Add backtick string literals and formatter support

lynglib/src/commonTest/kotlin/ScriptTest.kt

@@ -4395,6 +4395,30 @@ class ScriptTest {
         )
     }
 
+    @Test
+    fun backtickStringsMatchRegularStringSemantics() = runTest {
+        val d = '$'
+        eval(
+            """
+            val name = "Lyng"
+            val simple = `hello, $d{name}`
+            assertEquals("hello, Lyng", simple)
+            assertEquals("{\"name\":\"Lyng\"}", `{"name":"Lyng"}`)
+            assertEquals("use the `code` style", `use the \`code\` style`)
+            assertEquals("\\\"", `\"`)
+            assertEquals("\"", `"`)
+            assertEquals(
+                "first\n\"second\"\nthird",
+                `
+                    first
+                    "second"
+                    third
+                `
+            )
+        """.trimIndent()
+        )
+    }
+
     @Test
     fun testInlineArrayLiteral() = runTest {
         eval(

lynglib/src/commonTest/kotlin/UnicodeEscapeTest.kt

@@ -31,6 +31,13 @@ class UnicodeEscapeTest {
         assertEquals("☺", token.value)
     }
 
+    @Test
+    fun parserDecodesUnicodeEscapeInBacktickStringLiteral() {
+        val token = parseLyng("`\\u263A`".toSource()).first()
+        assertEquals(Token.Type.STRING, token.type)
+        assertEquals("☺", token.value)
+    }
+
     @Test
     fun parserDecodesUnicodeEscapeInCharLiteral() {
         val token = parseLyng("'\\u263A'".toSource()).first()
@@ -55,6 +62,7 @@ class UnicodeEscapeTest {
     @Test
     fun evalDecodesUnicodeEscapes() = runTest {
         assertEquals(ObjString("☺"), eval("\"\\u263A\""))
+        assertEquals(ObjString("☺"), eval("`\\u263A`"))
         assertEquals(ObjChar('☺'), eval("'\\u263A'"))
     }
 }

lynglib/src/commonTest/kotlin/net/sergeych/lyng/format/LyngFormatterTest.kt

@@ -18,9 +18,36 @@ package net.sergeych.lyng.format
 
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertTrue
 
 class LyngFormatterTest {
 
+    @Test
+    fun preferBackticksForQuoteHeavyStrings() {
+        val src = "val json = \"{\\\"name\\\":\\\"lyng\\\",\\\"kind\\\":\\\"lang\\\"}\""
+        val cfg = LyngFormatConfig(
+            applySpacing = true,
+            stringDelimiterPolicy = LyngStringDelimiterPolicy.PreferFewerEscapes
+        )
+
+        val out = LyngFormatter.format(src, cfg)
+
+        assertEquals("""val json = `{"name":"lyng","kind":"lang"}`""", out)
+        assertEquals(out, LyngFormatter.format(out, cfg))
+    }
+
+    @Test
+    fun preserveStringsWhenAlternativeWouldNotHelp() {
+        val src = "val sample = \"use `ticks` and keep \\` literal\""
+        val cfg = LyngFormatConfig(
+            stringDelimiterPolicy = LyngStringDelimiterPolicy.PreferFewerEscapes
+        )
+
+        val out = LyngFormatter.format(src, cfg)
+
+        assertEquals(src, out)
+    }
+
     @Test
     fun labelFormatting() {
         val src = "return  @label; break  @outer; continue  @inner"
@@ -79,9 +106,9 @@ class LyngFormatterTest {
 
         val formatted = LyngFormatter.format(src, LyngFormatConfig(applyWrapping = true, maxLineLength = 40, continuationIndentSize = 4))
         // Ensure the string literal remains intact
-        kotlin.test.assertTrue(formatted.contains(arg2), "String literal must be preserved")
+        assertTrue(formatted.contains(arg2), "String literal must be preserved")
         // Ensure end-of-line comment remains
-        kotlin.test.assertTrue(formatted.contains("// end comment"), "EOL comment must be preserved")
+        assertTrue(formatted.contains("// end comment"), "EOL comment must be preserved")
         // Idempotency
         val formatted2 = LyngFormatter.format(formatted, LyngFormatConfig(applyWrapping = true, maxLineLength = 40, continuationIndentSize = 4))
         assertEquals(formatted, formatted2)