go/build: skip rune literals when looking for go:embed

Fixes #49514

Change-Id: Id687eead731ba49974f11d2e5b489f11eff7d07b
Reviewed-on: https://go-review.googlesource.com/c/go/+/363275
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/embed/internal/embedtest/embed_test.go

@@ -60,6 +60,11 @@ func testDir(t *testing.T, f embed.FS, name string, expect ...string) {
 	}
 }
 
+// Tests for issue 49514.
+var _ = '"'
+var _ = '\''
+var _ = '🦆'
+
 func TestGlobal(t *testing.T) {
 	testFiles(t, global, "concurrency.txt", "Concurrency is not parallelism.\n")
 	testFiles(t, global, "testdata/hello.txt", "hello, world\n")

src/go/build/read.go

@@ -240,6 +240,27 @@ func (r *importReader) findEmbed(first bool) bool {
 				}
 			}
 
+		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
+				}
+			}
+
 		case '/':
 			c = r.readByteNoBuf()
 			switch c {