1 struct {
+	Block
+	index int
+}
+
+type blockSlice []block1
+
+func (b blockSlice) Len() int           { return len(b) }
+func (b blockSlice) Less(i, j int) bool { return b[i].startByte < b[j].startByte }
+func (b blockSlice) Swap(i, j int)      { b[i], b[j] = b[j], b[i] }
+
+// offset translates a token position into a 0-indexed byte offset.
+func (f *File) offset(pos token.Pos) int {
+	return f.fset.Position(pos).Offset
+}
+
+// addVariables adds to the end of the file the declarations to set up the counter and position variables.
+func (f *File) addVariables(w io.Writer) {
+	// Self-check: Verify that the instrumented basic blocks are disjoint.
+	t := make([]block1, len(f.blocks))
+	for i := range f.blocks {
+		t[i].Block = f.blocks[i]
+		t[i].index = i
+	}
+	sort.Sort(blockSlice(t))
+	for i := 1; i < len(t); i++ {
+		if t[i-1].endByte > t[i].startByte {
+			fmt.Fprintf(os.Stderr, "cover: internal error: block %d overlaps block %d\n", t[i-1].index, t[i].index)
+			// Note: error message is in byte positions, not token positions.
+			fmt.Fprintf(os.Stderr, "\t%s:#%d,#%d %s:#%d,#%d\n",
+				f.name, f.offset(t[i-1].startByte), f.offset(t[i-1].endByte),
+				f.name, f.offset(t[i].startByte), f.offset(t[i].endByte))
+		}
+	}
+
+	// Declare the coverage struct as a package-level variable.
+	fmt.Fprintf(w, "\nvar %s = struct {\n", *varVar)
+	fmt.Fprintf(w, "\tCount     [%d]uint32\n", len(f.blocks))
+	fmt.Fprintf(w, "\tPos       [3 * %d]uint32\n", len(f.blocks))
+	fmt.Fprintf(w, "\tNumStmt   [%d]uint16\n", len(f.blocks))
+	fmt.Fprintf(w, "} {\n")
+
+	// Initialize the position array field.
+	fmt.Fprintf(w, "\tPos: [3 * %d]uint32{\n", len(f.blocks))
+
+	// A nice long list of positions. Each position is encoded as follows to reduce size:
+	// - 32-bit starting line number
+	// - 32-bit ending line number
+	// - (16 bit ending column number << 16) | (16-bit starting column number).
+	for i, block := range f.blocks {
+		start := f.fset.Position(block.startByte)
+		end := f.fset.Position(block.endByte)
+
+		start, end = dedup(start, end)
+
+		fmt.Fprintf(w, "\t\t%d, %d, %#x, // [%d]\n", start.Line, end.Line, (end.Column&0xFFFF)<<16|(start.Column&0xFFFF), i)
+	}
+
+	// Close the position array.
+	fmt.Fprintf(w, "\t},\n")
+
+	// Initialize the position array field.
+	fmt.Fprintf(w, "\tNumStmt: [%d]uint16{\n", len(f.blocks))
+
+	// A nice long list of statements-per-block, so we can give a conventional
+	// valuation of "percent covered". To save space, it's a 16-bit number, so we
+	// clamp it if it overflows - won't matter in practice.
+	for i, block := range f.blocks {
+		n := block.numStmt
+		if n > 1<<16-1 {
+			n = 1<<16 - 1
+		}
+		fmt.Fprintf(w, "\t\t%d, // %d\n", n, i)
+	}
+
+	// Close the statements-per-block array.
+	fmt.Fprintf(w, "\t},\n")
+
+	// Close the struct initialization.
+	fmt.Fprintf(w, "}\n")
+
+	// Emit a reference to the atomic package to avoid
+	// import and not used error when there's no code in a file.
+	if *mode == "atomic" {
+		fmt.Fprintf(w, "var _ = %s.LoadUint32\n", atomicPackageName)
+	}
+}
+
+// It is possible for positions to repeat when there is a line
+// directive that does not specify column information and the input
+// has not been passed through gofmt.
+// See issues #27530 and #30746.
+// Tests are TestHtmlUnformatted and TestLineDup.
+// We use a map to avoid duplicates.
+
+// pos2 is a pair of token.Position values, used as a map key type.
+type pos2 struct {
+	p1, p2 token.Position
+}
+
+// seenPos2 tracks whether we have seen a token.Position pair.
+var seenPos2 = make(map[pos2]bool)
+
+// dedup takes a token.Position pair and returns a pair that does not
+// duplicate any existing pair. The returned pair will have the Offset
+// fields cleared.
+func dedup(p1, p2 token.Position) (r1, r2 token.Position) {
+	key := pos2{
+		p1: p1,
+		p2: p2,
+	}
+
+	// We want to ignore the Offset fields in the map,
+	// since cover uses only file/line/column.
+	key.p1.Offset = 0
+	key.p2.Offset = 0
+
+	for seenPos2[key] {
+		key.p2.Column++
+	}
+	seenPos2[key] = true
+
+	return key.p1, key.p2
+}
diff --git a/src/cmd/cover/cover_test.go b/src/cmd/cover/cover_test.go
new file mode 100644
index 0000000..86c95d1
--- /dev/null
+++ b/src/cmd/cover/cover_test.go
@@ -0,0 +1,573 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"bufio"
+	"bytes"
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"sync"
+	"testing"
+)
+
+const (
+	// Data directory, also the package directory for the test.
+	testdata = "testdata"
+)
+
+var (
+	// Input files.
+	testMain       = filepath.Join(testdata, "main.go")
+	testTest       = filepath.Join(testdata, "test.go")
+	coverProfile   = filepath.Join(testdata, "profile.cov")
+	toolexecSource = filepath.Join(testdata, "toolexec.go")
+
+	// The HTML test files are in a separate directory
+	// so they are a complete package.
+	htmlGolden = filepath.Join(testdata, "html", "html.golden")
+
+	// Temporary files.
+	tmpTestMain    string
+	coverInput     string
+	coverOutput    string
+	htmlProfile    string
+	htmlHTML       string
+	htmlUDir       string
+	htmlU          string
+	htmlUTest      string
+	htmlUProfile   string
+	htmlUHTML      string
+	lineDupDir     string
+	lineDupGo      string
+	lineDupTestGo  string
+	lineDupProfile string
+)
+
+var (
+	// testTempDir is a temporary directory created in TestMain.
+	testTempDir string
+
+	// testcover is a newly built version of the cover program.
+	testcover string
+
+	// toolexec is a program to use as the go tool's -toolexec argument.
+	toolexec string
+
+	// testcoverErr records an error building testcover or toolexec.
+	testcoverErr error
+
+	// testcoverOnce is used to build testcover once.
+	testcoverOnce sync.Once
+
+	// toolexecArg is the argument to pass to the go tool.
+	toolexecArg string
+)
+
+var debug = flag.Bool("debug", false, "keep rewritten files for debugging")
+
+// We use TestMain to set up a temporary directory and remove it when
+// the tests are done.
+func TestMain(m *testing.M) {
+	dir, err := os.MkdirTemp("", "go-testcover")
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	os.Setenv("GOPATH", filepath.Join(dir, "_gopath"))
+
+	testTempDir = dir
+
+	tmpTestMain = filepath.Join(dir, "main.go")
+	coverInput = filepath.Join(dir, "test_line.go")
+	coverOutput = filepath.Join(dir, "test_cover.go")
+	htmlProfile = filepath.Join(dir, "html.cov")
+	htmlHTML = filepath.Join(dir, "html.html")
+	htmlUDir = filepath.Join(dir, "htmlunformatted")
+	htmlU = filepath.Join(htmlUDir, "htmlunformatted.go")
+	htmlUTest = filepath.Join(htmlUDir, "htmlunformatted_test.go")
+	htmlUProfile = filepath.Join(htmlUDir, "htmlunformatted.cov")
+	htmlUHTML = filepath.Join(htmlUDir, "htmlunformatted.html")
+	lineDupDir = filepath.Join(dir, "linedup")
+	lineDupGo = filepath.Join(lineDupDir, "linedup.go")
+	lineDupTestGo = filepath.Join(lineDupDir, "linedup_test.go")
+	lineDupProfile = filepath.Join(lineDupDir, "linedup.out")
+
+	status := m.Run()
+
+	if !*debug {
+		os.RemoveAll(dir)
+	}
+
+	os.Exit(status)
+}
+
+// buildCover builds a version of the cover program for testing.
