Brace variable names in expression text that would not read back

Fixes #448

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/proto_expr.rs

@@ -143,7 +143,7 @@ impl Expr {
             return "…".into();
         }
         match self {
-            Expr::Value(VariableValue::Alias(id)) => vars.get(id).map_or_else(|| "(missing variable)".into(), |v| v.name.clone()),
+            Expr::Value(VariableValue::Alias(id)) => vars.get(id).map_or_else(|| "(missing variable)".into(), |v| var_ref(vars, v)),
             Expr::Value(VariableValue::String(s)) => quote(s),
             // Exact numbers and `#` colours, so the text reads back as the same expression.
             Expr::Value(VariableValue::Number(n)) => format!("{n}"),
@@ -178,6 +178,18 @@ impl Expr {
     }
 }
 
+/// A variable as written in an expression: its bare name when that reads back as this variable,
+/// otherwise `{name}` (or `{id}` when even the braced name wouldn't), so names like `1`, `2x` or
+/// `"q"` don't read back as literals or errors.
+fn var_ref(vars: &Variables, v: &crate::Variable) -> String {
+    let this = Expr::var(v.id.clone());
+    if Expr::parse(&v.name, vars).is_ok_and(|e| e == this) {
+        return v.name.clone();
+    }
+    let braced = !v.name.contains('}') && v.name.trim() == v.name && vars.get(&v.name).is_some_and(|x| x.id == v.id);
+    if braced { format!("{{{}}}", v.name) } else { format!("{{{}}}", v.id) }
+}
+
 /// A collection or mode name as written inside `mode(…)` or `[…]`: as it is, or quoted when it has
 /// spaces at its ends, quotes or brackets.
 fn name_arg(name: &str) -> String {

crates/doc/src/proto_expr_parse_tests.rs

@@ -176,3 +176,22 @@ fn hostile_text_is_bounded() {
         }
     }
 }
+
+#[test]
+fn names_that_look_like_literals_read_back_as_variables() {
+    for name in ["count", "count total", "2x", "1", "-3", ".5", "(x)", "#ff", "\"q\"", "not", "true"] {
+        let mut v = vars();
+        if let Some(var) = v.variables.iter_mut().find(|x| x.id == "n") {
+            var.name = name.into();
+        }
+        for e in [Expr::var("n"), Expr::op(ExprOp::Add, vec![Expr::var("n"), n(1.0)]), Expr::InMode { variable: "n".into(), mode: "dark".into() }] {
+            let shown = e.display(&v);
+            assert_eq!(Expr::parse(&shown, &v), Ok(e.clone()), "{name:?} shown as {shown:?}");
+        }
+    }
+    // Plain names stay bare, and literals still read as literals.
+    let v = vars();
+    assert_eq!(Expr::var("n").display(&v), "n-name");
+    assert_eq!(parse("1"), n(1.0));
+    assert_eq!(parse("\"q\""), Expr::Value(VariableValue::String("q".into())));
+}