fix(i18n): fill placeholders in one pass (#782) (#1063)

A value holding {other} was expanded again by a later argument. fmt now
walks the template once, so values stay exactly as they came.

crates/ui-egui/src/i18n/tests.rs

@@ -5,7 +5,19 @@ use std::collections::{BTreeSet, HashSet};
 
 use super::catalog::{Catalog, unescape};
 use super::lang::{name_sort_key, plural_czech, plural_east_slavic, plural_polish};
-use super::{LANGUAGES, Lang, LangInfo, lang_from_tag, message, tr, trn};
+use super::{LANGUAGES, Lang, LangInfo, fmt, lang_from_tag, message, tr, trn};
+
+#[test]
+fn a_filled_in_value_is_never_read_as_a_placeholder() {
+    assert_eq!(fmt("{name}: {error}", &[("name", "{error}"), ("error", "disk full")]), "{error}: disk full");
+    assert_eq!(fmt("{a} and {a}", &[("a", "{a}")]), "{a} and {a}");
+}
+
+#[test]
+fn placeholders_nobody_filled_stay_as_written() {
+    assert_eq!(fmt("{a} {b} { {} }{", &[("a", "1")]), "1 {b} { {} }{");
+    assert_eq!(fmt("{{a}", &[("a", "1")]), "{1");
+}
 
 /// Each catalog's rows as (line number, context, source, translation), split exactly as the
 /// parser does: `trim_end` only, since a plain row *starts* with a TAB.

crates/ui-egui/src/i18n/translate.rs

@@ -53,12 +53,22 @@ pub fn t_id<'a>(id: &str, label: &'a str) -> &'a str {
 #[cfg(test)]
 pub(crate) use strict::{StrictCjkDrawing, drew_cjk};
 
-/// Fill `{name}` placeholders. Unknown placeholders are left as written.
+/// Fill `{name}` placeholders in one pass. Values and unknown placeholders stay as written.
 pub fn fmt(template: &str, args: &[(&str, &str)]) -> String {
-    let mut out = template.to_string();
-    for (k, v) in args {
-        out = out.replace(&format!("{{{k}}}"), v);
+    let mut out = String::with_capacity(template.len());
+    let mut rest = template;
+    while let Some((before, after)) = rest.split_once('{') {
+        out.push_str(before);
+        let filled = after.split_once('}').and_then(|(name, tail)| args.iter().find(|(k, _)| *k == name).map(|(_, v)| (*v, tail)));
+        if let Some((value, tail)) = filled {
+            out.push_str(value);
+            rest = tail;
+        } else {
+            out.push('{');
+            rest = after;
+        }
     }
+    out.push_str(rest);
     out
 }