Update src/query.md

Co-authored-by: Boxy <rust@boxyuwu.dev>

Author: xizheyin

src/query.md

@@ -143,6 +143,17 @@ To register providers, each crate exposes a [`provide`][provide_fn] function tha
 
 [provide_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/hir/fn.provide.html
 
+```rust,ignore
+pub fn provide(providers: &mut query::Providers) {
+    *providers = query::Providers {
+        type_of,
+        // ... add more providers here
+        ..*providers
+    };
+}
+```
+
+Note that this function accepts `query::Providers` not `util::Providers`. It is exceedingly rare to need a `provide` function that doesn't just accept `query::Providers`. If more than the `queries` field of `util::Providers`  is being updated then `util::Providers` can be accepted instead:
 ```rust,ignore
 pub fn provide(providers: &mut rustc_middle::util::Providers) {
     providers.queries.type_of = type_of;
@@ -156,6 +167,8 @@ pub fn provide(providers: &mut rustc_middle::util::Providers) {
 }
 ```
 
+Note that `util::Providers` implements `DerefMut` to `query::Providers` so callers of the `provide` functions can pass in a `util::Providers` and it will just work for provider functions that accept `query::Providers` too
+
 - This function takes a mutable reference to the `Providers` struct and sets the fields to point to the correct provider functions.
 - You can assign fields individually for each provider type (local, external, and hooks).