diff panics (`into_string().unwrap()`) on a non-UTF-8 argument ending in `--width=N`

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
85/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Quiet
Tech stack
rust
Domain
cli

Research direction

Start in src/params.rs around the width and tabsize regexes at lines 62-63, then inspect the argument handling around line 116. Reproduce with the non-UTF-8 --width argument shown in the issue and verify that the argument is treated as a file operand rather than causing a panic or being accepted as an option; also check the xyz--width=5 case.

Written by the indexing model from the issue text.

Description

Steps to reproduce

$ printf 'a\nb\nc\n' > A; printf 'a\nX\nc\n' > B
$ diffutils diff $'\xff--width=5' A B
thread 'main' panicked at src/params.rs:116:45:
called `Result::unwrap()` on an `Err` value: "\xFF--width=5"
$ echo $?
134

diff aborts (panic, exit 134) when given a non-UTF-8 argument whose lossy form ends in --width=<digits> — e.g. an argument with a leading invalid byte like $'\xff--width=5'.

Expected behavior

Match GNU: the unrecognized argument is a file operand; with three operands diff reports the extra operand and exits 2.

$ /usr/bin/diff $'\xff--width=5' A B
diff: extra operand 'B'
$ echo $?
2

Root cause

The --width regex lacks a start anchor, unlike the sibling --tabsize one:

// src/params.rs:62-63
let tabsize_re = Regex::new(r"^--tabsize=(?<num>\d+)$").unwrap();   // anchored — safe
let width_re   = Regex::new(r"--width=(?P<long>\d+)$").unwrap();    // no leading ^

// src/params.rs:115-116
if width_re.is_match(param.to_string_lossy().as_ref()) {   // matches lossy form
    let param = param.into_string().unwrap();              // line 116: Err on non-UTF-8

to_string_lossy() maps invalid bytes to U+FFFD, so a non-UTF-8 argument whose tail is --width=N still matches; into_string() then fails on the real bytes.
Fix: anchor the regex at the start (^--width=…$, matching tabsize_re), and/or match on the bytes / avoid into_string().unwrap() so a non-UTF-8 argument falls through to the operand path. (The unanchored regex also makes diff xyz--width=5 silently accept a width option instead of treating it as a filename — same root cause, non-panic symptom.)

Found by our static analysis tooling.

Dominant language
Rust
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Forks
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Avg merge
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Merged PRs (30d)
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