Simplify the cache_on_disk_if modifier to just cache_on_disk

Queries with both `cache_on_disk` and `separate_provide_extern` will only
disk-cache values for local keys.

Other queries with `cache_on_disk` will disk-cache all values unconditionally.

Author: Zalathar

src/queries/incremental-compilation-in-detail.md

@@ -479,11 +479,11 @@ respect to incremental compilation:
       and the projection queries act as a "firewall", shielding their dependents
       from the unconditionally red `no_hash` node.
 
- - `cache_on_disk_if` - This attribute is what determines which query results
-   are persisted in the incremental compilation query result cache.
-   The attribute takes an expression that allows per query invocation decisions.
-   For example, it makes no sense to store values from upstream
-   crates in the cache because they are already available in the upstream crate's metadata.
+ - `cache_on_disk` - The query's return values are cached to disk, and can be
+   loaded by subsequent sessions if the corresponding dep node is green.
+   If the `separate_provide_extern` modifier is also present, values will only
+   be cached to disk for "local" keys, because values for external crates should
+   be loadable from crate metadata instead.
 
 [mod]: ../query.html#adding-a-new-kind-of-query
 

src/query.md

@@ -275,8 +275,9 @@ This looks something like:
 rustc_queries! {
     /// Records the type of every item.
     query type_of(key: DefId) -> Ty<'tcx> {
-        cache_on_disk_if { key.is_local() }
         desc { |tcx| "computing the type of `{}`", tcx.def_path_str(key) }
+        cache_on_disk
+        separate_provide_extern
     }
     ...
 }