parser: start enforcing non-associativity

parser/src/diagnostics.rs

@@ -73,6 +73,8 @@ pub enum ParsingError {
     SpecialVariableCall(Span, String),
     #[error("Can't use special variable `{}` for indirect function call.", .1)]
     SpecialVariableIndirectCall(Span, String),
+    #[error("Can't chain non-associative operators.")]
+    NonAssociativeOperator(Span),
 }
 
 impl ParsingError {
@@ -118,6 +120,7 @@ impl ParsingError {
             Self::UnexpectedTypedRegex(span) => Some(span.clone()),
             Self::SpecialVariableCall(span, _) => Some(span.clone()),
             Self::SpecialVariableIndirectCall(span, _) => Some(span.clone()),
+            Self::NonAssociativeOperator(span) => Some(span.clone()),
         }
     }
     fn hint(&self) -> Option<&'static str> {
@@ -151,6 +154,9 @@ impl ParsingError {
             Self::UnexpectedTypedRegex(_) => Some(
                 "This is only valid in some contexts, like a right-hand assignment or a function argument.",
             ),
+            Self::NonAssociativeOperator(_) => Some(
+                "Some operators can't be chained to avoid logical errors, such as comparison ones.\nExample: write `a == b && b == c` instead of `a == b == c`.",
+            ),
             _ => None,
         }
     }

parser/src/pratt.rs

@@ -231,6 +231,12 @@ impl<'a, 'b> Pratt<'a, 'b> {
         self.typecheck(lex, &lhs)?;
         // This is just a parsing construct; we only skip if it's a real token.
         lex.consume_with(|_| op != BinaryOperator::Concat);
+        // Checks invalids like `a == b == c`. The docs are ambiguous about the
+        // associativity of redirection operators, but I couldn't get awk to
+        // error out when chaining them.
+        if op.is_non_associative() && lhs.is_non_associative() {
+            return Err(ParsingError::NonAssociativeOperator(lex.span()));
+        }
         self.typed_regex = matches!(op, BinaryOperator::Matches | BinaryOperator::MatchesNot);
 
         let rhs = self.parse_expression(lex, op.binding_power().1)?;
@@ -352,6 +358,32 @@ impl<'a, 'b> Pratt<'a, 'b> {
     }
 }
 
+trait NonAssociativity {
+    fn is_non_associative(&self) -> bool;
+}
+
+impl NonAssociativity for Expr<'_> {
+    fn is_non_associative(&self) -> bool {
+        matches!(
+            self,
+            Expr::Node(x) if matches!(x.as_ref(), ExprNode::BinaryOperation(
+                op,
+                _,
+                _
+            ) if op.is_non_associative())
+        )
+    }
+}
+
+impl NonAssociativity for BinaryOperator {
+    fn is_non_associative(&self) -> bool {
+        matches!(
+            self,
+            Self::Eq | Self::NEq | Self::Gt | Self::Lt | Self::LtE | Self::GtE,
+        )
+    }
+}
+
 fn is_special_var(name: &str) -> bool {
     matches!(
         name,