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Titlecase the first-of-word code point without leading adjustment #8241

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Merged
youknowone merged 1 commit into from
Jul 8, 2026
+27 −1
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Titlecase the first-of-word code point without leading adjustment #8241

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28 changes: 27 additions & 1 deletion crates/unicode/src/case.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,7 @@ use alloc::{
vec::Vec,
};

use icu_casemap::options::{LeadingAdjustment, TitlecaseOptions};
use icu_casemap::{CaseMapper, TitlecaseMapper};
use icu_locale::LanguageIdentifier;
use icu_properties::props::{
Expand Down Expand Up @@ -236,8 +237,16 @@ fn titlecase_string(s: &str, out: &mut FmtWriter<'_>) {
}

fn titlecase_segment(s: &str, out: &mut FmtWriter<'_>) {
// Callers pass a single first-of-word code point, which Python titlecases
// unconditionally (applying its titlecase mapping). The default `Auto`
// leading adjustment looks for a head in Letter/Number/Symbol/Private_Use
// and skips anything else, dropping the titlecase mapping of cased marks
// such as U+0345 (`ͅ`, general category Mn) -> U+0399 (`Ι`). `None`
// titlecases the code point as given.
let mut options = TitlecaseOptions::default();
options.leading_adjustment = Some(LeadingAdjustment::None);
TitlecaseMapper::new()
.titlecase_segment(s, &LanguageIdentifier::UNKNOWN, Default::default())
.titlecase_segment(s, &LanguageIdentifier::UNKNOWN, options)
.write_to(out)
.expect("writing to an in-memory buffer cannot fail");
}
Expand Down Expand Up @@ -353,6 +362,23 @@ mod tests {
assert_eq!(swapcase_str("Hello"), "hELLO");
}

#[test]
fn titlecase_first_of_word_takes_titlecase_mapping() {
// A leading cased combining mark still takes its titlecase mapping:
// U+0345 (ͅ, general category Mn) titlecases to U+0399 (Ι), even though
// it is not a Letter/Number/Symbol head.
assert_eq!(title_str("\u{0345}"), "\u{0399}");
assert_eq!(capitalize_str("\u{0345}"), "\u{0399}");
assert_eq!(title_str("\u{0345}a"), "\u{0399}a");
// Full (one-to-many) titlecase mappings still apply to the first
// character of each word.
// cspell:ignore finnish NNISH dzungla Dzungla ßhello Sshello
assert_eq!(capitalize_str("finnish"), "Finnish");
assert_eq!(title_str("fiNNISH"), "Finnish");
assert_eq!(capitalize_str("dzungla"), "Dzungla");
assert_eq!(capitalize_str("ßhello"), "Sshello");
}

#[test]
fn wtf8_passes_surrogates_through() {
let mut buf = Wtf8Buf::from("ab cd");
Expand Down
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