marshal: don't escape quotes unnecessarily (#991)

Only 3 consecutive quotation marks need to be quoted. We choose to quote
all quotation marks in a sequence if there are 3 or more consecutive
present.

Fixes #990

---------

Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>

marshaler.go

@@ -517,12 +517,26 @@ func (enc *Encoder) encodeQuotedString(multiline bool, b []byte, v string) []byt
 		del = 0x7f
 	)
 
-	for _, r := range []byte(v) {
+	bv := []byte(v)
+	for i := 0; i < len(bv); i++ {
+		r := bv[i]
 		switch r {
 		case '\\':
 			b = append(b, `\\`...)
 		case '"':
+			if multiline {
+				// Quotation marks do not need to be quoted in multiline strings unless
+				// it contains 3 consecutive. If 3+ quotes appear, quote all of them
+				// because it's visually better
+				if i+2 > len(bv) || bv[i+1] != '"' || bv[i+2] != '"' {
+					b = append(b, r)
+				} else {
+					b = append(b, `\"\"\"`...)
+					i += 2
+				}
+			} else {
 				b = append(b, `\"`...)
+			}
 		case '\b':
 			b = append(b, `\b`...)
 		case '\f':

marshaler_test.go

@@ -387,6 +387,54 @@ name = 'Alice'
 			expected: `A = """
 hello
 world"""
+`,
+		},
+		{
+			desc: "multi-line quotation",
+			v: struct {
+				A string `toml:",multiline"`
+			}{
+				A: "hello\n\"world\"",
+			},
+			expected: `A = """
+hello
+"world""""
+`,
+		},
+		{
+			desc: "multi-line triple quotation",
+			v: struct {
+				A string `toml:",multiline"`
+			}{
+				A: "hello\n\"\"\"world\"",
+			},
+			expected: `A = """
+hello
+\"\"\"world""""
+`,
+		},
+		{
+			desc: "multi-line triple quotation",
+			v: struct {
+				A string `toml:",multiline"`
+			}{
+				A: "hello\n\"world\"\"\"",
+			},
+			expected: `A = """
+hello
+"world\"\"\""""
+`,
+		},
+		{
+			desc: "multi-line sextuple quotation",
+			v: struct {
+				A string `toml:",multiline"`
+			}{
+				A: "hello\n\"\"\"\"\"\"world\"",
+			},
+			expected: `A = """
+hello
+\"\"\"\"\"\"world""""
 `,
 		},
 		{