Remove BC escaped double quote behavior

When we switched to the current edition of the HCL parser, we
inadvertently broke some undocumented behavior where users could
successfully used escaped double quotes in a nested interpolation like
this:

    foo = "${somefunc(\"somearg\")}"

The proper syntax here is:

    foo = "${somefunc("somearg")}"

Because once you are inside the interpolation braces, the "quoting
context" is different.

At the time, while we didn't like the fact that the non-standard syntax
was in the wild, we didn't want to break BC, so we treated it as a bug
and fixed it in #62.

Now that we are at the moment of a major Terraform release, we can yank
the BC compatible behavior, which fixes a currently broken use case -
there is no way to express strings with double quotes in them from
inside interpolation braces.

    foo = "${somefunc("\"inquotes\"")}"

After merge and a dep upgrade, this will take care of
https://github.com/hashicorp/terraform/issues/5550

decoder_test.go

@@ -64,14 +64,7 @@ func TestDecode_interface(t *testing.T) {
 				"bar":          "new\nline",
 				"qax":          `slash\:colon`,
 				"nested":       `${HH\:mm\:ss}`,
-				"nestedquotes": `${"okay"}`,
-			},
-		},
-		{
-			"interpolate_escape.hcl",
-			false,
-			map[string]interface{}{
-				"foo": "${file(\"bing/bong.txt\")}",
+				"nestedquotes": `${"\"stringwrappedinquotes\""}`,
 			},
 		},
 		{

hcl/strconv/quote.go

@@ -49,8 +49,7 @@ func Unquote(s string) (t string, err error) {
 	for len(s) > 0 {
 		// If we're starting a '${}' then let it through un-unquoted.
 		// Specifically: we don't unquote any characters within the `${}`
-		// section, except for escaped quotes and escaped backslashes, which we
-		// handle specifically.
+		// section, except for escaped backslashes, which we handle specifically.
 		if s[0] == '$' && len(s) > 1 && s[1] == '{' {
 			buf = append(buf, '$', '{')
 			s = s[2:]
@@ -65,12 +64,12 @@ func Unquote(s string) (t string, err error) {
 
 				s = s[size:]
 
-				// We special case escaped double quotes and escaped backslashes in
-				// interpolations, converting them to their unescaped equivalents.
+				// We special case escaped backslashes in interpolations, converting
+				// them to their unescaped equivalents.
 				if r == '\\' {
 					q, _ := utf8.DecodeRuneInString(s)
 					switch q {
-					case '"', '\\':
+					case '\\':
 						continue
 					}
 				}

hcl/strconv/quote_test.go

@@ -36,7 +36,7 @@ var unquotetests = []unQuoteTest{
 	{`"\a\b\f\n\r\t\v\\\""`, "\a\b\f\n\r\t\v\\\""},
 	{`"'"`, "'"},
 	{`"${file("foo")}"`, `${file("foo")}`},
-	{`"${file(\"foo\")}"`, `${file("foo")}`},
+	{`"${file("\"foo\"")}"`, `${file("\"foo\"")}`},
 	{`"echo ${var.region}${element(split(",",var.zones),0)}"`,
 		`echo ${var.region}${element(split(",",var.zones),0)}`},
 	{`"${HH\\:mm\\:ss}"`, `${HH\:mm\:ss}`},

hcl/token/token_test.go

@@ -50,7 +50,7 @@ func TestTokenValue(t *testing.T) {
 		{Token{Type: IDENT, Text: `foo`}, "foo"},
 		{Token{Type: STRING, Text: `"foo"`}, "foo"},
 		{Token{Type: STRING, Text: `"foo\nbar"`}, "foo\nbar"},
-		{Token{Type: STRING, Text: `"${file(\"foo\")}"`}, `${file("foo")}`},
+		{Token{Type: STRING, Text: `"${file("foo")}"`}, `${file("foo")}`},
 		{Token{Type: HEREDOC, Text: "<<EOF\nfoo\nbar\nEOF"}, "foo\nbar"},
 	}
 

test-fixtures/escape.hcl

@@ -3,4 +3,4 @@ bar = "new\nline"
 qux = "back\\slash"
 qax = "slash\\:colon"
 nested = "${HH\\:mm\\:ss}"
-nestedquotes = "${\"okay\"}"
+nestedquotes = "${"\"stringwrappedinquotes\""}"

test-fixtures/interpolate_escape.hcl

@@ -1 +0,0 @@
-foo="${file(\"bing/bong.txt\")}"