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Document (and test) the behavior of the precision field with int types #51

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Merged
nicholasbishop merged 3 commits into from
Jan 25, 2026
Merged

Document (and test) the behavior of the precision field with int types #51

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4ff2b60
Make differences list formatting more consistent
nicholasbishop Jan 25, 2026
a816de6
Document behavior of precision with ints
nicholasbishop Jan 25, 2026
5e40a08
Add a test for the behavior of int precision
nicholasbishop Jan 25, 2026
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10 changes: 6 additions & 4 deletions src/output.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -221,14 +221,16 @@ macro_rules! define_unumeric {
///
/// - only valid UTF-8 data can be printed.
/// - an `X` format specifier with a `#` flag prints the hex data in uppercase,
/// but the leading `0x` is still lowercase
/// but the leading `0x` is still lowercase.
/// - an `o` format specifier with a `#` flag precedes the number with an `o`
/// instead of `0`
/// instead of `0`.
/// - `g`/`G` (shorted floating point) is aliased to `f`/`F`` (decimal floating
/// point)
/// - same for `a`/`A` (hex floating point)
/// point).
/// - same for `a`/`A` (hex floating point).
/// - the `n` format specifier, [`Specifier::WriteBytesWritten`], is not
/// implemented and will cause an error if encountered.
/// - precision is ignored for integral types, instead of specifying the
/// minimum number of digits.
pub fn fmt_write(w: &mut impl fmt::Write) -> impl FnMut(Argument) -> c_int + '_ {
use fmt::Write;
move |Argument {
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14 changes: 14 additions & 0 deletions tests/tests.rs
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Expand Up @@ -256,3 +256,17 @@ fn test_errors() {
assert_fmt_err(c"%");
assert_fmt_err(c"%1");
}

/// Test that specifying precision with an integral type does _not_
/// match C. This is one of the documented differences in
/// `fmt_write`. If this is fixed, make sure to update the
/// documentation.
///
/// https://github.com/lights0123/printf-compat/issues/50
#[test]
fn test_int_precision() {
unsafe {
assert_eq!(c_fmt!(c"%.3d", 1).1, "001");
assert_eq!(rust_fmt(c"%.3d".as_ptr(), 1).1, "1");
}
}