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display: fix fmt_hex_max! to not panic when given precision greater than byte length #261

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Merged
apoelstra merged 2 commits into from
Sep 19, 2026
Merged

display: fix fmt_hex_max! to not panic when given precision greater than byte length #261

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0fde95f
display: fix fmt_hex_max! to not panic when given precision greater t…
apoelstra Sep 17, 2026
86fe196
display: add regression test for fmt_hex_max! with precision between …
apoelstra Sep 17, 2026
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17 changes: 16 additions & 1 deletion src/display.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -561,7 +561,7 @@ where
Some(p) if p < N => {
let n = p.div_ceil(2);
encoder.put_bytes(bytes.into_iter().take(n));
&encoder.as_str()[..p]
encoder.as_str().get(..p).unwrap_or(encoder.as_str())
}
_ => {
encoder.put_bytes(bytes);
Expand Down Expand Up @@ -688,6 +688,21 @@ mod tests {
assert_eq!(format!("{:.65}", dummy), "2a".repeat(32));
}

#[test]
fn fmt_hex_max_short_bytes_with_precision() {
// `fmt_hex_max!` documents that `$bytes.len() <= $len` is acceptable. Ensure that
// if you have $bytes.len() strictly less than $len, and you attempt to use a
// precision value between the two lengths, nothing bad happens.
struct Short([u8; 2]);
impl fmt::Display for Short {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
fmt_hex_max!(f, 4, &self.0, Case::Lower)
}
}
let s = format!("{:.5}", Short([0x12, 0x34]));
assert_eq!(s, "1234");
}

struct TestHexUpperLower<'a>(&'a [u8], bool);

impl fmt::Display for TestHexUpperLower<'_> {
Expand Down