# Overview of the compiler

<!-- toc -->

This chapter is about the overall process of compiling a program -- how
everything fits together.

The Rust compiler is special in two ways: it does things to your code that
other compilers don't do (e.g. borrow-checking) and it has a lot of
unconventional implementation choices (e.g. queries). We will talk about these
in turn in this chapter, and in the rest of the guide, we will look at the
individual pieces in more detail.

## What the compiler does to your code

So first, let's look at what the compiler does to your code. For now, we will
avoid mentioning how the compiler implements these steps except as needed.

### Invocation

Compilation begins when a user writes a Rust source program in text and invokes
the `rustc` compiler on it. The work that the compiler needs to perform is
defined by command-line options. For example, it is possible to enable nightly
features (`-Z` flags), perform `check`-only builds, or emit the LLVM
Intermediate Representation (`LLVM-IR`) rather than executable machine code.
The `rustc` executable call may be indirect through the use of `cargo`.

Command line argument parsing occurs in the [`rustc_driver`]. This crate
defines the compile configuration that is requested by the user and passes it
to the rest of the compilation process as a [`rustc_interface::Config`].

### Lexing and parsing

The raw Rust source text is analyzed by a low-level *lexer* located in
[`rustc_lexer`]. At this stage, the source text is turned into a stream of
atomic source code units known as _tokens_.  The `lexer` supports the
Unicode character encoding.

The token stream passes through a higher-level lexer located in
[`rustc_parse`] to prepare for the next stage of the compile process. The
[`StringReader`] `struct` is used at this stage to perform a set of validations
and turn strings into interned symbols (_interning_ is discussed later).
[String interning] is a way of storing only one immutable
copy of each distinct string value.

The lexer has a small interface and doesn't depend directly on the diagnostic
infrastructure in `rustc`. Instead it provides diagnostics as plain data which
are emitted in [`rustc_parse::lexer`] as real diagnostics. The `lexer`
preserves full fidelity information for both IDEs and procedural macros
(sometimes referred to as "proc-macros").

The *parser* [translates the token stream from the `lexer` into an Abstract Syntax
Tree (AST)][parser]. It uses a recursive descent (top-down) approach to syntax
analysis. The crate entry points for the `parser` are the
[`Parser::parse_crate_mod()`][parse_crate_mod] and [`Parser::parse_mod()`][parse_mod]
methods found in [`rustc_parse::parser::Parser`]. The external module parsing
entry point is [`rustc_expand::module::parse_external_mod`][parse_external_mod].
And the macro-`parser` entry point is [`Parser::parse_nonterminal()`][parse_nonterminal].

Parsing is performed with a set of [`parser`] utility methods including [`bump`],
[`check`], [`eat`], [`expect`], [`look_ahead`].

Parsing is organized by semantic construct. Separate
`parse_*` methods can be found in the [`rustc_parse`][rustc_parse_parser_dir]
directory. The source file name follows the construct name. For example, the
following files are found in the `parser`:

- [`expr.rs`](https://github.com/rust-lang/rust/blob/master/compiler/rustc_parse/src/parser/expr.rs)
- [`pat.rs`](https://github.com/rust-lang/rust/blob/master/compiler/rustc_parse/src/parser/pat.rs)
- [`ty.rs`](https://github.com/rust-lang/rust/blob/master/compiler/rustc_parse/src/parser/ty.rs)
- [`stmt.rs`](https://github.com/rust-lang/rust/blob/master/compiler/rustc_parse/src/parser/stmt.rs)

This naming scheme is used across many compiler stages. You will find either a
file or directory with the same name across the parsing, lowering, type
checking, [Typed High-level Intermediate Representation (`THIR`)][thir] lowering, and
[Mid-level Intermediate Representation (`MIR`)][mir] building sources.

Macro-expansion, `AST`-validation, name-resolution, and early linting also take
place during the lexing and parsing stage.

The [`rustc_ast::ast`]::{[`Crate`], [`Expr`], [`Pat`], ...} `AST` nodes are
returned from the parser while the standard [`Diag`] API is used
for error handling. Generally Rust's compiler will try to recover from errors
by parsing a superset of Rust's grammar, while also emitting an error type.

