Handle Unicode double-quote characters when parsing arguments (#1553)

Closes #1545

src/framework/standard/args.rs

@@ -103,19 +103,49 @@ impl Token {
     }
 }
 
+// A utility enum to handle an edge case with Apple OSs.
+//
+// By default, a feature called "Smart Quotes" is enabled on MacOS and iOS devices. This feature
+// automatically substitutes the lame, but simple `"` ASCII character for quotation with the cool
+// `”` Unicode character. It can be disabled, but users may not want to do that as it is a global
+// setting (i.e. they might not want to disable it just for properly invoking commands of bots on
+// Discord).
+#[derive(Clone, Copy)]
+enum QuoteKind {
+    Ascii,
+    Apple,
+}
+
+impl QuoteKind {
+    fn new(c: char) -> Option<Self> {
+        match c {
+            '"' => Some(QuoteKind::Ascii),
+            '\u{201C}' => Some(QuoteKind::Apple),
+            _ => None,
+        }
+    }
+
+    fn is_ending_quote(self, c: char) -> bool {
+        match self {
+            QuoteKind::Ascii => c == '"',
+            QuoteKind::Apple => c == '\u{201D}',
+        }
+    }
+}
+
 fn lex(stream: &mut Stream<'_>, delims: &[Cow<'_, str>]) -> Option<Token> {
     if stream.is_empty() {
         return None;
     }
 
     let start = stream.offset();
-    if stream.current()? == b'"' {
-        stream.next();
+    if let Some(kind) = QuoteKind::new(stream.current_char()?) {
+        stream.next_char();
 
-        stream.take_until(|b| b == b'"');
+        stream.take_until_char(|c| kind.is_ending_quote(c));
 
-        let is_quote = stream.current().map_or(false, |b| b == b'"');
-        stream.next();
+        let is_quote = stream.current_char().map_or(false, |c| kind.is_ending_quote(c));
+        stream.next_char();
 
         let end = stream.offset();
 
@@ -150,12 +180,35 @@ fn lex(stream: &mut Stream<'_>, delims: &[Cow<'_, str>]) -> Option<Token> {
     Some(Token::new(TokenKind::Argument, start, end))
 }
 
+fn is_surrounded_with(s: &str, begin: char, end: char) -> bool {
+    s.starts_with(begin) && s.ends_with(end)
+}
+
+fn is_quoted(s: &str) -> bool {
+    if s.len() < 2 {
+        return false;
+    }
+
+    // Refer to `QuoteKind` why we check for Unicode quote characters.
+    is_surrounded_with(s, '"', '"') || is_surrounded_with(s, '\u{201C}', '\u{201D}')
+}
+
+fn strip(s: &str, begin: char, end: char) -> Option<&str> {
+    let s = s.strip_prefix(begin)?;
+    s.strip_suffix(end)
+}
+
 fn remove_quotes(s: &str) -> &str {
-    if s.starts_with('"') && s.ends_with('"') {
-        return &s[1..s.len() - 1];
+    if s.len() < 2 {
+        return s;
+    }
+
+    if let Some(s) = strip(s, '"', '"') {
+        return s;
     }
 
-    s
+    // Refer to `QuoteKind` why we check for Unicode quote characters.
+    strip(s, '\u{201C}', '\u{201D}').unwrap_or(s)
 }
 
 #[derive(Debug, Clone, Copy)]
@@ -299,11 +352,8 @@ impl Args {
             .collect::<Vec<_>>();
 
         let args = if delims.is_empty() && !message.is_empty() {
-            let kind = if message.starts_with('"') && message.ends_with('"') {
-                TokenKind::QuotedArgument
-            } else {
-                TokenKind::Argument
-            };
+            let kind =
+                if is_quoted(message) { TokenKind::QuotedArgument } else { TokenKind::Argument };
 
             // If there are no delimiters, then the only possible argument is the whole message.
             vec![Token::new(kind, 0, message.len())]