gh-93883: elide traceback indicators when possible (#93994)

* gh-93883: elide traceback indicators when possible

Elide traceback column indicators when the entire line of the
frame is implicated.  This reduces traceback length and draws
even more attention to the remaining (very relevant) indicators.

Example:
```
Traceback (most recent call last):
  File "query.py", line 99, in <module>
    bar()
  File "query.py", line 66, in bar
    foo()
  File "query.py", line 37, in foo
    magic_arithmetic('foo')
  File "query.py", line 18, in magic_arithmetic
    return add_counts(x) / 25
           ^^^^^^^^^^^^^
  File "query.py", line 24, in add_counts
    return 25 + query_user(user1) + query_user(user2)
                ^^^^^^^^^^^^^^^^^
  File "query.py", line 32, in query_user
    return 1 + query_count(db, response['a']['b']['c']['user'], retry=True)
                               ~~~~~~~~~~~~~~~~~~^^^^^
TypeError: 'NoneType' object is not subscriptable
```

Rather than going out of our way to provide indicator coverage
in every traceback test suite, the indicator test suite should
be responible for sufficient coverage (e.g. by adding a basic
exception group test to ensure that margin strings are covered).

1 files changed, 14 insertions, 4 deletions

diff --git a/Python/traceback.c b/Python/traceback.c
index 439689b32a..de658b9103 100644
--- a/Python/traceback.c
+++ b/Python/traceback.c
@@ -592,7 +592,6 @@ _Py_DisplaySourceLine(PyObject *f, PyObject *filename, int lineno, int indent,
  *  Traceback (most recent call last):
  *    File "/home/isidentical/cpython/cpython/t.py", line 10, in <module>
  *      add_values(1, 2, 'x', 3, 4)
- *      ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  *    File "/home/isidentical/cpython/cpython/t.py", line 2, in add_values
  *      return a + b + c + d + e
  *             ~~~~~~^~~
@@ -736,7 +735,7 @@ print_error_location_carets(PyObject *f, int offset, Py_ssize_t start_offset, Py
     int special_chars = (left_end_offset != -1 || right_start_offset != -1);
     const char *str;
     while (++offset <= end_offset) {
-        if (offset <= start_offset || offset > end_offset) {
+        if (offset <= start_offset) {
             str = " ";
         } else if (special_chars && left_end_offset < offset && offset <= right_start_offset) {
             str = secondary;
@@ -792,6 +791,7 @@ tb_displayline(PyTracebackObject* tb, PyObject *f, PyObject *filename, int linen
 
     int code_offset = tb->tb_lasti;
     PyCodeObject* code = frame->f_frame->f_code;
+    const Py_ssize_t source_line_len = PyUnicode_GET_LENGTH(source_line);
 
     int start_line;
     int end_line;
@@ -813,7 +813,7 @@ tb_displayline(PyTracebackObject* tb, PyObject *f, PyObject *filename, int linen
     //
     //  ERROR LINE ERROR LINE ERROR LINE ERROR LINE ERROR LINE ERROR LINE ERROR LINE
     //        ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^~~~~~~~~~~~~~~~~~~~
-    //        |              |-> left_end_offset     |                  |-> left_offset
+    //        |              |-> left_end_offset     |                  |-> end_offset
     //        |-> start_offset                       |-> right_start_offset
     //
     // In general we will only have (start_offset, end_offset) but we can gather more information
@@ -822,6 +822,9 @@ tb_displayline(PyTracebackObject* tb, PyObject *f, PyObject *filename, int linen
     // the different ranges (primary_error_char and secondary_error_char). If we cannot obtain the
     // AST information or we cannot identify special ranges within it, then left_end_offset and
     // right_end_offset will be set to -1.
+    //
+    // To keep the column indicators pertinent, they are not shown when the primary character
+    // spans the whole line.
 
     // Convert the utf-8 byte offset to the actual character offset so we print the right number of carets.
     assert(source_line);
@@ -859,7 +862,7 @@ tb_displayline(PyTracebackObject* tb, PyObject *f, PyObject *filename, int linen
             goto done;
         }
 
-        Py_ssize_t i = PyUnicode_GET_LENGTH(source_line);
+        Py_ssize_t i = source_line_len;
         while (--i >= 0) {
             if (!IS_WHITESPACE(source_line_str[i])) {
                 break;
@@ -869,6 +872,13 @@ tb_displayline(PyTracebackObject* tb, PyObject *f, PyObject *filename, int linen
         end_offset = i + 1;
     }
 
+    // Elide indicators if primary char spans the frame line
+    Py_ssize_t stripped_line_len = source_line_len - truncation - _TRACEBACK_SOURCE_LINE_INDENT;
+    bool has_secondary_ranges = (left_end_offset != -1 || right_start_offset != -1);
+    if (end_offset - start_offset == stripped_line_len && !has_secondary_ranges) {
+        goto done;
+    }
+
     if (_Py_WriteIndentedMargin(margin_indent, margin, f) < 0) {
         err = -1;
         goto done;