Add Section How queries interact with external crate metadata

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

Author: xizheyin

src/query.md

@@ -78,9 +78,6 @@ 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.
 
@@ -174,11 +171,53 @@ Suppose you want to add a new query called `fubar`. You would:
     ```rust,ignore
     pub fn provide(providers: &mut rustc_middle::util::Providers) {
         providers.queries.fubar = fubar;
-        // If you need an external provider:
-        providers.extern_queries.fubar = extern_fubar;
     }
     ```
 
+### How queries interact with external crate metadata
+
+When a query is made for an external crate (i.e., a dependency), the query system needs to load the information from that crate's metadata. 
+This is handled by the [`rustc_metadata` crate][rustc_metadata], which is responsible for decoding and providing the information stored in the `.rmeta` files.
+
+The process works like this:
+
+1. When a query is made, the query system first checks if the `DefId` refers to a local or external crate by checking if `def_id.krate == LOCAL_CRATE`.
+   This determines whether to use the local provider from [`Providers`][providers_struct] or the external provider from [`ExternProviders`][extern_providers_struct].
+
+2. For external crates, the query system will look for a provider in the [`ExternProviders`][extern_providers_struct] struct.
+   The `rustc_metadata` crate registers these external providers through the `provide_extern` function in `rustc_metadata/src/rmeta/decoder/cstore_impl.rs`. Just like:
+
+```rust
+pub fn provide_extern(providers: &mut ExternProviders) {
+    providers.foo = |tcx, def_id| {
+        // Load and decode metadata for external crate
+        let cdata = CStore::from_tcx(tcx).get_crate_data(def_id.krate);
+        cdata.foo(def_id.index)
+    };
+    // Register other external providers...
+}
+```
+
+1. The metadata is stored in a binary format in `.rmeta` files that contains pre-computed information about the external crate, such as types, function signatures, trait implementations, and other information needed by the compiler. When an external query is made, the `rustc_metadata` crate:
+   - Loads the `.rmeta` file for the external crate
+   - Decodes the metadata using the `Decodable` trait
+   - Returns the decoded information to the query system
+
+This approach avoids recompiling external crates, allows for faster compilation of dependent crates, and enables incremental compilation to work across crate boundaries.
+
+Here is a simplified example, when you call `tcx.type_of(def_id)` for a type defined in an external crate, the query system will:
+1. Detect that the `def_id` refers to an external crate by checking `def_id.krate != LOCAL_CRATE`
+2. Call the appropriate provider from `ExternProviders` which was registered by `rustc_metadata`
+3. The provider will load and decode the type information from the external crate's metadata
+4. Return the decoded type to the caller
+
+This is why most `rustc_*` crates only need to provide local providers - the external providers are handled by the metadata system. 
+The only exception is when a crate needs to provide special handling for external queries, in which case it would implement both local and external providers.
+
+[rustc_metadata]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/index.html
+[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
+
 ---
 
 ## Adding a new query