### `AST` lowering

Next the `AST` is converted into [High-Level Intermediate Representation
(`HIR`)][hir], a more compiler-friendly representation of the `AST`. This process
is called "lowering" and involves a lot of desugaring (the expansion and
formalizing of shortened or abbreviated syntax constructs) of things like loops
and `async fn`.

We then use the `HIR` to do [*type inference*] (the process of automatic
detection of the type of an expression), [*trait solving*] (the process of
pairing up an impl with each reference to a `trait`), and [*type checking*]. Type
checking is the process of converting the types found in the `HIR` ([`hir::Ty`]),
which represent what the user wrote, into the internal representation used by
the compiler ([`Ty<'tcx>`]). It's called type checking because the information
is used to verify the type safety, correctness and coherence of the types used
in the program.

### `MIR` lowering

The `HIR` is further lowered to `MIR`
(used for [borrow checking]) by constructing the `THIR`  (an even more desugared `HIR` used for
pattern and exhaustiveness checking) to convert into `MIR`.

We do [many optimizations on the MIR][mir-opt] because it is generic and that
improves later code generation and compilation speed. It is easier to do some
optimizations at `MIR` level than at `LLVM-IR` level. For example LLVM doesn't seem
to be able to optimize the pattern the [`simplify_try`] `MIR`-opt looks for.

Rust code is also [_monomorphized_] during code generation, which means making
copies of all the generic code with the type parameters replaced by concrete
types. To do this, we need to collect a list of what concrete types to generate
code for. This is called _monomorphization collection_ and it happens at the
`MIR` level.

[_monomorphized_]: https://en.wikipedia.org/wiki/Monomorphization

### Code generation

We then begin what is simply called _code generation_ or _codegen_. The [code
generation stage][codegen] is when higher-level representations of source are
turned into an executable binary. Since `rustc` uses LLVM for code generation,
the first step is to convert the `MIR` to `LLVM-IR`. This is where the `MIR` is
actually monomorphized. The `LLVM-IR` is passed to LLVM, which does a lot more
optimizations on it, emitting machine code which is basically assembly code
with additional low-level types and annotations added (e.g. an ELF object or
`WASM`). The different libraries/binaries are then linked together to produce
the final binary.

[*trait solving*]: traits/resolution.md
[*type checking*]: type-checking.md
[*type inference*]: type-inference.md
[`bump`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html#method.bump
[`check`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html#method.check
[`Crate`]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_ast/ast/struct.Crate.html
[`diag`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/struct.Diag.html
[`eat`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html#method.eat
[`expect`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html#method.expect
[`Expr`]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_ast/ast/struct.Expr.html
[`hir::Ty`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Ty.html
[`look_ahead`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html#method.look_ahead
[`Parser`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html
[`Pat`]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_ast/ast/struct.Pat.html
[`rustc_ast::ast`]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_ast/index.html
[`rustc_driver`]: rustc-driver.md
[`rustc_interface::Config`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_interface/interface/struct.Config.html
[`rustc_lexer`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html
[`rustc_parse::lexer`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/lexer/index.html
[`rustc_parse::parser::Parser`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html
[`rustc_parse`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/index.html
[`simplify_try`]: https://github.com/rust-lang/rust/pull/66282
[`StringReader`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/lexer/struct.StringReader.html
[`Ty<'tcx>`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html
[borrow checking]: borrow_check.md
[codegen]: backend/codegen.md
[hir]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/index.html
[lex]: the-parser.md
[mir-opt]: mir/optimizations.md
[mir]: mir/index.md
[parse_crate_mod]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html#method.parse_crate_mod
[parse_external_mod]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/module/fn.parse_external_mod.html
[parse_mod]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html#method.parse_mod
[parse_nonterminal]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html#method.parse_nonterminal
[parser]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/index.html
[rustc_parse_parser_dir]: https://github.com/rust-lang/rust/tree/master/compiler/rustc_parse/src/parser
[String interning]: https://en.wikipedia.org/wiki/String_interning
[thir]: ./thir.md

## How it does it

Now that we have a high-level view of what the compiler does to your code,
let's take a high-level view of _how_ it does all that stuff. There are a lot
of constraints and conflicting goals that the compiler needs to
satisfy/optimize for. For example,

