hint for unknown variable containing dash (#924)

src/eval/scope.rs

@@ -42,7 +42,7 @@ impl<'a> Scopes<'a> {
             .chain(self.scopes.iter().rev())
             .chain(self.base.map(|base| base.global.scope()))
             .find_map(|scope| scope.get(var))
-            .ok_or_else(|| eco_format!("unknown variable: {}", var))
+            .ok_or_else(|| unknown_variable(var))
     }
 
     /// Try to access a variable immutably in math.
@@ -62,12 +62,22 @@ impl<'a> Scopes<'a> {
             .ok_or_else(|| {
                 match self.base.and_then(|base| base.global.scope().get(var)) {
                     Some(_) => eco_format!("cannot mutate a constant: {}", var),
-                    _ => eco_format!("unknown variable: {}", var),
+                    _ => unknown_variable(var),
                 }
             })?
     }
 }
 
+/// The error message when a variable is not found.
+#[cold]
+fn unknown_variable(var: &str) -> EcoString {
+    if var.contains('-') {
+        eco_format!("unknown variable: {} – if you meant to use subtraction, try adding spaces around the minus sign.", var)
+    } else {
+        eco_format!("unknown variable: {}", var)
+    }
+}
+
 /// A map from binding names to values.
 #[derive(Default, Clone, Hash)]
 pub struct Scope(BTreeMap<EcoString, Slot>, bool);

tests/typ/compiler/hint.typ

@@ -4,3 +4,19 @@
 ---
 // Error: 1:17-1:19 expected length, found integer: a length needs a unit – did you mean 12pt?
 #set text(size: 12)
+
+---
+#{
+  let a = 2
+  a = 1-a
+  a = a -1
+
+  // Error: 7-10 unknown variable: a-1 – if you meant to use subtraction, try adding spaces around the minus sign.
+  a = a-1
+}
+
+---
+#{
+  // Error: 3-6 unknown variable: a-1 – if you meant to use subtraction, try adding spaces around the minus sign.
+  a-1 = 2
+}