+// This ensures that "go test cmd/cover" tests the current cmd/cover.
+func buildCover(t *testing.T) {
+	t.Helper()
+	testenv.MustHaveGoBuild(t)
+	testcoverOnce.Do(func() {
+		var wg sync.WaitGroup
+		wg.Add(2)
+
+		var err1, err2 error
+		go func() {
+			defer wg.Done()
+			testcover = filepath.Join(testTempDir, "cover.exe")
+			t.Logf("running [go build -o %s]", testcover)
+			out, err := exec.Command(testenv.GoToolPath(t), "build", "-o", testcover).CombinedOutput()
+			if len(out) > 0 {
+				t.Logf("%s", out)
+			}
+			err1 = err
+		}()
+
+		go func() {
+			defer wg.Done()
+			toolexec = filepath.Join(testTempDir, "toolexec.exe")
+			t.Logf("running [go -build -o %s %s]", toolexec, toolexecSource)
+			out, err := exec.Command(testenv.GoToolPath(t), "build", "-o", toolexec, toolexecSource).CombinedOutput()
+			if len(out) > 0 {
+				t.Logf("%s", out)
+			}
+			err2 = err
+		}()
+
+		wg.Wait()
+
+		testcoverErr = err1
+		if err2 != nil && err1 == nil {
+			testcoverErr = err2
+		}
+
+		toolexecArg = "-toolexec=" + toolexec + " " + testcover
+	})
+	if testcoverErr != nil {
+		t.Fatal("failed to build testcover or toolexec program:", testcoverErr)
+	}
+}
+
+// Run this shell script, but do it in Go so it can be run by "go test".
+//
+//	replace the word LINE with the line number < testdata/test.go > testdata/test_line.go
+// 	go build -o testcover
+// 	testcover -mode=count -var=CoverTest -o ./testdata/test_cover.go testdata/test_line.go
+//	go run ./testdata/main.go ./testdata/test.go
+//
+func TestCover(t *testing.T) {
+	t.Parallel()
+	testenv.MustHaveGoRun(t)
+	buildCover(t)
+
+	// Read in the test file (testTest) and write it, with LINEs specified, to coverInput.
+	file, err := os.ReadFile(testTest)
+	if err != nil {
+		t.Fatal(err)
+	}
+	lines := bytes.Split(file, []byte("\n"))
+	for i, line := range lines {
+		lines[i] = bytes.ReplaceAll(line, []byte("LINE"), []byte(fmt.Sprint(i+1)))
+	}
+
+	// Add a function that is not gofmt'ed. This used to cause a crash.
+	// We don't put it in test.go because then we would have to gofmt it.
+	// Issue 23927.
+	lines = append(lines, []byte("func unFormatted() {"),
+		[]byte("\tif true {"),
+		[]byte("\t}else{"),
+		[]byte("\t}"),
+		[]byte("}"))
+	lines = append(lines, []byte("func unFormatted2(b bool) {if b{}else{}}"))
+
+	if err := os.WriteFile(coverInput, bytes.Join(lines, []byte("\n")), 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	// testcover -mode=count -var=thisNameMustBeVeryLongToCauseOverflowOfCounterIncrementStatementOntoNextLineForTest -o ./testdata/test_cover.go testdata/test_line.go
+	cmd := exec.Command(testcover, "-mode=count", "-var=thisNameMustBeVeryLongToCauseOverflowOfCounterIncrementStatementOntoNextLineForTest", "-o", coverOutput, coverInput)
+	run(cmd, t)
+
+	cmd = exec.Command(testcover, "-mode=set", "-var=Not_an-identifier", "-o", coverOutput, coverInput)
+	err = cmd.Run()
+	if err == nil {
+		t.Error("Expected cover to fail with an error")
+	}
+
+	// Copy testmain to testTempDir, so that it is in the same directory
+	// as coverOutput.
+	b, err := os.ReadFile(testMain)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(tmpTestMain, b, 0444); err != nil {
+		t.Fatal(err)
+	}
+
+	// go run ./testdata/main.go ./testdata/test.go
+	cmd = exec.Command(testenv.GoToolPath(t), "run", tmpTestMain, coverOutput)
+	run(cmd, t)
+
+	file, err = os.ReadFile(coverOutput)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// compiler directive must appear right next to function declaration.
+	if got, err := regexp.MatchString(".*\n//go:nosplit\nfunc someFunction().*", string(file)); err != nil || !got {
+		t.Error("misplaced compiler directive")
+	}
+	// "go:linkname" compiler directive should be present.
+	if got, err := regexp.MatchString(`.*go\:linkname some\_name some\_name.*`, string(file)); err != nil || !got {
+		t.Error("'go:linkname' compiler directive not found")
+	}
+
+	// Other comments should be preserved too.
+	c := ".*// This comment didn't appear in generated go code.*"
+	if got, err := regexp.MatchString(c, string(file)); err != nil || !got {
+		t.Errorf("non compiler directive comment %q not found", c)
+	}
+}
+
+// TestDirectives checks that compiler directives are preserved and positioned
+// correctly. Directives that occur before top-level declarations should remain
+// above those declarations, even if they are not part of the block of
+// documentation comments.
+func TestDirectives(t *testing.T) {
+	t.Parallel()
+	buildCover(t)
+
+	// Read the source file and find all the directives. We'll keep
+	// track of whether each one has been seen in the output.
+	testDirectives := filepath.Join(testdata, "directives.go")
+	source, err := os.ReadFile(testDirectives)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sourceDirectives := findDirectives(source)
+
+	// testcover -mode=atomic ./testdata/directives.go
+	cmd := exec.Command(testcover, "-mode=atomic", testDirectives)
+	cmd.Stderr = os.Stderr
+	output, err := cmd.Output()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Check that all directives are present in the output.
+	outputDirectives := findDirectives(output)
+	foundDirective := make(map[string]bool)
+	for _, p := range sourceDirectives {
+		foundDirective[p.name] = false
+	}
+	for _, p := range outputDirectives {
+		if found, ok := foundDirective[p.name]; !ok {
+			t.Errorf("unexpected directive in output: %s", p.text)
+		} else if found {
+			t.Errorf("directive found multiple times in output: %s", p.text)
+		}
+		foundDirective[p.name] = true
+	}
+	for name, found := range foundDirective {
+		if !found {
+			t.Errorf("missing directive: %s", name)
+		}
+	}
+
+	// Check that directives that start with the name of top-level declarations
+	// come before the beginning of the named declaration and after the end
+	// of the previous declaration.
+	fset := token.NewFileSet()
+	astFile, err := parser.ParseFile(fset, testDirectives, output, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	prevEnd := 0
+	for _, decl := range astFile.Decls {
+		var name string
+		switch d := decl.(type) {
+		case *ast.FuncDecl:
+			name = d.Name.Name
+		case *ast.GenDecl:
+			if len(d.Specs) == 0 {
+				// An empty group declaration. We still want to check that
+				// directives can be associated with it, so we make up a name
+				// to match directives in the test data.
+				name = "_empty"
+			} else if spec, ok := d.Specs[0].(*ast.TypeSpec); ok {
+				name = spec.Name.Name
+			}
+		}
+		pos := fset.Position(decl.Pos()).Offset
+		end := fset.Position(decl.End()).Offset
+		if name == "" {
+			prevEnd = end
+			continue
+		}
+		for _, p := range outputDirectives {
+			if !strings.HasPrefix(p.name, name) {
+				continue
+			}
+			if p.offset < prevEnd || pos < p.offset {
+				t.Errorf("directive %s does not appear before definition %s", p.text, name)
+			}
+		}
+		prevEnd = end
+	}
+}
+
+type directiveInfo struct {
+	text   string // full text of the comment, not including newline
+	name   string // text after //go:
+	offset int    // byte offset of first slash in comment
+}
+
+func findDirectives(source []byte) []directiveInfo {
+	var directives []directiveInfo
+	directivePrefix := []byte("\n//go:")
+	offset := 0
+	for {
+		i := bytes.Index(source[offset:], directivePrefix)
+		if i < 0 {
+			break
+		}
+		i++ // skip newline
+		p := source[offset+i:]
+		j := bytes.IndexByte(p, '\n')
+		if j < 0 {
+			// reached EOF
+			j = len(p)
+		}
+		directive := directiveInfo{
+			text:   string(p[:j]),
+			name:   string(p[len(directivePrefix)-1 : j]),
+			offset: offset + i,
+		}
+		directives = append(directives, directive)
+		offset += i + j
+	}
+	return directives
+}
+
+// Makes sure that `cover -func=profile.cov` reports accurate coverage.
