fix: use utf-8 length instead of byte length

Author: kamadorueda

CHANGELOG.md

@@ -17,6 +17,18 @@ Types of changes
 - Security in case of vulnerabilities.
 -->
 
+### Fixed
+
+- Multiline strings are handled as utf-8 correctly, preventing panics
+  on utf-8 whitespace like:
+
+  ```nix
+    ''
+    foo
+  \u{2002}bar
+  ''
+  ```
+
 ## [0.5.0] - 2022-02-23
 
 ### Changed

src/alejandra_engine/src/rules/string.rs

@@ -73,7 +73,7 @@ pub fn rule(
             if !line.is_empty() {
                 indentation = usize::min(
                     indentation,
-                    line.len() - line.trim_start().len(),
+                    line.chars().count() - line.trim_start().chars().count(),
                 );
             }
         }
@@ -85,8 +85,8 @@ pub fn rule(
         lines = lines
             .iter()
             .map(|line| {
-                if indentation < line.len() {
-                    line[indentation..line.len()].to_string()
+                if indentation < line.chars().count() {
+                    line.chars().skip(indentation).collect::<String>()
                 } else {
                     line.to_string()
                 }

src/alejandra_engine/tests/cases/string/in

@@ -1,4 +1,8 @@
 [
+  ''
+  foo
+ bar
+''
   ""
 ###
   "

src/alejandra_engine/tests/cases/string/out

@@ -1,4 +1,8 @@
 [
+  ''
+     foo
+    bar
+  ''
   ""
   ###
   "