Disallow range with upper bound in lhs of binop

src/expr.rs

@@ -1391,7 +1391,11 @@ pub(crate) mod parsing {
     ) -> Result<Expr> {
         loop {
             let ahead = input.fork();
-            if let Ok(op) = ahead.parse::<BinOp>() {
+            if let Expr::Range(ExprRange { end: Some(_), .. }) = lhs {
+                // A range with an upper bound cannot be the left-hand side of
+                // another binary operator.
+                break;
+            } else if let Ok(op) = ahead.parse::<BinOp>() {
                 let precedence = Precedence::of(&op);
                 if precedence < base {
                     break;

tests/test_expr.rs

@@ -379,30 +379,11 @@ fn test_range_precedence() {
     }
     "###);
 
-    // FIXME: should fail to parse. A range with a lower bound cannot be the
-    // upper bound of another range.
-    snapshot!(".. () .." as Expr, @r###"
-    Expr::Range {
-        limits: RangeLimits::HalfOpen,
-        end: Some(Expr::Range {
-            start: Some(Expr::Tuple),
-            limits: RangeLimits::HalfOpen,
-        }),
-    }
-    "###);
-
-    // FIXME: should fail to parse. A range with an upper bound cannot be the
-    // lower bound of another range.
-    snapshot!("() .. () .." as Expr, @r###"
-    Expr::Range {
-        start: Some(Expr::Range {
-            start: Some(Expr::Tuple),
-            limits: RangeLimits::HalfOpen,
-            end: Some(Expr::Tuple),
-        }),
-        limits: RangeLimits::HalfOpen,
-    }
-    "###);
+    // A range with a lower bound cannot be the upper bound of another range,
+    // and a range with an upper bound cannot be the lower bound of another
+    // range.
+    syn::parse_str::<Expr>(".. x ..").unwrap_err();
+    syn::parse_str::<Expr>("x .. x ..").unwrap_err();
 }
 
 #[test]