+// Issue #20515.
+func TestCoverFunc(t *testing.T) {
+	t.Parallel()
+	buildCover(t)
+	// testcover -func ./testdata/profile.cov
+	cmd := exec.Command(testcover, "-func", coverProfile)
+	out, err := cmd.Output()
+	if err != nil {
+		if ee, ok := err.(*exec.ExitError); ok {
+			t.Logf("%s", ee.Stderr)
+		}
+		t.Fatal(err)
+	}
+
+	if got, err := regexp.Match(".*total:.*100.0.*", out); err != nil || !got {
+		t.Logf("%s", out)
+		t.Errorf("invalid coverage counts. got=(%v, %v); want=(true; nil)", got, err)
+	}
+}
+
+// Check that cover produces correct HTML.
+// Issue #25767.
+func TestCoverHTML(t *testing.T) {
+	t.Parallel()
+	testenv.MustHaveGoRun(t)
+	buildCover(t)
+
+	// go test -coverprofile testdata/html/html.cov cmd/cover/testdata/html
+	cmd := exec.Command(testenv.GoToolPath(t), "test", toolexecArg, "-coverprofile", htmlProfile, "cmd/cover/testdata/html")
+	run(cmd, t)
+	// testcover -html testdata/html/html.cov -o testdata/html/html.html
+	cmd = exec.Command(testcover, "-html", htmlProfile, "-o", htmlHTML)
+	run(cmd, t)
+
+	// Extract the parts of the HTML with comment markers,
+	// and compare against a golden file.
+	entireHTML, err := os.ReadFile(htmlHTML)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var out bytes.Buffer
+	scan := bufio.NewScanner(bytes.NewReader(entireHTML))
+	in := false
+	for scan.Scan() {
+		line := scan.Text()
+		if strings.Contains(line, "// START") {
+			in = true
+		}
+		if in {
+			fmt.Fprintln(&out, line)
+		}
+		if strings.Contains(line, "// END") {
+			in = false
+		}
+	}
+	if scan.Err() != nil {
+		t.Error(scan.Err())
+	}
+	golden, err := os.ReadFile(htmlGolden)
+	if err != nil {
+		t.Fatalf("reading golden file: %v", err)
+	}
+	// Ignore white space differences.
+	// Break into lines, then compare by breaking into words.
+	goldenLines := strings.Split(string(golden), "\n")
+	outLines := strings.Split(out.String(), "\n")
+	// Compare at the line level, stopping at first different line so
+	// we don't generate tons of output if there's an inserted or deleted line.
+	for i, goldenLine := range goldenLines {
+		if i >= len(outLines) {
+			t.Fatalf("output shorter than golden; stops before line %d: %s\n", i+1, goldenLine)
+		}
+		// Convert all white space to simple spaces, for easy comparison.
+		goldenLine = strings.Join(strings.Fields(goldenLine), " ")
+		outLine := strings.Join(strings.Fields(outLines[i]), " ")
+		if outLine != goldenLine {
+			t.Fatalf("line %d differs: got:\n\t%s\nwant:\n\t%s", i+1, outLine, goldenLine)
+		}
+	}
+	if len(goldenLines) != len(outLines) {
+		t.Fatalf("output longer than golden; first extra output line %d: %q\n", len(goldenLines)+1, outLines[len(goldenLines)])
+	}
+}
+
+// Test HTML processing with a source file not run through gofmt.
+// Issue #27350.
+func TestHtmlUnformatted(t *testing.T) {
+	t.Parallel()
+	testenv.MustHaveGoRun(t)
+	buildCover(t)
+
+	if err := os.Mkdir(htmlUDir, 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := os.WriteFile(filepath.Join(htmlUDir, "go.mod"), []byte("module htmlunformatted\n"), 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	const htmlUContents = `
+package htmlunformatted
+
+var g int
+
+func F() {
+//line x.go:1
+	{ { F(); goto lab } }
+lab:
+}`
+
+	const htmlUTestContents = `package htmlunformatted`
+
+	if err := os.WriteFile(htmlU, []byte(htmlUContents), 0444); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(htmlUTest, []byte(htmlUTestContents), 0444); err != nil {
+		t.Fatal(err)
+	}
+
+	// go test -covermode=count -coverprofile TMPDIR/htmlunformatted.cov
+	cmd := exec.Command(testenv.GoToolPath(t), "test", toolexecArg, "-covermode=count", "-coverprofile", htmlUProfile)
+	cmd.Dir = htmlUDir
+	run(cmd, t)
+
+	// testcover -html TMPDIR/htmlunformatted.cov -o unformatted.html
+	cmd = exec.Command(testcover, "-html", htmlUProfile, "-o", htmlUHTML)
+	cmd.Dir = htmlUDir
+	run(cmd, t)
+}
+
+// lineDupContents becomes linedup.go in TestFuncWithDuplicateLines.
+const lineDupContents = `
+package linedup
+
+var G int
+
+func LineDup(c int) {
+	for i := 0; i < c; i++ {
+//line ld.go:100
+		if i % 2 == 0 {
+			G++
+		}
+		if i % 3 == 0 {
+			G++; G++
+		}
+//line ld.go:100
+		if i % 4 == 0 {
+			G++; G++; G++
+		}
+		if i % 5 == 0 {
+			G++; G++; G++; G++
+		}
+	}
+}
+`
+
+// lineDupTestContents becomes linedup_test.go in TestFuncWithDuplicateLines.
+const lineDupTestContents = `
+package linedup
+
+import "testing"
+
+func TestLineDup(t *testing.T) {
+	LineDup(100)
+}
+`
+
+// Test -func with duplicate //line directives with different numbers
+// of statements.
+func TestFuncWithDuplicateLines(t *testing.T) {
+	t.Parallel()
+	testenv.MustHaveGoRun(t)
+	buildCover(t)
+
+	if err := os.Mkdir(lineDupDir, 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := os.WriteFile(filepath.Join(lineDupDir, "go.mod"), []byte("module linedup\n"), 0666); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(lineDupGo, []byte(lineDupContents), 0444); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(lineDupTestGo, []byte(lineDupTestContents), 0444); err != nil {
+		t.Fatal(err)
+	}
+
+	// go test -cover -covermode count -coverprofile TMPDIR/linedup.out
+	cmd := exec.Command(testenv.GoToolPath(t), "test", toolexecArg, "-cover", "-covermode", "count", "-coverprofile", lineDupProfile)
+	cmd.Dir = lineDupDir
+	run(cmd, t)
+
+	// testcover -func=TMPDIR/linedup.out
+	cmd = exec.Command(testcover, "-func", lineDupProfile)
+	cmd.Dir = lineDupDir
+	run(cmd, t)
+}
+
+func run(c *exec.Cmd, t *testing.T) {
+	t.Helper()
+	t.Log("running", c.Args)
+	out, err := c.CombinedOutput()
+	if len(out) > 0 {
+		t.Logf("%s", out)
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/src/cmd/cover/doc.go b/src/cmd/cover/doc.go
new file mode 100644
index 0000000..e2c8494
--- /dev/null
+++ b/src/cmd/cover/doc.go
@@ -0,0 +1,25 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Cover is a program for analyzing the coverage profiles generated by
+'go test -coverprofile=cover.out'.
+
+Cover is also used by 'go test -cover' to rewrite the source code with
+annotations to track which parts of each function are executed.
+It operates on one Go source file at a time, computing approximate
+basic block information by studying the source. It is thus more portable
+than binary-rewriting coverage tools, but also a little less capable.
