KotlinStringLiteralTextEscaper: provides offsets for already decoded (KT-27380)

compiler/psi/src/org/jetbrains/kotlin/psi/KotlinStringLiteralTextEscaper.kt

@@ -41,6 +41,9 @@ class KotlinStringLiteralTextEscaper(host: KtStringTemplateExpression) : Literal
                 is KtEscapeStringTemplateEntry -> {
                     if (!rangeInsideHost.contains(childRange)) {
                         //don't allow injection if its range starts or ends inside escaped sequence
+                        //but still process offsets for the already decoded part
+                        sourceOffsetsList.add(sourceOffset)
+                        sourceOffsets = sourceOffsetsList.toNativeArray()
                         return false
                     }
                     val unescaped = child.unescapedValue

idea/tests/org/jetbrains/kotlin/psi/injection/StringInjectionHostTest.kt

@@ -104,6 +104,28 @@ class StringInjectionHostTest : KotlinTestWithEnvironment() {
         }
     }
 
+    fun testProvideOffsetsForDecodablePartOfUndecodableString() {
+        val undecodable = stringExpression(""""{\\d\}"""")
+        val escaper = undecodable.createLiteralTextEscaper()
+        val undecodableRange = undecodable.text.rangeOf("""\\d\""")
+
+        val decoded = StringBuilder()
+        assertFalse(escaper.decode(undecodableRange, decoded))
+        assertEquals("""\d""", decoded.toString())
+
+        val mapping = (0..undecodableRange.length).keysToMap { escaper.getOffsetInHost(it, undecodableRange) }
+        assertEquals(
+            mapOf(
+                0 to 2,
+                1 to 4,
+                2 to 5,
+                3 to -1,
+                4 to -1
+            ),
+            mapping
+        )
+    }
+
     private fun quoted(s: String): KtStringTemplateExpression {
         return stringExpression("\"$s\"")
     }
@@ -163,3 +185,5 @@ class StringInjectionHostTest : KotlinTestWithEnvironment() {
 
     override fun createEnvironment() = createEnvironmentWithMockJdk(ConfigurationKind.JDK_ONLY)
 }
+
+private fun String.rangeOf(inner: String): TextRange = indexOf(inner).let { TextRange.from(it, inner.length) }