Clarify aspects of query macros

compiler/rustc_middle/src/dep_graph/dep_node.rs

@@ -324,11 +324,12 @@ impl StableOrd for WorkProductId {
     const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
 }
 
+// Note: `$K` and `$V` are unused but present so this can be called by `rustc_with_all_queries`.
 macro_rules! define_dep_nodes {
     (
         $(
             $(#[$attr:meta])*
-            [$($modifiers:tt)*] fn $variant:ident($($K:tt)*) -> $V:ty,
+            [$($modifiers:tt)*] fn $variant:ident($K:ty) -> $V:ty,
         )*
     ) => {
 
@@ -382,20 +383,20 @@ macro_rules! define_dep_nodes {
 }
 
 // Create various data structures for each query, and also for a few things
-// that aren't queries.
+// that aren't queries. The key and return types aren't used, hence the use of `()`.
 rustc_with_all_queries!(define_dep_nodes![
     /// We use this for most things when incr. comp. is turned off.
-    [] fn Null() -> (),
+    [] fn Null(()) -> (),
     /// We use this to create a forever-red node.
-    [] fn Red() -> (),
+    [] fn Red(()) -> (),
     /// We use this to create a side effect node.
-    [] fn SideEffect() -> (),
+    [] fn SideEffect(()) -> (),
     /// We use this to create the anon node with zero dependencies.
-    [] fn AnonZeroDeps() -> (),
-    [] fn TraitSelect() -> (),
-    [] fn CompileCodegenUnit() -> (),
-    [] fn CompileMonoItem() -> (),
-    [] fn Metadata() -> (),
+    [] fn AnonZeroDeps(()) -> (),
+    [] fn TraitSelect(()) -> (),
+    [] fn CompileCodegenUnit(()) -> (),
+    [] fn CompileMonoItem(()) -> (),
+    [] fn Metadata(()) -> (),
 ]);
 
 // WARNING: `construct` is generic and does not know that `CompileCodegenUnit` takes `Symbol`s as keys.