Fix str.expandtabs aborting on a tab size of zero (#8562)

`"a\tb".expandtabs(0)` panicked with a capacity overflow. CPython returns
`'ab'`: with no width to advance to, the tabs come out and nothing else
moves. `expandtabs(-1)` is the same call, since `ExpandTabsArgs::tabsize`
sends every negative value to 0.

`expandtabs` keeps the tab stop in `tab_size` and the current column in
`col_count`, and on a tab it does `tab_size - col_count`. With a tab size of
zero both start at 0, the first character makes `col_count` 1 while
`tab_size` stays 0, and the subtraction underflows. The run of spaces asked
for next is `usize::MAX`, and the allocation aborts the process. A tab has to
follow something on the line to reach it: `"\ta".expandtabs(0)` subtracts 0
from 0 and comes out right by accident.

`BytesInner::expandtabs` already returns early for this and filters the tabs
out. The string version now does the same.

Assisted-by: Claude Code:claude-opus-5

crates/common/src/str.rs

@@ -743,6 +743,15 @@ pub mod levenshtein {
 /// Replace all tabs in a string with spaces, using the given tab size.
 #[must_use]
 pub fn expandtabs(input: &str, tab_size: usize) -> String {
+    // A tab size of zero, which is also where a negative one lands, leaves no
+    // column for a tab to advance to: the tabs come out and nothing else moves.
+    // Going through the arithmetic anyway subtracts the current column from a
+    // tab stop of zero and underflows on the first tab, so the width asked for
+    // next is `usize::MAX`. The bytes version of this already returns here.
+    if tab_size == 0 {
+        return input.chars().filter(|ch| *ch != '\t').collect();
+    }
+
     let tab_stop = tab_size;
     let mut expanded_str = String::with_capacity(input.len());
     let mut tab_size = tab_stop;
@@ -905,4 +914,28 @@ mod tests {
         let s = "0😀😃😄😁😆😅😂🤣9";
         assert_eq!(get_chars(s, 3..7), "😄😁😆😅");
     }
+
+    #[test]
+    fn expandtabs_with_zero_tab_size_drops_tabs() {
+        // A tab that follows a character used to subtract that column from a
+        // tab stop of zero, so the width of the run of spaces came out as
+        // `usize::MAX` and the allocation aborted the process.
+        assert_eq!(expandtabs("a\tb", 0), "ab");
+        assert_eq!(expandtabs("ab\tcd\tef", 0), "abcdef");
+        assert_eq!(expandtabs("a\nb\tc", 0), "a\nbc");
+        assert_eq!(expandtabs("á\tb", 0), "áb");
+        assert_eq!(expandtabs("\ta", 0), "a");
+        assert_eq!(expandtabs("\t", 0), "");
+        assert_eq!(expandtabs("", 0), "");
+        assert_eq!(expandtabs("no tabs", 0), "no tabs");
+    }
+
+    #[test]
+    fn expandtabs_with_a_real_tab_size_is_unchanged() {
+        assert_eq!(expandtabs("a\tb", 8), "a       b");
+        assert_eq!(expandtabs("a\tb", 1), "a b");
+        assert_eq!(expandtabs("abcd\te", 4), "abcd    e");
+        assert_eq!(expandtabs("a\nb\tc", 4), "a\nb   c");
+        assert_eq!(expandtabs("\ta", 4), "    a");
+    }
 }

extra_tests/snippets/builtin_str.py

@@ -951,3 +951,31 @@ def test_replace_empty_pattern():
 
 
 test_replace_empty_pattern()
+
+
+def test_expandtabs_zero_tabsize():
+    # With no width to advance to, the tabs come out and nothing else moves.
+    # A tab that followed a character used to ask for a run of usize::MAX
+    # spaces and take the interpreter down with it.
+    for tabsize in (0, -1, -8):
+        assert "a\tb".expandtabs(tabsize) == "ab"
+        assert "ab\tcd\tef".expandtabs(tabsize) == "abcdef"
+        assert "a\nb\tc".expandtabs(tabsize) == "a\nbc"
+        assert "a\r\nb\tc".expandtabs(tabsize) == "a\r\nbc"
+        assert "á\tb".expandtabs(tabsize) == "áb"
+        assert "😀\tb".expandtabs(tabsize) == "😀b"
+        assert "\ta".expandtabs(tabsize) == "a"
+        assert "\t".expandtabs(tabsize) == ""
+        assert "".expandtabs(tabsize) == ""
+        assert "no tabs".expandtabs(tabsize) == "no tabs"
+        assert b"a\tb".expandtabs(tabsize) == b"ab"
+        assert bytearray(b"a\tb").expandtabs(tabsize) == bytearray(b"ab")
+
+    # A tab size that is actually there keeps working.
+    assert "a\tb".expandtabs(8) == "a       b"
+    assert "a\tb".expandtabs(1) == "a b"
+    assert "abcd\te".expandtabs(4) == "abcd    e"
+    assert "a\nb\tc".expandtabs(4) == "a\nb   c"
+
+
+test_expandtabs_zero_tabsize()