eagerly evaluate \@if conditions

src/atrule/if_rule.rs

@@ -22,12 +22,12 @@ pub(crate) struct If {
 
 #[derive(Debug, Clone)]
 pub(crate) struct Branch {
-    pub cond: Vec<Token>,
+    pub cond: Spanned<Value>,
     pub toks: Vec<Token>,
 }
 
 impl Branch {
-    pub fn new(cond: Vec<Token>, toks: Vec<Token>) -> Branch {
+    pub fn new(cond: Spanned<Value>, toks: Vec<Token>) -> Branch {
         Branch { cond, toks }
     }
 }
@@ -35,14 +35,17 @@ impl Branch {
 impl If {
     pub fn from_tokens<I: Iterator<Item = Token>>(
         toks: &mut PeekMoreIterator<I>,
+        scope: &Scope,
+        super_selector: &Selector,
         span_before: Span,
     ) -> SassResult<If> {
         devour_whitespace_or_comment(toks)?;
         let mut branches = Vec::new();
-        let init_cond = read_until_open_curly_brace(toks);
-        if toks.next().is_none() {
+        let init_toks = read_until_open_curly_brace(toks);
+        if init_toks.is_empty() || toks.next().is_none() {
             return Err(("Expected expression.", span_before).into());
         }
+        let init_cond = Value::from_vec(init_toks, scope, super_selector)?;
         devour_whitespace_or_comment(toks)?;
         let mut init_toks = read_until_closing_curly_brace(toks);
         if let Some(tok) = toks.next() {
@@ -76,7 +79,7 @@ impl If {
                 match tok.kind.to_ascii_lowercase() {
                     'i' if toks.next().unwrap().kind.to_ascii_lowercase() == 'f' => {
                         toks.next();
-                        let cond = read_until_open_curly_brace(toks);
+                        let cond = Value::from_vec(read_until_open_curly_brace(toks), scope, super_selector)?;
                         toks.next();
                         devour_whitespace(toks);
                         branches.push(Branch::new(cond, read_until_closing_curly_brace(toks)));
@@ -111,8 +114,7 @@ impl If {
         let mut toks = Vec::new();
         let mut found_true = false;
         for branch in self.branches {
-            let val = Value::from_vec(branch.cond, scope, super_selector)?;
-            if val.node.is_true(val.span)? {
+            if branch.cond.node.is_true(branch.cond.span)? {
                 toks = branch.toks;
                 found_true = true;
                 break;

src/atrule/mod.rs

@@ -216,7 +216,7 @@ impl AtRule {
                 span: kind_span,
             },
             AtRuleKind::If => Spanned {
-                node: AtRule::If(If::from_tokens(toks, kind_span)?),
+                node: AtRule::If(If::from_tokens(toks, scope, super_selector, kind_span)?),
                 span: kind_span,
             },
             AtRuleKind::For => Spanned {

tests/if.rs

@@ -113,8 +113,18 @@ error!(
     "@if",
     "Error: Expected expression."
 );
+error!(
+    no_condition,
+    "@if{}",
+    "Error: Expected expression."
+);
 error!(
     nothing_after_open_curly,
     "@if foo {",
     "Error: expected \"}\"."
 );
+error!(
+    condition_is_evaluated_eagerly,
+    "@if 1 + 1 =s {\n}",
+    "Error: expected \"=\"."
+);