tr: a large `[c*n]` repeat count is materialized unbounded — `capacity overflow` panic / allocation abort (exit 134)

tr accepts any repeat count in the [c*n] construct up to u64::MAX, then eagerly materializes the repeat into a Vec<u8> — one byte per repetition. A count in the 10^14 range aborts the process with an allocation failure; a count at or above 2^63 aborts with a capacity overflow panic.

Steps to reproduce

$ tr '[a*9223372036854775808]' b </dev/null
thread 'main' panicked at library/alloc/src/raw_vec/mod.rs:28:5:
capacity overflow
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Aborted (core dumped)
$ echo $?
134

$ tr '[a*99999999999999]' b </dev/null
memory allocation of 99999999999999 bytes failed
Aborted (core dumped)
$ echo $?
134

Root cause

Sequence::flatten expands a repeat into an iterator of n bytes:

// src/uu/tr/src/operation.rs:210, 215 
pub fn flatten(&self) -> Box<dyn Iterator<Item = u8>> {
    match self {
        // ...
        Self::CharRepeat(c, n) => Box::new(std::iter::repeat_n(*c, *n)),

The two consumers of that iterator fail in two different ways:

// src/uu/tr/src/operation.rs:295  -- SET1: allocates n bytes up front
let mut set1_solved: Vec<u8> = set1.iter().flat_map(Self::flatten).collect();

// src/uu/tr/src/operation.rs:301-308 -- SET2: iterates n times
let set2_len = set2
            .iter()
            .filter_map(|s| match s {
                Self::CharStar(_) => None,
                r => Some(r),
            })
            .flat_map(Self::flatten)
            .count();

• collect() (SET1) — Box<dyn Iterator> forwards size_hint, and repeat_n reports it exactly, so Vec's SpecFromIterNested reserves all n bytes in one request. That is the abort.

Start in src/uu/tr/src/operation.rs at Sequence::flatten and the SET1 and SET2 consumers around the cited lines. Run the two reproduction commands to observe the allocation failure and capacity-overflow panic; done means large repeat counts no longer abort the tr process, while the existing repeat behavior remains correct.