- Compilation speed: how fast is it to compile a program? More/better
  compile-time analyses often means compilation is slower.
  - Also, we want to support incremental compilation, so we need to take that
    into account. How can we keep track of what work needs to be redone and
    what can be reused if the user modifies their program?
    - Also we can't store too much stuff in the incremental cache because
      it would take a long time to load from disk and it could take a lot
      of space on the user's system...
- Compiler memory usage: while compiling a program, we don't want to use more
  memory than we need.
- Program speed: how fast is your compiled program? More/better compile-time
  analyses often means the compiler can do better optimizations.
- Program size: how large is the compiled binary? Similar to the previous
  point.
- Compiler compilation speed: how long does it take to compile the compiler?
  This impacts contributors and compiler maintenance.
- Implementation complexity: building a compiler is one of the hardest
  things a person/group can do, and Rust is not a very simple language, so how
  do we make the compiler's code base manageable?
- Compiler correctness: the binaries produced by the compiler should do what
  the input programs says they do, and should continue to do so despite the
  tremendous amount of change constantly going on.
- Integration: a number of other tools need to use the compiler in
  various ways (e.g. `cargo`, `clippy`, `MIRI`) that must be supported.
- Compiler stability: the compiler should not crash or fail ungracefully on the
  stable channel.
- Rust stability: the compiler must respect Rust's stability guarantees by not
  breaking programs that previously compiled despite the many changes that are
  always going on to its implementation.
- Limitations of other tools: `rustc` uses LLVM in its backend, and LLVM has some
  strengths we leverage and some aspects we need to work around.

So, as you continue your journey through the rest of the guide, keep these
things in mind. They will often inform decisions that we make.

### Intermediate representations

As with most compilers, `rustc` uses some intermediate representations (IRs) to
facilitate computations. In general, working directly with the source code is
extremely inconvenient and error-prone. Source code is designed to be human-friendly while at
the same time being unambiguous, but it's less convenient for doing something
like, say, type checking.

Instead most compilers, including `rustc`, build some sort of IR out of the
source code which is easier to analyze. `rustc` has a few IRs, each optimized
for different purposes:

- Token stream: the lexer produces a stream of tokens directly from the source
  code. This stream of tokens is easier for the parser to deal with than raw
  text.
- Abstract Syntax Tree (`AST`): the abstract syntax tree is built from the stream
  of tokens produced by the lexer. It represents
  pretty much exactly what the user wrote. It helps to do some syntactic sanity
  checking (e.g. checking that a type is expected where the user wrote one).
- High-level IR (HIR): This is a sort of desugared `AST`. It's still close
  to what the user wrote syntactically, but it includes some implicit things
  such as some elided lifetimes, etc. This IR is amenable to type checking.
- Typed `HIR` (THIR) _formerly High-level Abstract IR (HAIR)_: This is an
  intermediate between `HIR` and MIR. It is like the `HIR` but it is fully typed
  and a bit more desugared (e.g. method calls and implicit dereferences are
  made fully explicit). As a result, it is easier to lower to `MIR` from `THIR`  than
  from HIR.
- Middle-level IR (`MIR`): This IR is basically a Control-Flow Graph (CFG). A CFG
  is a type of diagram that shows the basic blocks of a program and how control
  flow can go between them. Likewise, `MIR` also has a bunch of basic blocks with
  simple typed statements inside them (e.g. assignment, simple computations,
  etc) and control flow edges to other basic blocks (e.g., calls, dropping
  values). `MIR` is used for borrow checking and other
  important dataflow-based checks, such as checking for uninitialized values.
  It is also used for a series of optimizations and for constant evaluation (via
  `MIRI`). Because `MIR` is still generic, we can do a lot of analyses here more
  efficiently than after monomorphization.
- `LLVM-IR`: This is the standard form of all input to the LLVM compiler. `LLVM-IR`
  is a sort of typed assembly language with lots of annotations. It's
  a standard format that is used by all compilers that use LLVM (e.g. the clang
  C compiler also outputs `LLVM-IR`). `LLVM-IR` is designed to be easy for other
  compilers to emit and also rich enough for LLVM to run a bunch of
  optimizations on it.

One other thing to note is that many values in the compiler are _interned_.
This is a performance and memory optimization in which we allocate the values in
a special allocator called an
_[arena]_. Then, we pass
around references to the values allocated in the arena. This allows us to make
sure that identical values (e.g. types in your program) are only allocated once
and can be compared cheaply by comparing pointers. Many of the intermediate
representations are interned.

