Add link to Providers and ExternProviders in Query System of rdg

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

Author: xizheyin

src/query.md

@@ -78,6 +78,9 @@ during compiler initialization. The macro system generates both structs,
 which act as function tables for all query implementations, where each
 field is a function pointer to the actual provider.
 
+[providers_struct]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.Providers.html
+[extern_providers_struct]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.ExternProviders.html
+
 **Note:** Both the `Providers` and `ExternProviders` structs are generated by macros and act as function tables for all query implementations.
 They are **not** Rust traits, but plain structs with function pointer fields.
 
@@ -268,4 +271,5 @@ More discussion and issues:
 ["Red/Green" dependency tracking in compiler]: https://github.com/rust-lang/rust/issues/42293
 [GitHub issue #42633]: https://github.com/rust-lang/rust/issues/42633
 [Incremental Compilation Beta]: https://internals.rust-lang.org/t/incremental-compilation-beta/4721
-[Incremental Compilation Announcement]: https://blog.rust-lang.org/2016/09/08/incremental.html
+[Incremental Compilation Announcement]: https://blog.rust-lang.org/2016/09/08/incremental.html
+