give a detailed error message in case of indentation mismatches in multiline jsonnet blocks

core/lexer.cpp

@@ -411,6 +411,27 @@ static int whitespace_check(const char *a, const char *b)
     return i;
 }
 
+static void describe_whitespace(std::stringstream& msg, const std::string& ws) {
+  int spaces = 0;
+  int tabs = 0;
+  for (char c : ws) {
+    if (c == ' ')
+      spaces++;
+    else if (c == '\t')
+      tabs++;
+  }
+  if (spaces > 0 && tabs > 0) {
+    msg << spaces << (spaces == 1 ? " space" : " spaces") << " and " << tabs
+        << (tabs == 1 ? " tab" : " tabs");
+  } else if (spaces > 0) {
+    msg << spaces << (spaces == 1 ? " space" : " spaces");
+  } else if (tabs > 0) {
+    msg << tabs << (tabs == 1 ? " tab" : " tabs");
+  } else {
+    msg << "no indentation";
+  }
+}
+
 /*
 static void add_whitespace(Fodder &fodder, const char *s, size_t n)
 {
@@ -755,25 +776,52 @@ Tokens jsonnet_lex(const std::string &filename, const char *input)
                             // Examine next line
                             ws_chars = whitespace_check(first_line, c);
                             if (ws_chars == 0) {
-                                // End of text block
-                                // Skip over any whitespace
-                                while (*c == ' ' || *c == '\t') {
-                                    string_block_term_indent += *c;
-                                    ++c;
-                                }
-                                // Expect |||
-                                if (!(*c == '|' && *(c + 1) == '|' && *(c + 2) == '|')) {
-                                    auto msg = "text block not terminated with |||";
-                                    throw StaticError(filename, begin, msg);
+                              // End of text block (or indentation error).
+                              // Count actual whitespace on this line.
+                              int actual_ws = 0;
+                              while (c[actual_ws] == ' ' ||
+                                     c[actual_ws] == '\t') {
+                                actual_ws++;
+                              }
+
+                              // Check if this is the terminator |||
+                              bool is_terminator = (c[actual_ws] == '|' &&
+                                                    c[actual_ws + 1] == '|' &&
+                                                    c[actual_ws + 2] == '|');
+
+                              if (!is_terminator) {
+                                // Not a terminator - check if it's an
+                                // indentation issue.
+                                if (actual_ws > 0) {
+                                  // Has whitespace but doesn't match expected
+                                  // indentation.
+                                  std::stringstream msg;
+                                  msg << "text block indentation mismatch: "
+                                         "expected at least ";
+                                  describe_whitespace(msg, string_block_indent);
+                                  msg << ", found ";
+                                  describe_whitespace(
+                                      msg, std::string(c, actual_ws));
+                                  throw StaticError(filename, begin, msg.str());
+                                } else {
+                                  // No whitespace and no ||| - missing
+                                  // terminator.
+                                  auto msg =
+                                      "text block not terminated with |||";
+                                  throw StaticError(filename, begin, msg);
                                 }
-                                c += 3;  // Leave after the last |
-                                data = block.str();
-                                kind = Token::STRING_BLOCK;
-                                if (chomp_trailing_nl) {
-                                    assert(data.back() == '\n');
-                                    data.pop_back();
-                                }
-                                break;  // Out of the while loop.
+                              }
+
+                              // Valid termination - skip over any whitespace.
+                              while (*c == ' ' || *c == '\t') {
+                                string_block_term_indent += *c;
+                                ++c;
+                              }
+                              // Skip the |||
+                              c += 3;  // Leave after the last |
+                              data = block.str();
+                              kind = Token::STRING_BLOCK;
+                              break;  // Out of the while loop.
                             }
                         }
 

test_suite/error.parse.text_block_bad_whitespace.jsonnet.golden

@@ -1 +1 @@
-STATIC ERROR: error.parse.text_block_bad_whitespace.jsonnet:17:1: text block not terminated with |||
+STATIC ERROR: error.parse.text_block_bad_whitespace.jsonnet:17:1: text block indentation mismatch: expected at least 4 spaces, found 1 tab

test_suite/error.parse.text_block_indent_spaces.jsonnet

@@ -0,0 +1,20 @@
+/*
+Copyright 2026 Google Inc. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUTHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+|||
+    foo
+  bar
+|||

test_suite/error.parse.text_block_indent_spaces.jsonnet.golden

@@ -0,0 +1 @@
+STATIC ERROR: error.parse.text_block_indent_spaces.jsonnet:17:1: text block indentation mismatch: expected at least 4 spaces, found 2 spaces