Add warnings when labels are ignored or unattached (#4783)

crates/typst/src/eval/markup.rs

@@ -1,6 +1,8 @@
 use crate::diag::{warning, SourceResult};
 use crate::eval::{Eval, Vm};
-use crate::foundations::{Content, Label, NativeElement, Smart, Unlabellable, Value};
+use crate::foundations::{
+    Content, Label, NativeElement, Repr, Smart, Unlabellable, Value,
+};
 use crate::math::EquationElem;
 use crate::model::{
     EmphElem, EnumItem, HeadingElem, LinkElem, ListItem, ParbreakElem, RefElem,
@@ -52,7 +54,20 @@ fn eval_markup<'a>(
                     if let Some(elem) =
                         seq.iter_mut().rev().find(|node| !node.can::<dyn Unlabellable>())
                     {
+                        if elem.label().is_some() {
+                            vm.engine.sink.warn(warning!(
+                                elem.span(), "content labelled multiple times";
+                                hint: "only the last label is used, the rest are ignored",
+                            ));
+                        }
+
                         *elem = std::mem::take(elem).labelled(label);
+                    } else {
+                        vm.engine.sink.warn(warning!(
+                            expr.span(),
+                            "label `{}` is not attached to anything",
+                            label.repr()
+                        ));
                     }
                 }
                 value => seq.push(value.display().spanned(expr.span())),

tests/suite/foundations/label.typ

@@ -50,6 +50,7 @@ _Visible_
 --- label-in-block ---
 // Test that label only works within one content block.
 #show <strike>: strike
+// Warning: 13-21 label `<strike>` is not attached to anything
 *This is* #[<strike>] *protected.*
 *This is not.* <strike>
 
@@ -74,3 +75,25 @@ _Visible_
 // Hint: 7-7 labels can only be applied in markup mode
 // Hint: 7-7 try wrapping your code in a markup block (`[ ]`)
 #{ [A] <a> }
+
+--- label-multiple-ignored-warn ---
+// Warning: 1-8 content labelled multiple times
+// Hint: 1-8 only the last label is used, the rest are ignored
+= Hello <a> <b>
+
+// Warning: 12-19 content labelled multiple times
+// Hint: 12-19 only the last label is used, the rest are ignored
+#let f = [#block()<c>]
+#f<d>
+
+// Warning: 6-13 content labelled multiple times
+// Hint: 6-13 only the last label is used, the rest are ignored
+#[#[#block()]<e>]<f>
+
+// Error: 1-3 label `<a>` does not exist in the document
+@a
+
+--- label-unattached-warn ---
+#set heading(numbering: "1.")
+// Warning: 1-4 label `<a>` is not attached to anything
+<a>