+For instance, it does not probe inside && and || expressions, and can
+be mildly confused by single statements with multiple function literals.
+
+When computing coverage of a package that uses cgo, the cover tool
+must be applied to the output of cgo preprocessing, not the input,
+because cover deletes comments that are significant to cgo.
+
+For usage information, please see:
+	go help testflag
+	go tool cover -help
+*/
+package main
diff --git a/src/cmd/cover/func.go b/src/cmd/cover/func.go
new file mode 100644
index 0000000..ce7c771
--- /dev/null
+++ b/src/cmd/cover/func.go
@@ -0,0 +1,246 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements the visitor that computes the (line, column)-(line-column) range for each function.
+
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	exec "internal/execabs"
+	"io"
+	"os"
+	"path"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"text/tabwriter"
+)
+
+// funcOutput takes two file names as arguments, a coverage profile to read as input and an output
+// file to write ("" means to write to standard output). The function reads the profile and produces
+// as output the coverage data broken down by function, like this:
+//
+//	fmt/format.go:30:	init			100.0%
+//	fmt/format.go:57:	clearflags		100.0%
+//	...
+//	fmt/scan.go:1046:	doScan			100.0%
+//	fmt/scan.go:1075:	advance			96.2%
+//	fmt/scan.go:1119:	doScanf			96.8%
+//	total:		(statements)			91.9%
+
+func funcOutput(profile, outputFile string) error {
+	profiles, err := ParseProfiles(profile)
+	if err != nil {
+		return err
+	}
+
+	dirs, err := findPkgs(profiles)
+	if err != nil {
+		return err
+	}
+
+	var out *bufio.Writer
+	if outputFile == "" {
+		out = bufio.NewWriter(os.Stdout)
+	} else {
+		fd, err := os.Create(outputFile)
+		if err != nil {
+			return err
+		}
+		defer fd.Close()
+		out = bufio.NewWriter(fd)
+	}
+	defer out.Flush()
+
+	tabber := tabwriter.NewWriter(out, 1, 8, 1, '\t', 0)
+	defer tabber.Flush()
+
+	var total, covered int64
+	for _, profile := range profiles {
+		fn := profile.FileName
+		file, err := findFile(dirs, fn)
+		if err != nil {
+			return err
+		}
+		funcs, err := findFuncs(file)
+		if err != nil {
+			return err
+		}
+		// Now match up functions and profile blocks.
+		for _, f := range funcs {
+			c, t := f.coverage(profile)
+			fmt.Fprintf(tabber, "%s:%d:\t%s\t%.1f%%\n", fn, f.startLine, f.name, percent(c, t))
+			total += t
+			covered += c
+		}
+	}
+	fmt.Fprintf(tabber, "total:\t(statements)\t%.1f%%\n", percent(covered, total))
+
+	return nil
+}
+
+// findFuncs parses the file and returns a slice of FuncExtent descriptors.
+func findFuncs(name string) ([]*FuncExtent, error) {
+	fset := token.NewFileSet()
+	parsedFile, err := parser.ParseFile(fset, name, nil, 0)
+	if err != nil {
+		return nil, err
+	}
+	visitor := &FuncVisitor{
+		fset:    fset,
+		name:    name,
+		astFile: parsedFile,
+	}
+	ast.Walk(visitor, visitor.astFile)
+	return visitor.funcs, nil
+}
+
+// FuncExtent describes a function's extent in the source by file and position.
+type FuncExtent struct {
+	name      string
+	startLine int
+	startCol  int
+	endLine   int
+	endCol    int
+}
+
+// FuncVisitor implements the visitor that builds the function position list for a file.
+type FuncVisitor struct {
+	fset    *token.FileSet
+	name    string // Name of file.
+	astFile *ast.File
+	funcs   []*FuncExtent
+}
+
+// Visit implements the ast.Visitor interface.
+func (v *FuncVisitor) Visit(node ast.Node) ast.Visitor {
+	switch n := node.(type) {
+	case *ast.FuncDecl:
+		if n.Body == nil {
+			// Do not count declarations of assembly functions.
+			break
+		}
+		start := v.fset.Position(n.Pos())
+		end := v.fset.Position(n.End())
+		fe := &FuncExtent{
+			name:      n.Name.Name,
+			startLine: start.Line,
+			startCol:  start.Column,
+			endLine:   end.Line,
+			endCol:    end.Column,
+		}
+		v.funcs = append(v.funcs, fe)
+	}
+	return v
+}
+
+// coverage returns the fraction of the statements in the function that were covered, as a numerator and denominator.
+func (f *FuncExtent) coverage(profile *Profile) (num, den int64) {
+	// We could avoid making this n^2 overall by doing a single scan and annotating the functions,
+	// but the sizes of the data structures is never very large and the scan is almost instantaneous.
+	var covered, total int64
+	// The blocks are sorted, so we can stop counting as soon as we reach the end of the relevant block.
+	for _, b := range profile.Blocks {
+		if b.StartLine > f.endLine || (b.StartLine == f.endLine && b.StartCol >= f.endCol) {
+			// Past the end of the function.
+			break
+		}
+		if b.EndLine < f.startLine || (b.EndLine == f.startLine && b.EndCol <= f.startCol) {
+			// Before the beginning of the function
+			continue
+		}
+		total += int64(b.NumStmt)
+		if b.Count > 0 {
+			covered += int64(b.NumStmt)
+		}
+	}
+	return covered, total
+}
+
+// Pkg describes a single package, compatible with the JSON output from 'go list'; see 'go help list'.
+type Pkg struct {
+	ImportPath string
+	Dir        string
+	Error      *struct {
+		Err string
+	}
+}
+
+func findPkgs(profiles []*Profile) (map[string]*Pkg, error) {
+	// Run go list to find the location of every package we care about.
+	pkgs := make(map[string]*Pkg)
+	var list []string
+	for _, profile := range profiles {
+		if strings.HasPrefix(profile.FileName, ".") || filepath.IsAbs(profile.FileName) {
+			// Relative or absolute path.
+			continue
+		}
+		pkg := path.Dir(profile.FileName)
+		if _, ok := pkgs[pkg]; !ok {
+			pkgs[pkg] = nil
+			list = append(list, pkg)
+		}
+	}
+
+	if len(list) == 0 {
+		return pkgs, nil
+	}
+
+	// Note: usually run as "go tool cover" in which case $GOROOT is set,
+	// in which case runtime.GOROOT() does exactly what we want.
+	goTool := filepath.Join(runtime.GOROOT(), "bin/go")
+	cmd := exec.Command(goTool, append([]string{"list", "-e", "-json"}, list...)...)
+	var stderr bytes.Buffer
+	cmd.Stderr = &stderr
+	stdout, err := cmd.Output()
+	if err != nil {
+		return nil, fmt.Errorf("cannot run go list: %v\n%s", err, stderr.Bytes())
+	}
+	dec := json.NewDecoder(bytes.NewReader(stdout))
+	for {
+		var pkg Pkg
+		err := dec.Decode(&pkg)
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			return nil, fmt.Errorf("decoding go list json: %v", err)
+		}
+		pkgs[pkg.ImportPath] = &pkg
+	}
+	return pkgs, nil
+}
+
+// findFile finds the location of the named file in GOROOT, GOPATH etc.
+func findFile(pkgs map[string]*Pkg, file string) (string, error) {
+	if strings.HasPrefix(file, ".") || filepath.IsAbs(file) {
+		// Relative or absolute path.
+		return file, nil
+	}
+	pkg := pkgs[path.Dir(file)]
+	if pkg != nil {
+		if pkg.Dir != "" {
+			return filepath.Join(pkg.Dir, path.Base(file)), nil
+		}
+		if pkg.Error != nil {
+			return "", errors.New(pkg.Error.Err)
+		}
+	}
+	return "", fmt.Errorf("did not find package for %s in go list output", file)
+}
+
+func percent(covered, total int64) float64 {
+	if total == 0 {
+		total = 1 // Avoid zero denominator.
