go/build: skip string literal while findEmbed

The findEmbed function looking for comment by readbyte,
however it might have constant or variables that contains
comment.
Maybe we should use ast parser in the future.

Fixes #43373

Change-Id: I92544384fc4c11363d8b2f6b9898c8dea1602767
Reviewed-on: https://go-review.googlesource.com/c/go/+/280332
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/go/build/read.go

@@ -171,6 +171,41 @@ func (r *importReader) findEmbed(first bool) bool {
 		case ' ', '\t':
 			// leave startLine alone
 
+		case '"':
+			startLine = false
+			for r.err == nil {
+				if r.eof {
+					r.syntaxError()
+				}
+				c = r.readByteNoBuf()
+				if c == '\\' {
+					r.readByteNoBuf()
+					if r.err != nil {
+						r.syntaxError()
+						return false
+					}
+					continue
+				}
+				if c == '"' {
+					c = r.readByteNoBuf()
+					goto Reswitch
+				}
+			}
+			goto Reswitch
+
+		case '`':
+			startLine = false
+			for r.err == nil {
+				if r.eof {
+					r.syntaxError()
+				}
+				c = r.readByteNoBuf()
+				if c == '`' {
+					c = r.readByteNoBuf()
+					goto Reswitch
+				}
+			}
+
 		case '/':
 			c = r.readByteNoBuf()
 			switch c {

src/go/build/read_test.go

@@ -255,6 +255,26 @@ var readEmbedTests = []struct {
 		"package p\nimport \"embed\"\n//go:embed x y z\nvar files embed.FS",
 		[]string{"x", "y", "z"},
 	},
+	{
+		"package p\nimport \"embed\"\nvar s = \"/*\"\n//go:embed x\nvar files embed.FS",
+		[]string{"x"},
+	},
+	{
+		`package p
+		 import "embed"
+		 var s = "\"\\\\"
+		 //go:embed x
+		 var files embed.FS`,
+		[]string{"x"},
+	},
+	{
+		"package p\nimport \"embed\"\nvar s = `/*`\n//go:embed x\nvar files embed.FS",
+		[]string{"x"},
+	},
+	{
+		"package p\nimport \"embed\"\nvar s = z/ *y\n//go:embed pointer\nvar pointer embed.FS",
+		[]string{"pointer"},
+	},
 	{
 		"package p\n//go:embed x y z\n", // no import, no scan
 		nil,