Diagnostic multiline message

cli/src/main/kotlin/tools/samt/cli/DiagnosticFormatter.kt

@@ -115,20 +115,40 @@ internal class DiagnosticFormatter(
         DiagnosticSeverity.Info -> formatTextForSeverity("INFO:", severity, withBold = true)
     }
 
-    private fun formatGlobalMessage(message: DiagnosticGlobalMessage): String = buildString {
-        append(formatSeverityIndicator(message.severity))
+    private fun formatSeverityAndMessage(severity: DiagnosticSeverity, message: String): String = buildString {
+        // <severity>: <message>
+        append(formatSeverityIndicator(severity))
         append(" ")
-        append(message.message)
-        appendLine()
+
+        // these magic numbers relate to the amount of space each severity indicator takes up when formatted
+        // e.g. "ERROR: ".length == 7
+        //      "WARNING: ".length == 9
+        //      "INFO: ".length == 6
+        val indentLength = when (severity) {
+            DiagnosticSeverity.Error -> 7
+            DiagnosticSeverity.Warning -> 9
+            DiagnosticSeverity.Info -> 6
+        }
+
+        val lines = message.lines()
+        if (lines.size > 1) {
+            lines.forEachIndexed { index, it ->
+                if (index > 0) {
+                    append(" ".repeat(indentLength))
+                }
+                appendLine(it)
+            }
+        } else {
+            appendLine(message)
+        }
     }
 
-    private fun formatMessage(message: DiagnosticMessage, context: DiagnosticContext): String = buildString {
+    private fun formatGlobalMessage(message: DiagnosticGlobalMessage): String = buildString {
+        append(formatSeverityAndMessage(message.severity, message.message))
+    }
 
-        // <severity>: <message>
-        append(formatSeverityIndicator(message.severity))
-        append(" ")
-        append(message.message)
-        appendLine()
+    private fun formatMessage(message: DiagnosticMessage, context: DiagnosticContext): String = buildString {
+        append(formatSeverityAndMessage(message.severity, message.message))
 
         val errorSourceFilePath = if (message.highlights.isNotEmpty()) {
             message.highlights.first().location.source.path

cli/src/test/kotlin/tools/samt/cli/DiagnosticFormatterTest.kt

@@ -478,6 +478,48 @@ class DiagnosticFormatterTest {
         """.trimIndent().trim(), outputWithoutColors.trimIndent().trim())
     }
 
+    @Test
+    fun `multiline error messages`() {
+        val source = """
+            package debug
+            enum Test {
+                Foo
+            }
+        """.trimIndent()
+
+        val (fileNode, context, controller) = parse(source)
+        assert(fileNode.statements.single() is EnumDeclarationNode)
+        val enumNode = fileNode.statements.single() as EnumDeclarationNode
+
+        context.error {
+            message("some error\nsome error\nsome error")
+            highlight("some highlight", enumNode.location, highlightBeginningOnly = true)
+        }
+
+        val output = DiagnosticFormatter.format(controller, 0, 0, terminalWidth = 40)
+        val outputWithoutColors = output.replace(Regex("\u001B\\[[;\\d]*m"), "")
+
+        assertEquals("""
+            ────────────────────────────────────────
+            ERROR: some error
+                   some error
+                   some error
+             ---> file:///tmp/DiagnosticFormatterTest.samt:2:1
+
+                  1 │ package debug
+                  2 │ enum Test {
+                    │ ^
+                    │ |
+                    │ some highlight
+                    │ 
+                  3 │     Foo
+                  4 │ }
+
+            ────────────────────────────────────────
+            FAILED in 0ms (1 error(s), 0 warning(s))
+        """.trimIndent().trim(), outputWithoutColors.trimIndent().trim())
+    }
+
     private fun parse(source: String): Triple<FileNode, DiagnosticContext, DiagnosticController> {
         val baseDirectory = URI("file:///tmp")
         val filePath = URI("file:///tmp/DiagnosticFormatterTest.samt")