+	}
+	return 100.0 * float64(covered) / float64(total)
+}
diff --git a/src/cmd/cover/html.go b/src/cmd/cover/html.go
new file mode 100644
index 0000000..b2865c4
--- /dev/null
+++ b/src/cmd/cover/html.go
@@ -0,0 +1,305 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"cmd/internal/browser"
+	"fmt"
+	"html/template"
+	"io"
+	"math"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// htmlOutput reads the profile data from profile and generates an HTML
+// coverage report, writing it to outfile. If outfile is empty,
+// it writes the report to a temporary file and opens it in a web browser.
+func htmlOutput(profile, outfile string) error {
+	profiles, err := ParseProfiles(profile)
+	if err != nil {
+		return err
+	}
+
+	var d templateData
+
+	dirs, err := findPkgs(profiles)
+	if err != nil {
+		return err
+	}
+
+	for _, profile := range profiles {
+		fn := profile.FileName
+		if profile.Mode == "set" {
+			d.Set = true
+		}
+		file, err := findFile(dirs, fn)
+		if err != nil {
+			return err
+		}
+		src, err := os.ReadFile(file)
+		if err != nil {
+			return fmt.Errorf("can't read %q: %v", fn, err)
+		}
+		var buf strings.Builder
+		err = htmlGen(&buf, src, profile.Boundaries(src))
+		if err != nil {
+			return err
+		}
+		d.Files = append(d.Files, &templateFile{
+			Name:     fn,
+			Body:     template.HTML(buf.String()),
+			Coverage: percentCovered(profile),
+		})
+	}
+
+	var out *os.File
+	if outfile == "" {
+		var dir string
+		dir, err = os.MkdirTemp("", "cover")
+		if err != nil {
+			return err
+		}
+		out, err = os.Create(filepath.Join(dir, "coverage.html"))
+	} else {
+		out, err = os.Create(outfile)
+	}
+	if err != nil {
+		return err
+	}
+	err = htmlTemplate.Execute(out, d)
+	if err2 := out.Close(); err == nil {
+		err = err2
+	}
+	if err != nil {
+		return err
+	}
+
+	if outfile == "" {
+		if !browser.Open("file://" + out.Name()) {
+			fmt.Fprintf(os.Stderr, "HTML output written to %s\n", out.Name())
+		}
+	}
+
+	return nil
+}
+
+// percentCovered returns, as a percentage, the fraction of the statements in
+// the profile covered by the test run.
+// In effect, it reports the coverage of a given source file.
+func percentCovered(p *Profile) float64 {
+	var total, covered int64
+	for _, b := range p.Blocks {
+		total += int64(b.NumStmt)
+		if b.Count > 0 {
+			covered += int64(b.NumStmt)
+		}
+	}
+	if total == 0 {
+		return 0
+	}
+	return float64(covered) / float64(total) * 100
+}
+
+// htmlGen generates an HTML coverage report with the provided filename,
+// source code, and tokens, and writes it to the given Writer.
+func htmlGen(w io.Writer, src []byte, boundaries []Boundary) error {
+	dst := bufio.NewWriter(w)
+	for i := range src {
+		for len(boundaries) > 0 && boundaries[0].Offset == i {
+			b := boundaries[0]
+			if b.Start {
+				n := 0
+				if b.Count > 0 {
+					n = int(math.Floor(b.Norm*9)) + 1
+				}
+				fmt.Fprintf(dst, `<span class="cov%v" title="%v">`, n, b.Count)
+			} else {
+				dst.WriteString("</span>")
+			}
+			boundaries = boundaries[1:]
+		}
+		switch b := src[i]; b {
+		case '>':
+			dst.WriteString("&gt;")
+		case '<':
+			dst.WriteString("&lt;")
+		case '&':
+			dst.WriteString("&amp;")
+		case '\t':
+			dst.WriteString("        ")
+		default:
+			dst.WriteByte(b)
+		}
+	}
+	return dst.Flush()
+}
+
+// rgb returns an rgb value for the specified coverage value
+// between 0 (no coverage) and 10 (max coverage).
+func rgb(n int) string {
+	if n == 0 {
+		return "rgb(192, 0, 0)" // Red
+	}
+	// Gradient from gray to green.
+	r := 128 - 12*(n-1)
+	g := 128 + 12*(n-1)
+	b := 128 + 3*(n-1)
+	return fmt.Sprintf("rgb(%v, %v, %v)", r, g, b)
+}
+
+// colors generates the CSS rules for coverage colors.
+func colors() template.CSS {
+	var buf bytes.Buffer
+	for i := 0; i < 11; i++ {
+		fmt.Fprintf(&buf, ".cov%v { color: %v }\n", i, rgb(i))
+	}
+	return template.CSS(buf.String())
+}
+
+var htmlTemplate = template.Must(template.New("html").Funcs(template.FuncMap{
+	"colors": colors,
+}).Parse(tmplHTML))
+
+type templateData struct {
+	Files []*templateFile
+	Set   bool
+}
+
+// PackageName returns a name for the package being shown.
+// It does this by choosing the penultimate element of the path
+// name, so foo.bar/baz/foo.go chooses 'baz'. This is cheap
+// and easy, avoids parsing the Go file, and gets a better answer
+// for package main. It returns the empty string if there is
+// a problem.
+func (td templateData) PackageName() string {
+	if len(td.Files) == 0 {
+		return ""
+	}
+	fileName := td.Files[0].Name
+	elems := strings.Split(fileName, "/") // Package path is always slash-separated.
+	// Return the penultimate non-empty element.
+	for i := len(elems) - 2; i >= 0; i-- {
+		if elems[i] != "" {
+			return elems[i]
+		}
+	}
+	return ""
+}
+
+type templateFile struct {
+	Name     string
+	Body     template.HTML
+	Coverage float64
+}
+
+const tmplHTML = `
+<!DOCTYPE html>
+<html>
+	<head>
+		<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+		<title>{{$pkg := .PackageName}}{{if $pkg}}{{$pkg}}: {{end}}Go Coverage Report</title>
+		<style>
+			body {
+				background: black;
+				color: rgb(80, 80, 80);
+			}
+			body, pre, #legend span {
+				font-family: Menlo, monospace;
+				font-weight: bold;
+			}
+			#topbar {
+				background: black;
+				position: fixed;
+				top: 0; left: 0; right: 0;
+				height: 42px;
+				border-bottom: 1px solid rgb(80, 80, 80);
+			}
+			#content {
+				margin-top: 50px;
+			}
+			#nav, #legend {
+				float: left;
+				margin-left: 10px;
+			}
+			#legend {
+				margin-top: 12px;
+			}
+			#nav {
+				margin-top: 10px;
+			}
+			#legend span {
+				margin: 0 5px;
+			}
+			{{colors}}
+		</style>
+	</head>
+	<body>
+		<div id="topbar">
+			<div id="nav">
+				<select id="files">
+				{{range $i, $f := .Files}}
+				<option value="file{{$i}}">{{$f.Name}} ({{printf "%.1f" $f.Coverage}}%)</option>
+				{{end}}
+				</select>
+			</div>
+			<div id="legend">
+				<span>not tracked</span>
+			{{if .Set}}
+				<span class="cov0">not covered</span>
+				<span class="cov8">covered</span>
+			{{else}}
+				<span class="cov0">no coverage</span>
+				<span class="cov1">low coverage</span>
+				<span class="cov2">*</span>
+				<span class="cov3">*</span>
+				<span class="cov4">*</span>
+				<span class="cov5">*</span>
+				<span class="cov6">*</span>
+				<span class="cov7">*</span>
+				<span class="cov8">*</span>
+				<span class="cov9">*</span>
+				<span class="cov10">high coverage</span>
+			{{end}}
+			</div>
+		</div>
+		<div id="content">
+		{{range $i, $f := .Files}}
+		<pre class="file" id="file{{$i}}" style="display: none">{{$f.Body}}</pre>
+		{{end}}
+		</div>
+	</body>
+	<script>
+	(function() {
+		var files = document.getElementById('files');
+		var visible;
+		files.addEventListener('change', onChange, false);
+		function select(part) {
+			if (visible)
+				visible.style.display = 'none';
+			visible = document.getElementById(part);
+			if (!visible)
+				return;
+			files.value = part;
+			visible.style.display = 'block';
+			location.hash = part;
+		}
+		function onChange() {
+			select(files.value);
+			window.scrollTo(0, 0);
+		}
+		if (location.hash != "") {
+			select(location.hash.substr(1));
+		}
+		if (!visible) {
+			select("file0");
+		}
+	})();
+	</script>
+</html>
+`
diff --git a/src/cmd/cover/pkgname_test.go b/src/cmd/cover/pkgname_test.go
new file mode 100644
index 0000000..1c731ad
--- /dev/null
+++ b/src/cmd/cover/pkgname_test.go
@@ -0,0 +1,31 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "testing"
+
+func TestPackageName(t *testing.T) {
+	var tests = []struct {
+		fileName, pkgName string
+	}{
+		{"", ""},
+		{"///", ""},
+		{"fmt", ""}, // No Go file, improper form.
