Make sure to escape formatting of char literals

Closes #4682

crates/compiler/fmt/src/expr.rs

@@ -298,17 +298,7 @@ impl<'a> Formattable for Expr<'a> {
             }
             SingleQuote(string) => {
                 buf.indent(indent);
-                buf.push('\'');
-                for c in string.chars() {
-                    if c == '"' {
-                        buf.push_char_literal('"')
-                    } else {
-                        for escaped in c.escape_default() {
-                            buf.push_char_literal(escaped);
-                        }
-                    }
-                }
-                buf.push('\'');
+                format_sq_literal(buf, string);
             }
             &NonBase10Int {
                 base,
@@ -438,6 +428,20 @@ impl<'a> Formattable for Expr<'a> {
     }
 }
 
+pub(crate) fn format_sq_literal(buf: &mut Buf, s: &str) {
+    buf.push('\'');
+    for c in s.chars() {
+        if c == '"' {
+            buf.push_char_literal('"')
+        } else {
+            for escaped in c.escape_default() {
+                buf.push_char_literal(escaped);
+            }
+        }
+    }
+    buf.push('\'');
+}
+
 fn starts_with_newline(expr: &Expr) -> bool {
     use roc_parse::ast::Expr::*;
 

crates/compiler/fmt/src/pattern.rs

@@ -1,5 +1,5 @@
 use crate::annotation::{Formattable, Newlines, Parens};
-use crate::expr::fmt_str_literal;
+use crate::expr::{fmt_str_literal, format_sq_literal};
 use crate::spaces::{fmt_comments_only, fmt_spaces, NewlineAt};
 use crate::Buf;
 use roc_parse::ast::{Base, CommentOrNewline, Pattern};
@@ -155,9 +155,7 @@ impl<'a> Formattable for Pattern<'a> {
             StrLiteral(literal) => fmt_str_literal(buf, *literal, indent),
             SingleQuote(string) => {
                 buf.indent(indent);
-                buf.push('\'');
-                buf.push_str(string);
-                buf.push('\'');
+                format_sq_literal(buf, string);
             }
             Underscore(name) => {
                 buf.indent(indent);

crates/compiler/fmt/tests/test_fmt.rs

@@ -5673,6 +5673,18 @@ mod test_fmt {
         ));
     }
 
+    #[test]
+    fn format_char_pattern() {
+        expr_formats_same(indoc!(
+            r#"
+            when x is
+                ' ' -> x
+                '\n' -> x
+                '\t' -> x
+            "#
+        ));
+    }
+
     #[test]
     fn format_nested_pipeline() {
         expr_formats_same(indoc!(