Adjust an impl of IntoQueryParam.

`IntoQueryParam` is a trait that lets query callers be a bit sloppy with
arguments:
- types similar to `DefId` and `LocalDefId` will be auto-converted
- reference types will be auto-derefed

I find this sloppiness a bit odd and distasteful; I'm not sure why
queries get special treatment like this. Anyway, it turns out that the
auto-derefing is only used for `&DefId`. This commit replaces the
blanket impl for `&'a P` with one for `&DefId`, which enables the next
commit.

Author: nnethercote

compiler/rustc_middle/src/query/plumbing.rs

@@ -627,24 +627,26 @@ mod sealed {
 
     use super::{DefId, LocalDefId, OwnerId};
 
-    /// An analogue of the `Into` trait that's intended only for query parameters.
+    /// An analogue of the `Into` trait that's intended only for certain query keys.
     ///
-    /// This exists to allow queries to accept either `DefId` or `LocalDefId` while requiring that the
-    /// user call `to_def_id` to convert between them everywhere else.
+    /// This lets queries with `DefId` or `LocalDefId` keys be called with closely-related types
+    /// without requiring an explicit conversion, as a mild conveniences for common cases.
     pub trait IntoQueryParam<P> {
         fn into_query_param(self) -> P;
     }
 
+    // This blanket impl provides a no-op "conversion" for all non-`DefId`/`LocalDefId` types.
     impl<P> IntoQueryParam<P> for P {
         #[inline(always)]
         fn into_query_param(self) -> P {
             self
         }
     }
 
-    impl<'a, P: Copy> IntoQueryParam<P> for &'a P {
+    #[allow(rustc::pass_by_value)]
+    impl IntoQueryParam<DefId> for &DefId {
         #[inline(always)]
-        fn into_query_param(self) -> P {
+        fn into_query_param(self) -> DefId {
             *self
         }
     }