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add some docs for specify

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XFFXFF 2022-08-27 06:22:04 +08:00
parent e99e45d35a
commit dbe8561721
1 changed files with 12 additions and 0 deletions
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12
components/salsa-2022/src/function/specify.rs
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@ -33,6 +33,18 @@ where
None => panic!("can only use `set` with an active query"),
};
// `specify` only works if the key is a tracked struct created in the current query.
//
// The reason is this. We want to ensure that the same result is reached regardless of
// the "path" that the user takes through the execution graph.
// If you permit values to be specified from other queries, you can have a situation like this:
// * Q0 creates the tracked struct T0
// * Q1 specifies the value for F(T0)
// * Q2 invokes F(T0)
// * Q3 invokes Q1 and then Q2
// * Q4 invokes Q2 and then Q1
//
// Now, if We invoke Q3 first, We get one result for Q2, but if We invoke Q4 first, We get a different value. That's no good.
let database_key_index = key.database_key_index(db);
if !runtime.is_output_of_active_query(database_key_index) {
panic!("can only use `set` on entities created during current query");