start with the name as given, and cap the number to be switched

src/cargo/sources/registry/index.rs

@@ -105,21 +105,22 @@ impl<'cfg> RegistryIndex<'cfg> {
  3 => format!("3/{}/{}", &fs_name[..1], fs_name),
  _ => format!("{}/{}/{}", &fs_name[0..2], &fs_name[2..4], fs_name),
  };
- let num_hyphen_underscore = raw_path.chars()
- .filter(|&c| c == '_' || c == '-')
- .count();
+ let num_hyphen_underscore = ::std::cmp::min(
+ raw_path.chars().filter(|&c| c == '_' || c == '-').count(),
+ 10,
+ ) as u16;
  let mut ret = Vec::new();
  // Crates.io treats hyphen and underscores as interchangeable
  // but, the index and old cargo do not. So the index must store uncanonicalized version
  // of the name so old cargos can find it.
- // This loop tries all possible combinations of
+ // This loop tries all possible combinations of switching
  // hyphen and underscores to find the uncanonicalized one.
  for hyphen_combination_num in 0u16..(1 << num_hyphen_underscore) {
  let path = raw_path
  .chars()
- .scan(0u32, |s, c| {
- if c == '_' || c == '-' {
- let out = Some(if (hyphen_combination_num & (1u16 << *s)) > 0 {
+ .scan(0u16, |s, c| {
+ if (c == '_' || c == '-') && *s <= num_hyphen_underscore {
+ let out = Some(if (c == '_') ^ ((hyphen_combination_num & (1u16 << *s)) > 0) {
  '_'
  } else {
  '-'