Update field mutation error message (typst#1742)

crates/typst/src/eval/mod.rs

@@ -68,7 +68,7 @@ use unicode_segmentation::UnicodeSegmentation;
 
 use self::func::{CapturesVisitor, Closure};
 use crate::diag::{
-    bail, error, warning, At, FileError, SourceDiagnostic, SourceResult, StrResult,
+    bail, error, warning, At, FileError, Hint, SourceDiagnostic, SourceResult, StrResult,
     Trace, Tracepoint,
 };
 use crate::model::{
@@ -1962,11 +1962,24 @@ fn access_dict<'a>(
 ) -> SourceResult<&'a mut Dict> {
     match access.target().access(vm)? {
         Value::Dict(dict) => Ok(dict),
-        value => bail!(
-            access.target().span(),
-            "expected dictionary, found {}",
-            value.type_name(),
-        ),
+        value => {
+            let type_name = value.type_name();
+            let span = access.target().span();
+            if matches!(
+                value, // those types have their own field getters
+                Value::Symbol(_) | Value::Content(_) | Value::Module(_) | Value::Func(_)
+            ) {
+                bail!(span, "cannot mutate fields on {type_name}");
+            } else if fields::fields_on(type_name).is_empty() {
+                bail!(span, "{type_name} does not have accessible fields");
+            } else {
+                // type supports static fields, which don't yet have
+                // setters
+                Err(eco_format!("fields on {type_name} are not yet mutable"))
+                    .hint(eco_format!("try creating a new {type_name} with the updated field value instead"))
+                    .at(span)
+            }
+        }
     }
 }
 

tests/typ/compiler/dict.typ

@@ -92,10 +92,3 @@
 ---
 // Error: 3-15 cannot mutate a temporary value
 #((key: "val").other = "some")
-
----
-#{
-  let object = none
-  // Error: 3-9 expected dictionary, found none
-  object.property = "value"
-}

tests/typ/compiler/field.typ

@@ -148,3 +148,53 @@
 #test((top + start).y, top)
 #test((bottom + end).y, bottom)
 #test((horizon + center).y, horizon)
+
+---
+#{
+  let object = sym.eq.not
+  // Error: 3-9 cannot mutate fields on symbol
+  object.property = "value"
+}
+
+---
+#{
+  let object = [hi]
+  // Error: 3-9 cannot mutate fields on content
+  object.property = "value"
+}
+
+---
+#{
+  let object = calc
+  // Error: 3-9 cannot mutate fields on module
+  object.property = "value"
+}
+
+---
+#{
+  let object = calc.sin
+  // Error: 3-9 cannot mutate fields on function
+  object.property = "value"
+}
+
+---
+#{
+  let object = none
+  // Error: 3-9 none does not have accessible fields
+  object.property = "value"
+}
+
+---
+#{
+  let object = 10
+  // Error: 3-9 integer does not have accessible fields
+  object.property = "value"
+}
+
+---
+#{
+  let s = 1pt + red
+  // Error: 3-4 fields on stroke are not yet mutable
+  // Hint: 3-4 try creating a new stroke with the updated field value instead
+  s.thickness = 5pt
+}