+		{"fmt/foo.go", "fmt"},
+		{"encoding/binary/foo.go", "binary"},
+		{"encoding/binary/////foo.go", "binary"},
+	}
+	var tf templateFile
+	for _, test := range tests {
+		tf.Name = test.fileName
+		td := templateData{
+			Files: []*templateFile{&tf},
+		}
+		got := td.PackageName()
+		if got != test.pkgName {
+			t.Errorf("%s: got %s want %s", test.fileName, got, test.pkgName)
+		}
+	}
+}
diff --git a/src/cmd/cover/profile.go b/src/cmd/cover/profile.go
new file mode 100644
index 0000000..656c862
--- /dev/null
+++ b/src/cmd/cover/profile.go
@@ -0,0 +1,220 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file provides support for parsing coverage profiles
+// generated by "go test -coverprofile=cover.out".
+// It is a copy of golang.org/x/tools/cover/profile.go.
+
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"math"
+	"os"
+	"regexp"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// Profile represents the profiling data for a specific file.
+type Profile struct {
+	FileName string
+	Mode     string
+	Blocks   []ProfileBlock
+}
+
+// ProfileBlock represents a single block of profiling data.
+type ProfileBlock struct {
+	StartLine, StartCol int
+	EndLine, EndCol     int
+	NumStmt, Count      int
+}
+
+type byFileName []*Profile
+
+func (p byFileName) Len() int           { return len(p) }
+func (p byFileName) Less(i, j int) bool { return p[i].FileName < p[j].FileName }
+func (p byFileName) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
+
+// ParseProfiles parses profile data in the specified file and returns a
+// Profile for each source file described therein.
+func ParseProfiles(fileName string) ([]*Profile, error) {
+	pf, err := os.Open(fileName)
+	if err != nil {
+		return nil, err
+	}
+	defer pf.Close()
+
+	files := make(map[string]*Profile)
+	buf := bufio.NewReader(pf)
+	// First line is "mode: foo", where foo is "set", "count", or "atomic".
+	// Rest of file is in the format
+	//	encoding/base64/base64.go:34.44,37.40 3 1
+	// where the fields are: name.go:line.column,line.column numberOfStatements count
+	s := bufio.NewScanner(buf)
+	mode := ""
+	for s.Scan() {
+		line := s.Text()
+		if mode == "" {
+			const p = "mode: "
+			if !strings.HasPrefix(line, p) || line == p {
+				return nil, fmt.Errorf("bad mode line: %v", line)
+			}
+			mode = line[len(p):]
+			continue
+		}
+		m := lineRe.FindStringSubmatch(line)
+		if m == nil {
+			return nil, fmt.Errorf("line %q doesn't match expected format: %v", m, lineRe)
+		}
+		fn := m[1]
+		p := files[fn]
+		if p == nil {
+			p = &Profile{
+				FileName: fn,
+				Mode:     mode,
+			}
+			files[fn] = p
+		}
+		p.Blocks = append(p.Blocks, ProfileBlock{
+			StartLine: toInt(m[2]),
+			StartCol:  toInt(m[3]),
+			EndLine:   toInt(m[4]),
+			EndCol:    toInt(m[5]),
+			NumStmt:   toInt(m[6]),
+			Count:     toInt(m[7]),
+		})
+	}
+	if err := s.Err(); err != nil {
+		return nil, err
+	}
+	for _, p := range files {
+		sort.Sort(blocksByStart(p.Blocks))
+		// Merge samples from the same location.
+		j := 1
+		for i := 1; i < len(p.Blocks); i++ {
+			b := p.Blocks[i]
+			last := p.Blocks[j-1]
+			if b.StartLine == last.StartLine &&
+				b.StartCol == last.StartCol &&
+				b.EndLine == last.EndLine &&
+				b.EndCol == last.EndCol {
+				if b.NumStmt != last.NumStmt {
+					return nil, fmt.Errorf("inconsistent NumStmt: changed from %d to %d", last.NumStmt, b.NumStmt)
+				}
+				if mode == "set" {
+					p.Blocks[j-1].Count |= b.Count
+				} else {
+					p.Blocks[j-1].Count += b.Count
+				}
+				continue
+			}
+			p.Blocks[j] = b
+			j++
+		}
+		p.Blocks = p.Blocks[:j]
+	}
+	// Generate a sorted slice.
+	profiles := make([]*Profile, 0, len(files))
+	for _, profile := range files {
+		profiles = append(profiles, profile)
+	}
+	sort.Sort(byFileName(profiles))
+	return profiles, nil
+}
+
+type blocksByStart []ProfileBlock
+
+func (b blocksByStart) Len() int      { return len(b) }
+func (b blocksByStart) Swap(i, j int) { b[i], b[j] = b[j], b[i] }
+func (b blocksByStart) Less(i, j int) bool {
+	bi, bj := b[i], b[j]
+	return bi.StartLine < bj.StartLine || bi.StartLine == bj.StartLine && bi.StartCol < bj.StartCol
+}
+
+var lineRe = regexp.MustCompile(`^(.+):([0-9]+).([0-9]+),([0-9]+).([0-9]+) ([0-9]+) ([0-9]+)$`)
+
+func toInt(s string) int {
+	i, err := strconv.Atoi(s)
+	if err != nil {
+		panic(err)
+	}
+	return i
+}
+
+// Boundary represents the position in a source file of the beginning or end of a
+// block as reported by the coverage profile. In HTML mode, it will correspond to
+// the opening or closing of a <span> tag and will be used to colorize the source
+type Boundary struct {
+	Offset int     // Location as a byte offset in the source file.
+	Start  bool    // Is this the start of a block?
+	Count  int     // Event count from the cover profile.
+	Norm   float64 // Count normalized to [0..1].
+	Index  int     // Order in input file.
+}
+
+// Boundaries returns a Profile as a set of Boundary objects within the provided src.
+func (p *Profile) Boundaries(src []byte) (boundaries []Boundary) {
+	// Find maximum count.
+	max := 0
+	for _, b := range p.Blocks {
+		if b.Count > max {
+			max = b.Count
+		}
+	}
+	// Divisor for normalization.
+	divisor := math.Log(float64(max))
+
+	// boundary returns a Boundary, populating the Norm field with a normalized Count.
+	index := 0
+	boundary := func(offset int, start bool, count int) Boundary {
+		b := Boundary{Offset: offset, Start: start, Count: count, Index: index}
+		index++
+		if !start || count == 0 {
+			return b
+		}
+		if max <= 1 {
+			b.Norm = 0.8 // Profile is in "set" mode; we want a heat map. Use cov8 in the CSS.
+		} else if count > 0 {
+			b.Norm = math.Log(float64(count)) / divisor
+		}
+		return b
+	}
+
+	line, col := 1, 2 // TODO: Why is this 2?
+	for si, bi := 0, 0; si < len(src) && bi < len(p.Blocks); {
+		b := p.Blocks[bi]
+		if b.StartLine == line && b.StartCol == col {
+			boundaries = append(boundaries, boundary(si, true, b.Count))
+		}
+		if b.EndLine == line && b.EndCol == col || line > b.EndLine {
+			boundaries = append(boundaries, boundary(si, false, 0))
+			bi++
+			continue // Don't advance through src; maybe the next block starts here.
