doc: Describe relevant error modes during rule compilation (#18663)

src/python/pants/engine/internals/scheduler_test.py

@@ -243,9 +243,31 @@ def test_outlined_get() -> None:
     with pytest.raises(ExecutionError) as exc:
         rule_runner.request(int, [])
     assert (
-        "Get(int, str, hello) was not detected in your @rule body at rule compile time. "
-        "Was the `Get` constructor called in a separate function, or perhaps "
-        "dynamically? If so, it must be inlined into the @rule body."
+        "Get(int, str, hello) was not detected in your @rule body at rule compile time."
+    ) in str(exc.value.args[0])
+
+
+@rule
+async def uses_rule_helper_before_definition() -> int:
+    return await get_after_rule()
+
+
+async def get_after_rule() -> int:
+    return await Get(int, str, "hello")
+
+
+def test_rule_helper_after_rule_definition_fails() -> None:
+    rule_runner = RuleRunner(
+        rules=[
+            uses_rule_helper_before_definition,
+            QueryRule(int, []),
+        ],
+    )
+
+    with pytest.raises(ExecutionError) as exc:
+        rule_runner.request(int, [])
+    assert (
+        "Get(int, str, hello) was not detected in your @rule body at rule compile time."
     ) in str(exc.value.args[0])
 
 

src/rust/engine/src/nodes.rs

@@ -1146,8 +1146,9 @@ impl Task {
                 // `type(input) != input_type`.
                 throw(format!(
                   "{get} was not detected in your @rule body at rule compile time. \
-                  Was the `Get` constructor called in a separate function, or perhaps \
-                  dynamically? If so, it must be inlined into the @rule body.",
+                  Was the `Get` constructor called in a non async-function, or \
+                  was it inside an async function defined after the @rule? \
+                  Make sure the `Get` is defined before or inside the @rule body.",
                 ))
               }
             })?;