[arena]: https://en.wikipedia.org/wiki/Region-based_memory_management

### Queries

The first big implementation choice is Rust's use of the _query_ system in its
compiler. The Rust compiler _is not_ organized as a series of passes over the
code which execute sequentially. The Rust compiler does this to make
incremental compilation possible -- that is, if the user makes a change to
their program and recompiles, we want to do as little redundant work as
possible to output the new binary.

In `rustc`, all the major steps above are organized as a bunch of queries that
call each other. For example, there is a query to ask for the type of something
and another to ask for the optimized `MIR` of a function. These queries can call
each other and are all tracked through the query system. The results of the
queries are cached on disk so that the compiler can tell which queries' results
changed from the last compilation and only redo those. This is how incremental
compilation works.

In principle, for the query-field steps, we do each of the above for each item
individually. For example, we will take the `HIR` for a function and use queries
to ask for the `LLVM-IR` for that HIR. This drives the generation of optimized
`MIR`, which drives the borrow checker, which drives the generation of `MIR`, and
so on.

... except that this is very over-simplified. In fact, some queries are not
cached on disk, and some parts of the compiler have to run for all code anyway
for correctness even if the code is dead code (e.g. the borrow checker). For
example, [currently the `mir_borrowck` query is first executed on all functions
of a crate.][passes] Then the codegen backend invokes the
`collect_and_partition_mono_items` query, which first recursively requests the
`optimized_mir` for all reachable functions, which in turn runs `mir_borrowck`
for that function and then creates codegen units. This kind of split will need
to remain to ensure that unreachable functions still have their errors emitted.

[passes]: https://github.com/rust-lang/rust/blob/e69c7306e2be08939d95f14229e3f96566fb206c/compiler/rustc_interface/src/passes.rs#L791

Moreover, the compiler wasn't originally built to use a query system; the query
system has been retrofitted into the compiler, so parts of it are not query-field
yet. Also, LLVM isn't our code, so that isn't querified either. The plan is to
eventually query-fy all of the steps listed in the previous section,
but as of <!-- date-check --> November 2022, only the steps between `HIR` and
`LLVM-IR` are query-field. That is, lexing, parsing, name resolution, and macro
expansion are done all at once for the whole program.

One other thing to mention here is the all-important "typing context",
[`TyCtxt`], which is a giant struct that is at the center of all things.
(Note that the name is mostly historic. This is _not_ a "typing context" in the
sense of `Γ` or `Δ` from type theory. The name is retained because that's what
the name of the struct is in the source code.) All
queries are defined as methods on the [`TyCtxt`] type, and the in-memory query
cache is stored there too. In the code, there is usually a variable called
`tcx` which is a handle on the typing context. You will also see lifetimes with
the name `'tcx`, which means that something is tied to the lifetime of the
[`TyCtxt`] (usually it is stored or interned there).

[`TyCtxt`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html

### `ty::Ty`

Types are really important in Rust, and they form the core of a lot of compiler
analyses. The main type (in the compiler) that represents types (in the user's
program) is [`rustc_middle::ty::Ty`][ty]. This is so important that we have a whole chapter
on [`ty::Ty`][ty], but for now, we just want to mention that it exists and is the way
`rustc` represents types!

Also note that the [`rustc_middle::ty`] module defines the [`TyCtxt`] struct we mentioned before.

[ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html
[`rustc_middle::ty`]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_middle/ty/index.html

### Parallelism

Compiler performance is a problem that we would like to improve on
(and are always working on). One aspect of that is parallelizing
`rustc` itself.

Currently, there is only one part of rustc that is parallel by default: 
[code generation](./parallel-rustc.md#Codegen).

However, the rest of the compiler is still not yet parallel. There have been
lots of efforts spent on this, but it is generally a hard problem. The current
approach is to turn [`RefCell`]s into [`Mutex`]s -- that is, we
switch to thread-safe internal mutability. However, there are ongoing
challenges with lock contention, maintaining query-system invariants under
concurrency, and the complexity of the code base. One can try out the current
work by enabling parallel compilation in `config.toml`. It's still early days,
but there are already some promising performance improvements.