+		}
+		if src[si] == '\n' {
+			line++
+			col = 0
+		}
+		col++
+		si++
+	}
+	sort.Sort(boundariesByPos(boundaries))
+	return
+}
+
+type boundariesByPos []Boundary
+
+func (b boundariesByPos) Len() int      { return len(b) }
+func (b boundariesByPos) Swap(i, j int) { b[i], b[j] = b[j], b[i] }
+func (b boundariesByPos) Less(i, j int) bool {
+	if b[i].Offset == b[j].Offset {
+		// Boundaries at the same offset should be ordered according to
+		// their original position.
+		return b[i].Index < b[j].Index
+	}
+	return b[i].Offset < b[j].Offset
+}
diff --git a/src/cmd/cover/testdata/directives.go b/src/cmd/cover/testdata/directives.go
new file mode 100644
index 0000000..dfb7b8e
--- /dev/null
+++ b/src/cmd/cover/testdata/directives.go
@@ -0,0 +1,40 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is processed by the cover command, then a test verifies that
+// all compiler directives are preserved and positioned appropriately.
+
+//go:a
+
+//go:b
+package main
+
+//go:c1
+
+//go:c2
+//doc
+func c() {
+}
+
+//go:d1
+
+//doc
+//go:d2
+type d int
+
+//go:e1
+
+//doc
+//go:e2
+type (
+	e int
+	f int
+)
+
+//go:_empty1
+//doc
+//go:_empty2
+type ()
+
+//go:f
diff --git a/src/cmd/cover/testdata/html/html.go b/src/cmd/cover/testdata/html/html.go
new file mode 100644
index 0000000..2057825
--- /dev/null
+++ b/src/cmd/cover/testdata/html/html.go
@@ -0,0 +1,30 @@
+package html
+
+import "fmt"
+
+// This file is tested by html_test.go.
+// The comments below are markers for extracting the annotated source
+// from the HTML output.
+
+// This is a regression test for incorrect sorting of boundaries
+// that coincide, specifically for empty select clauses.
+// START f
+func f() {
+	ch := make(chan int)
+	select {
+	case <-ch:
+	default:
+	}
+}
+
+// END f
+
+// https://golang.org/issue/25767
+// START g
+func g() {
+	if false {
+		fmt.Printf("Hello")
+	}
+}
+
+// END g
diff --git a/src/cmd/cover/testdata/html/html.golden b/src/cmd/cover/testdata/html/html.golden
new file mode 100644
index 0000000..84377d1
--- /dev/null
+++ b/src/cmd/cover/testdata/html/html.golden
@@ -0,0 +1,18 @@
+// START f
+func f() <span class="cov8" title="1">{
+	ch := make(chan int)
+	select </span>{
+	case &lt;-ch:<span class="cov0" title="0"></span>
+	default:<span class="cov8" title="1"></span>
+	}
+}
+
+// END f
+// START g
+func g() <span class="cov8" title="1">{
+	if false </span><span class="cov0" title="0">{
+		fmt.Printf("Hello")
+	}</span>
+}
+
+// END g
diff --git a/src/cmd/cover/testdata/html/html_test.go b/src/cmd/cover/testdata/html/html_test.go
new file mode 100644
index 0000000..c15561f
--- /dev/null
+++ b/src/cmd/cover/testdata/html/html_test.go
@@ -0,0 +1,8 @@
+package html
+
+import "testing"
+
+func TestAll(t *testing.T) {
+	f()
+	g()
+}
diff --git a/src/cmd/cover/testdata/main.go b/src/cmd/cover/testdata/main.go
new file mode 100644
index 0000000..be74b4a
--- /dev/null
+++ b/src/cmd/cover/testdata/main.go
@@ -0,0 +1,116 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test runner for coverage test. This file is not coverage-annotated; test.go is.
+// It knows the coverage counter is called
+// "thisNameMustBeVeryLongToCauseOverflowOfCounterIncrementStatementOntoNextLineForTest".
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	testAll()
+	verify()
+}
+
+type block struct {
+	count uint32
+	line  uint32
+}
+
+var counters = make(map[block]bool)
+
+// shorthand for the long counter variable.
+var coverTest = &thisNameMustBeVeryLongToCauseOverflowOfCounterIncrementStatementOntoNextLineForTest
+
+// check records the location and expected value for a counter.
+func check(line, count uint32) {
+	b := block{
+		count,
+		line,
+	}
+	counters[b] = true
+}
+
+// checkVal is a version of check that returns its extra argument,
+// so it can be used in conditionals.
+func checkVal(line, count uint32, val int) int {
+	b := block{
+		count,
+		line,
+	}
+	counters[b] = true
+	return val
+}
+
+var PASS = true
+
+// verify checks the expected counts against the actual. It runs after the test has completed.
+func verify() {
+	for b := range counters {
+		got, index := count(b.line)
+		if b.count == anything && got != 0 {
+			got = anything
+		}
+		if got != b.count {
+			fmt.Fprintf(os.Stderr, "test_go:%d expected count %d got %d [counter %d]\n", b.line, b.count, got, index)
+			PASS = false
+		}
+	}
+	verifyPanic()
+	if !PASS {
+		fmt.Fprintf(os.Stderr, "FAIL\n")
+		os.Exit(2)
+	}
+}
+
+// verifyPanic is a special check for the known counter that should be
+// after the panic call in testPanic.
+func verifyPanic() {
+	if coverTest.Count[panicIndex-1] != 1 {
+		// Sanity check for test before panic.
+		fmt.Fprintf(os.Stderr, "bad before panic")
+		PASS = false
+	}
+	if coverTest.Count[panicIndex] != 0 {
+		fmt.Fprintf(os.Stderr, "bad at panic: %d should be 0\n", coverTest.Count[panicIndex])
+		PASS = false
+	}
+	if coverTest.Count[panicIndex+1] != 1 {
+		fmt.Fprintf(os.Stderr, "bad after panic")
+		PASS = false
+	}
+}
+
+// count returns the count and index for the counter at the specified line.
+func count(line uint32) (uint32, int) {
+	// Linear search is fine. Choose perfect fit over approximate.
+	// We can have a closing brace for a range on the same line as a condition for an "else if"
+	// and we don't want that brace to steal the count for the condition on the "if".
+	// Therefore we test for a perfect (lo==line && hi==line) match, but if we can't
+	// find that we take the first imperfect match.
+	index := -1
+	indexLo := uint32(1e9)
+	for i := range coverTest.Count {
+		lo, hi := coverTest.Pos[3*i], coverTest.Pos[3*i+1]
+		if lo == line && line == hi {
+			return coverTest.Count[i], i
+		}
+		// Choose the earliest match (the counters are in unpredictable order).
+		if lo <= line && line <= hi && indexLo > lo {
+			index = i
+			indexLo = lo
+		}
+	}
+	if index == -1 {
+		fmt.Fprintln(os.Stderr, "cover_test: no counter for line", line)
+		PASS = false
+		return 0, 0
+	}
+	return coverTest.Count[index], index
+}
diff --git a/src/cmd/cover/testdata/p.go b/src/cmd/cover/testdata/p.go
new file mode 100644
index 0000000..ce3a8c0
--- /dev/null
+++ b/src/cmd/cover/testdata/p.go
@@ -0,0 +1,27 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// A package such that there are 3 functions with zero total and covered lines.
+// And one with 1 total and covered lines. Reproduces issue #20515.
+package p
+
+//go:noinline
+func A() {
+
+}
+
+//go:noinline
+func B() {
+
+}
+
+//go:noinline
+func C() {
+
+}
+
+//go:noinline
+func D() int64 {
+	return 42
+}
diff --git a/src/cmd/cover/testdata/profile.cov b/src/cmd/cover/testdata/profile.cov
new file mode 100644
index 0000000..db08602
--- /dev/null
+++ b/src/cmd/cover/testdata/profile.cov
@@ -0,0 +1,5 @@
+mode: set
+./testdata/p.go:10.10,12.2 0 0
+./testdata/p.go:15.10,17.2 0 0
+./testdata/p.go:20.10,22.2 0 0
+./testdata/p.go:25.16,27.2 1 1
diff --git a/src/cmd/cover/testdata/test.go b/src/cmd/cover/testdata/test.go
new file mode 100644
index 0000000..b794962
--- /dev/null
+++ b/src/cmd/cover/testdata/test.go
@@ -0,0 +1,299 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This program is processed by the cover command, and then testAll is called.
