go.tools/godoc: ignore //line comments in source code

Per suggestion from Gary Burd.

Fixes golang/go#5247.

LGTM=rsc
R=bgarcia, rsc
CC=golang-codereviews
https://golang.org/cl/84050044

godoc/cmdline_test.go

@@ -169,6 +169,14 @@ func First() {
 func Second() {
 }
 `,
+		"src/pkg/gen/gen.go": `// Package gen
+package gen
+
+//line notgen.go:3
+// F doc //line 1 should appear
+// line 2 should appear
+func F()
+//line foo.go:100`, // no newline on end to check corner cases!
 		"src/pkg/vet/vet.go": `// Package vet
 package vet
 `,
@@ -230,6 +238,11 @@ package main
 			args: []string{"src/foo", "Second"},
 			exp:  "// Second function is second.\nfunc Second() {\n}",
 		},
+		{
+			desc: "package w. //line comments",
+			args: []string{"gen", "F"},
+			exp:  "PACKAGE \nfunc F()\n    F doc //line 1 should appear line 2 should appear\n",
+		},
 		{
 			desc: "command",
 			args: []string{"go"},
@@ -259,7 +272,7 @@ package main
 		w := new(bytes.Buffer)
 		err := CommandLine(w, fs, p, tc.args)
 		if got, want := w.String(), tc.exp; got != want || tc.err == (err == nil) {
-			t.Errorf("%s: CommandLine(%v) = %q,%v; want %q,%v",
+			t.Errorf("%s: CommandLine(%v) = %q (%v); want %q (%v)",
 				tc.desc, tc.args, got, err, want, tc.err)
 		}
 	}

godoc/parser.go

@@ -8,6 +8,7 @@
 package godoc
 
 import (
+	"bytes"
 	"go/ast"
 	"go/parser"
 	"go/token"
@@ -16,11 +17,45 @@ import (
 	"code.google.com/p/go.tools/godoc/vfs"
 )
 
+var linePrefix = []byte("//line ")
+
+// This function replaces source lines starting with "//line " with a blank line.
+// It does this irrespective of whether the line is truly a line comment or not;
+// e.g., the line may be inside a string, or a /*-style comment; however that is
+// rather unlikely (proper testing would require a full Go scan which we want to
+// avoid for performance).
+func replaceLinePrefixCommentsWithBlankLine(src []byte) {
+	for {
+		i := bytes.Index(src, linePrefix)
+		if i < 0 {
+			break // we're done
+		}
+		// 0 <= i && i+len(linePrefix) <= len(src)
+		if i == 0 || src[i-1] == '\n' {
+			// at beginning of line: blank out line
+			for i < len(src) && src[i] != '\n' {
+				src[i] = ' '
+				i++
+			}
+		} else {
+			// not at beginning of line: skip over prefix
+			i += len(linePrefix)
+		}
+		// i <= len(src)
+		src = src[i:]
+	}
+}
+
 func (c *Corpus) parseFile(fset *token.FileSet, filename string, mode parser.Mode) (*ast.File, error) {
 	src, err := vfs.ReadFile(c.fs, filename)
 	if err != nil {
 		return nil, err
 	}
+
+	// Temporary ad-hoc fix for issue 5247.
+	// TODO(gri) Remove this in favor of a better fix, eventually (see issue 7702).
+	replaceLinePrefixCommentsWithBlankLine(src)
+
 	return parser.ParseFile(fset, filename, src, mode)
 }