From 2c7c85c2803a1845270afea2c7b6da7ac730fb81 Mon Sep 17 00:00:00 2001
From: Ian Hobson <ian.r.hobson@gmail.com>
Date: Fri, 5 Apr 2024 08:11:45 +0200
Subject: [PATCH] Fix support for single-char-but-multi-byte fill characters

---
 crates/parser/src/string_format_options.rs | 12 +++++++++++-
 docs/language_guide.md                     |  7 ++++++-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/crates/parser/src/string_format_options.rs b/crates/parser/src/string_format_options.rs
index 9b1fd2ef0..a2947d18a 100644
--- a/crates/parser/src/string_format_options.rs
+++ b/crates/parser/src/string_format_options.rs
@@ -46,7 +46,8 @@ impl StringFormatOptions {
             match (next, chars.peek(), position) {
                 // Check for single-char fill character at the start of the string
                 (_, Some('<' | '^' | '>'), Start) => {
-                    result.fill_character = Some(add_string_constant(&format_string[0..1])?);
+                    result.fill_character =
+                        Some(add_string_constant(&format_string[0..next.len_utf8()])?);
                     result.alignment = char_to_alignment(chars.next().unwrap());
                     position = MinWidth;
                 }
@@ -205,6 +206,15 @@ mod tests {
                     ..Default::default()
                 },
             ),
+            (
+                "𝜇<.9",
+                StringFormatOptions {
+                    alignment: StringAlignment::Left,
+                    fill_character: Some(0),
+                    precision: Some(9),
+                    ..Default::default()
+                },
+            ),
             (
                 "🫶🏽>20.10",
                 StringFormatOptions {
diff --git a/docs/language_guide.md b/docs/language_guide.md
index a8e77aee5..b0aabedac 100644
--- a/docs/language_guide.md
+++ b/docs/language_guide.md
@@ -1336,7 +1336,12 @@ check! ('l', 'l', 'ø')
 Interpolated string expressions can be formatted using formatting options
 similar to [Rust's][rust-format-options].
 
-Options can be provided after a `:` separator inside the `{}` expression.
+Inside an interpolated expression, options are provided after a `:` separator.
+
+```koto
+print! '{number.pi:𝜋^8.2}'
+check! 𝜋𝜋3.14𝜋𝜋
+```
 
 ### Minimum Width and Alignment