more robustly parse @else if

src/parse/control_flow.rs

@@ -69,13 +69,16 @@ impl<'a> Parser<'a> {
             self.whitespace_or_comment();
             if let Some(tok) = self.toks.peek() {
                 match tok.kind {
-                    'i' if matches!(
-                        self.toks.peek_forward(1),
-                        Some(Token { kind: 'f', .. }) | Some(Token { kind: 'F', .. })
-                    ) =>
-                    {
-                        self.toks.next();
-                        self.toks.next();
+                    'i' | 'I' | '\\' => {
+                        self.span_before = tok.pos;
+                        let mut ident = self.parse_identifier_no_interpolation(false)?;
+
+                        ident.node.make_ascii_lowercase();
+
+                        if ident.node != "if" {
+                            return Err(("expected \"{\".", ident.span).into());
+                        }
+
                         let cond = if found_true {
                             self.throw_away_until_open_curly_brace()?;
                             false
@@ -84,6 +87,7 @@ impl<'a> Parser<'a> {
                             self.expect_char('{')?;
                             v
                         };
+
                         if cond {
                             found_true = true;
                             self.scopes.enter_new_scope();

tests/charset.rs

@@ -18,11 +18,9 @@ test!(
 );
 error!(
     invalid_charset_value,
-    "@charset 1;",
-    "Error: Expected string."
+    "@charset 1;", "Error: Expected string."
 );
 error!(
     invalid_charset_value_unquoted_string,
-    "@charset a;",
-    "Error: Expected string."
+    "@charset a;", "Error: Expected string."
 );

tests/if.rs

@@ -236,3 +236,27 @@ error!(
     denies_interpolated_at_rule,
     "@#{if} true { a {   color: red; } }", "Error: expected \"(\"."
 );
+test!(
+    else_if_escaped_lower_i,
+    r"@if false {
+    } @else \69 f true {
+      /**/
+    }",
+    "/**/\n"
+);
+test!(
+    else_if_escaped_lower_both,
+    r"@if false {
+    } @else \69 \66  true {
+      /**/
+    }",
+    "/**/\n"
+);
+test!(
+    else_if_uppercase,
+    r"@if false {
+    } @else IF true {
+      /**/
+    }",
+    "/**/\n"
+);