Parallel codegen (#1206)

* Described underlying data structures in parallel code generation and crates they are used in
* Added links
* replace crate information with description of types

Author: Sl1mb0

src/parallel-rustc.md

@@ -10,15 +10,24 @@ These next few sections describe where and how parallelism is currently used,
 and the current status of making parallel compilation the default in `rustc`.
 
 The underlying thread-safe data-structures used in the parallel compiler 
-can be found in `rustc_data_structures/sync.rs`. Some of these data structures
-use the `parking_lot` API.
+can be found in the `rustc_data_structures::sync` module. Some of these data structures
+use the `parking_lot` crate as well.
 
 ## Codegen
 
+There are two underlying thread safe data structures used in code generation:
+
+- `Lrc`
+    -  Which is an [`Arc`][Arc] if `parallel_compiler` is true, and a [`Rc`][Rc]
+       if it is not.
+- `MetadataRef` -> [`OwningRef<Box<dyn Erased + Send + Sync>, [u8]>`][OwningRef]
+    - This data structure is specific to `rustc`.
+
 During [monomorphization][monomorphization] the compiler splits up all the code to 
 be generated into smaller chunks called _codegen units_. These are then generated by 
 independent instances of LLVM running in parallel. At the end, the linker 
-is run to combine all the codegen units together into one binary.
+is run to combine all the codegen units together into one binary. This process
+occurs in the `rustc_codegen_ssa::base` module.
 
 ## Query System 
 
@@ -92,3 +101,6 @@ are a bit out of date):
 [tracking]: https://github.com/rust-lang/rust/issues/48685
 [monomorphization]:https://rustc-dev-guide.rust-lang.org/backend/monomorph.html
 [parallel-rustdoc]:https://github.com/rust-lang/rust/issues/82741
+[Arc]:https://doc.rust-lang.org/std/sync/struct.Arc.html
+[Rc]:https://doc.rust-lang.org/std/rc/struct.Rc.html
+[OwningRef]:https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/owning_ref/index.html