Examples of Parallel Iterator Usage in Rust Compiler

3rd chunk of `src/parallel-rustc.md`

ac9878cc32cb5b831643daea4dd5de5cab159968c280507e000000010000093b

| **ModuleItems::par_foreign_items**(&self, f: impl Fn(ForeignItemId)) | run `f` on all foreign items in the module                   | rustc_middle::hir          |

There are a lot of loops in the compiler which can possibly be parallelized
using these functions. As of <!-- date-check--> August 2022, scenarios where
the parallel iterator function has been used are as follows:

| caller                                                  | scenario                                                     | callee                   |
| ------------------------------------------------------- | ------------------------------------------------------------ | ------------------------ |
| rustc_metadata::rmeta::encoder::prefetch_mir            | Prefetch queries which will be needed later by metadata encoding | par_iter                 |
| rustc_monomorphize::collector::collect_crate_mono_items | Collect monomorphized items reachable from non-generic items | par_for_each_in          |
| rustc_interface::passes::analysis                       | Check the validity of the match statements                   | Map::par_body_owners     |
| rustc_interface::passes::analysis                       | MIR borrow check                                             | Map::par_body_owners     |
| rustc_typeck::check::typeck_item_bodies                 | Type check                                                   | Map::par_body_owners     |
| rustc_interface::passes::hir_id_validator::check_crate  | Check the validity of hir                                    | Map::par_for_each_module |
| rustc_interface::passes::analysis                       | Check the validity of loops body, attributes, naked functions, unstable abi, const bodys | Map::par_for_each_module |
| rustc_interface::passes::analysis                       | Liveness and intrinsic checking of MIR                       | Map::par_for_each_module |
| rustc_interface::passes::analysis                       | Deathness checking                                           | Map::par_for_each_module |
| rustc_interface::passes::analysis                       | Privacy checking                                             | Map::par_for_each_module |
| rustc_lint::late::check_crate                           | Run per-module lints                                         | Map::par_for_each_module |

Title: Examples of Parallel Iterator Usage in Rust Compiler

Summary

This section provides examples of how parallel iterator functions are used within the Rust compiler as of August 2022. It lists the caller functions, the scenarios in which parallel iteration is applied, and the specific parallel iterator functions (callee) being used. The table details scenarios such as prefetching queries for metadata encoding, collecting monomorphized items, various analysis passes, type checking, HIR validation, and running per-module lints.