Report unexpected indentation characters in margin of multiline string

ktlint-ruleset-standard/src/main/kotlin/com/pinterest/ktlint/ruleset/standard/IndentationRule.kt

@@ -798,6 +798,18 @@ class IndentationRule : Rule("indent"), Rule.Modifier.RestrictToRootLast {
     ) {
         val psi = node.psi as KtStringTemplateExpression
         if (psi.isMultiLine() && psi.isFollowedByTrimIndent()) {
+            if (node.containsMixedIndentationCharacters()) {
+                // It can not be determined with certainty how mixed indentation characters should be interpreted.
+                // The trimIndent function handles tabs and spaces equally (one tabs equals one space) while the user
+                // might expect that the tab size in the indentation is more than one space.
+                emit(
+                    node.startOffset,
+                    "Indentation of multiline string should not contain both tab(s) and space(s)",
+                    false
+                )
+                return
+            }
+
             val prefixLength = node.children()
                 .filterNot { it.elementType == OPEN_QUOTE }
                 .filterNot { it.elementType == CLOSING_QUOTE }
@@ -1092,6 +1104,27 @@ class IndentationRule : Rule("indent"), Rule.Modifier.RestrictToRootLast {
                 children().any { c -> c.textContains('\n') && c.elementType !in ignoreElementTypes }
         }
     }
+
+    private fun ASTNode.containsMixedIndentationCharacters(): Boolean {
+        assert((this.psi as KtStringTemplateExpression).isMultiLine())
+        val nonBlankLines = this
+            .text
+            .split("\n")
+            .filterNot { it.startsWith("\"\"\"") }
+            .filterNot { it.endsWith("\"\"\"") }
+            .filterNot { it.isBlank() }
+        val prefixLength = nonBlankLines
+            .map { it.indentLength() }
+            .min() ?: 0
+        val distinctIndentCharacters = nonBlankLines
+            .joinToString(separator = "") {
+                it.splitIndentAt(prefixLength).first
+            }
+            .toCharArray()
+            .distinct()
+            .count()
+        return distinctIndentCharacters > 1
+    }
 }
 
 private fun ASTNode.isIndentBeforeClosingQuote() =

ktlint-ruleset-standard/src/test/kotlin/com/pinterest/ktlint/ruleset/standard/IndentationRuleTest.kt

@@ -1271,6 +1271,31 @@ internal class IndentationRuleTest {
         ).isEmpty()
     }
 
+    @Test
+    fun `multiline string with mixed indentation characters, can not be autocorrected`() {
+        val code =
+            """
+                val foo = $MULTILINE_STRING_QUOTE
+                      line1
+                ${TAB}line2
+                    $MULTILINE_STRING_QUOTE.trimIndent()
+                """
+                .trimIndent()
+        assertThat(
+            IndentationRule().lint(code)
+        ).isEqualTo(
+            listOf(
+                LintError(
+                    line = 1,
+                    col = 11,
+                    ruleId = "indent",
+                    detail = "Indentation of multiline string should not contain both tab(s) and space(s)"
+                ),
+            )
+        )
+        assertThat(IndentationRule().format(code)).isEqualTo(code)
+    }
+
     private companion object {
         const val MULTILINE_STRING_QUOTE = "${'"'}${'"'}${'"'}"
         const val TAB = "${'\t'}"