+// The test driver in main.go can then compare the coverage statistics with expectation.
+
+// The word LINE is replaced by the line number in this file. When the file is executed,
+// the coverage processing has changed the line numbers, so we can't use runtime.Caller.
+
+package main
+
+import _ "unsafe" // for go:linkname
+
+//go:linkname some_name some_name
+
+const anything = 1e9 // Just some unlikely value that means "we got here, don't care how often"
+
+func testAll() {
+	testSimple()
+	testBlockRun()
+	testIf()
+	testFor()
+	testRange()
+	testSwitch()
+	testTypeSwitch()
+	testSelect1()
+	testSelect2()
+	testPanic()
+	testEmptySwitches()
+	testFunctionLiteral()
+	testGoto()
+}
+
+// The indexes of the counters in testPanic are known to main.go
+const panicIndex = 3
+
+// This test appears first because the index of its counters is known to main.go
+func testPanic() {
+	defer func() {
+		recover()
+	}()
+	check(LINE, 1)
+	panic("should not get next line")
+	check(LINE, 0) // this is GoCover.Count[panicIndex]
+	// The next counter is in testSimple and it will be non-zero.
+	// If the panic above does not trigger a counter, the test will fail
+	// because GoCover.Count[panicIndex] will be the one in testSimple.
+}
+
+func testSimple() {
+	check(LINE, 1)
+}
+
+func testIf() {
+	if true {
+		check(LINE, 1)
+	} else {
+		check(LINE, 0)
+	}
+	if false {
+		check(LINE, 0)
+	} else {
+		check(LINE, 1)
+	}
+	for i := 0; i < 3; i++ {
+		if checkVal(LINE, 3, i) <= 2 {
+			check(LINE, 3)
+		}
+		if checkVal(LINE, 3, i) <= 1 {
+			check(LINE, 2)
+		}
+		if checkVal(LINE, 3, i) <= 0 {
+			check(LINE, 1)
+		}
+	}
+	for i := 0; i < 3; i++ {
+		if checkVal(LINE, 3, i) <= 1 {
+			check(LINE, 2)
+		} else {
+			check(LINE, 1)
+		}
+	}
+	for i := 0; i < 3; i++ {
+		if checkVal(LINE, 3, i) <= 0 {
+			check(LINE, 1)
+		} else if checkVal(LINE, 2, i) <= 1 {
+			check(LINE, 1)
+		} else if checkVal(LINE, 1, i) <= 2 {
+			check(LINE, 1)
+		} else if checkVal(LINE, 0, i) <= 3 {
+			check(LINE, 0)
+		}
+	}
+	if func(a, b int) bool { return a < b }(3, 4) {
+		check(LINE, 1)
+	}
+}
+
+func testFor() {
+	for i := 0; i < 10; func() { i++; check(LINE, 10) }() {
+		check(LINE, 10)
+	}
+}
+
+func testRange() {
+	for _, f := range []func(){
+		func() { check(LINE, 1) },
+	} {
+		f()
+		check(LINE, 1)
+	}
+}
+
+func testBlockRun() {
+	check(LINE, 1)
+	{
+		check(LINE, 1)
+	}
+	{
+		check(LINE, 1)
+	}
+	check(LINE, 1)
+	{
+		check(LINE, 1)
+	}
+	{
+		check(LINE, 1)
+	}
+	check(LINE, 1)
+}
+
+func testSwitch() {
+	for i := 0; i < 5; func() { i++; check(LINE, 5) }() {
+		goto label2
+	label1:
+		goto label1
+	label2:
+		switch i {
+		case 0:
+			check(LINE, 1)
+		case 1:
+			check(LINE, 1)
+		case 2:
+			check(LINE, 1)
+		default:
+			check(LINE, 2)
+		}
+	}
+}
+
+func testTypeSwitch() {
+	var x = []interface{}{1, 2.0, "hi"}
+	for _, v := range x {
+		switch func() { check(LINE, 3) }(); v.(type) {
+		case int:
+			check(LINE, 1)
+		case float64:
+			check(LINE, 1)
+		case string:
+			check(LINE, 1)
+		case complex128:
+			check(LINE, 0)
+		default:
+			check(LINE, 0)
+		}
+	}
+}
+
+func testSelect1() {
+	c := make(chan int)
+	go func() {
+		for i := 0; i < 1000; i++ {
+			c <- i
+		}
+	}()
+	for {
+		select {
+		case <-c:
+			check(LINE, anything)
+		case <-c:
+			check(LINE, anything)
+		default:
+			check(LINE, 1)
+			return
+		}
+	}
+}
+
+func testSelect2() {
+	c1 := make(chan int, 1000)
+	c2 := make(chan int, 1000)
+	for i := 0; i < 1000; i++ {
+		c1 <- i
+		c2 <- i
+	}
+	for {
+		select {
+		case <-c1:
+			check(LINE, 1000)
+		case <-c2:
+			check(LINE, 1000)
+		default:
+			check(LINE, 1)
+			return
+		}
+	}
+}
+
+// Empty control statements created syntax errors. This function
+// is here just to be sure that those are handled correctly now.
+func testEmptySwitches() {
+	check(LINE, 1)
+	switch 3 {
+	}
+	check(LINE, 1)
+	switch i := (interface{})(3).(int); i {
+	}
+	check(LINE, 1)
+	c := make(chan int)
+	go func() {
+		check(LINE, 1)
+		c <- 1
+		select {}
+	}()
+	<-c
+	check(LINE, 1)
+}
+
+func testFunctionLiteral() {
+	a := func(f func()) error {
+		f()
+		f()
+		return nil
+	}
+
+	b := func(f func()) bool {
+		f()
+		f()
+		return true
+	}
+
+	check(LINE, 1)
+	a(func() {
+		check(LINE, 2)
+	})
+
+	if err := a(func() {
+		check(LINE, 2)
+	}); err != nil {
+	}
+
+	switch b(func() {
+		check(LINE, 2)
+	}) {
+	}
+
+	x := 2
+	switch x {
+	case func() int { check(LINE, 1); return 1 }():
+		check(LINE, 0)
+		panic("2=1")
+	case func() int { check(LINE, 1); return 2 }():
+		check(LINE, 1)
+	case func() int { check(LINE, 0); return 3 }():
+		check(LINE, 0)
+		panic("2=3")
+	}
+}
+
+func testGoto() {
+	for i := 0; i < 2; i++ {
+		if i == 0 {
+			goto Label
+		}
+		check(LINE, 1)
+	Label:
+		check(LINE, 2)
+	}
+	// Now test that we don't inject empty statements
+	// between a label and a loop.
+loop:
+	for {
+		check(LINE, 1)
+		break loop
+	}
+}
+
+// This comment didn't appear in generated go code.
+func haha() {
+	// Needed for cover to add counter increment here.
+	_ = 42
+}
+
+// Some someFunction.
+//
+//go:nosplit
+func someFunction() {
+}
diff --git a/src/cmd/cover/testdata/toolexec.go b/src/cmd/cover/testdata/toolexec.go
new file mode 100644
index 0000000..386de79
--- /dev/null
+++ b/src/cmd/cover/testdata/toolexec.go
@@ -0,0 +1,33 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The toolexec program is a helper program for cmd/cover tests.
+// It is used so that the go tool will call the newly built version
+// of the cover program, rather than the installed one.
+//
+// The tests arrange to run the go tool with the argument
+//    -toolexec="/path/to/toolexec /path/to/testcover"
+// The go tool will invoke this program (compiled into /path/to/toolexec)
+// with the arguments shown above followed by the command to run.
+// This program will check whether it is expected to run the cover
+// program, and if so replace it with /path/to/testcover.
+package main
+
+import (
+	"os"
+	exec "internal/execabs"
+	"strings"
+)
+
+func main() {
+	if strings.HasSuffix(strings.TrimSuffix(os.Args[2], ".exe"), "cover") {
+		os.Args[2] = os.Args[1]
+	}
+	cmd := exec.Command(os.Args[2], os.Args[3:]...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		os.Exit(1)
+	}
+}