[`RefCell`]: https://doc.rust-lang.org/std/cell/struct.RefCell.html
[`Mutex`]: https://doc.rust-lang.org/std/sync/struct.Mutex.html

### Bootstrapping

`rustc` itself is written in Rust. So how do we compile the compiler? We use an
older compiler to compile the newer compiler. This is called [_bootstrapping_].

Bootstrapping has a lot of interesting implications. For example, it means
that one of the major users of Rust is the Rust compiler, so we are
constantly testing our own software ("eating our own dogfood").

For more details on bootstrapping, see
[the bootstrapping section of the guide][rustc-bootstrap].

[_bootstrapping_]: https://en.wikipedia.org/wiki/Bootstrapping_(compilers)
[rustc-bootstrap]: building/bootstrapping/intro.md

<!--
# Unresolved Questions

- Does LLVM ever do optimizations in debug builds?
- How do I explore phases of the compile process in my own sources (lexer,
  parser, HIR, etc)? - e.g., `cargo rustc -- -Z unpretty=hir-tree` allows you to
  view `HIR` representation
- What is the main source entry point for `X`?
- Where do phases diverge for cross-compilation to machine code across
  different platforms?
-->

# References

- Command line parsing
  - Guide: [The Rustc Driver and Interface](rustc-driver.md)
  - Driver definition: [`rustc_driver`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_driver/)
  - Main entry point: [`rustc_session::config::build_session_options`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/config/fn.build_session_options.html)
- Lexical Analysis: Lex the user program to a stream of tokens
  - Guide: [Lexing and Parsing](the-parser.md)
  - Lexer definition: [`rustc_lexer`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html)
  - Main entry point: [`rustc_lexer::cursor::Cursor::advance_token`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/cursor/struct.Cursor.html#method.advance_token)
- Parsing: Parse the stream of tokens to an Abstract Syntax Tree (AST)
  - Guide: [Lexing and Parsing](the-parser.md)
  - Guide: [Macro Expansion](macro-expansion.md)
  - Guide: [Name Resolution](name-resolution.md)
  - Parser definition: [`rustc_parse`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/index.html)
  - Main entry points:
    - [Entry point for first file in crate](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_interface/passes/fn.parse.html)
    - [Entry point for outline module parsing](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/module/fn.parse_external_mod.html)
    - [Entry point for macro fragments][parse_nonterminal]
  - `AST` definition: [`rustc_ast`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html)
  - Feature gating: **TODO**
  - Early linting: **TODO**
- The High Level Intermediate Representation (HIR)
  - Guide: [The HIR](hir.md)
  - Guide: [Identifiers in the HIR](hir.md#identifiers-in-the-hir)
  - Guide: [The `HIR` Map](hir.md#the-hir-map)
  - Guide: [Lowering `AST` to `HIR`](ast-lowering.md)
  - How to view `HIR` representation for your code `cargo rustc -- -Z unpretty=hir-tree`
  - Rustc `HIR` definition: [`rustc_hir`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/index.html)
  - Main entry point: **TODO**
  - Late linting: **TODO**
- Type Inference
  - Guide: [Type Inference](type-inference.md)
  - Guide: [The ty Module: Representing Types](ty.md) (semantics)
  - Main entry point (type inference): [`InferCtxtBuilder::enter`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxtBuilder.html#method.enter)
  - Main entry point (type checking bodies): [the `typeck` query](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.typeck)
    - These two functions can't be decoupled.
- The Mid Level Intermediate Representation (MIR)
  - Guide: [The `MIR` (Mid level IR)](mir/index.md)
  - Definition: [`rustc_middle/src/mir`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/index.html)
  - Definition of sources that manipulates the MIR: [`rustc_mir_build`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_build/index.html), [`rustc_mir_dataflow`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/index.html), [`rustc_mir_transform`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/index.html)
- The Borrow Checker
  - Guide: [MIR Borrow Check](borrow_check.md)
  - Definition: [`rustc_borrowck`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/index.html)
  - Main entry point: [`mir_borrowck` query](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/fn.mir_borrowck.html)
- `MIR` Optimizations
  - Guide: [MIR Optimizations](mir/optimizations.md)
  - Definition: [`rustc_mir_transform`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/index.html)
  - Main entry point: [`optimized_mir` query](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/fn.optimized_mir.html)
- Code Generation
  - Guide: [Code Generation](backend/codegen.md)
  - Generating Machine Code from `LLVM-IR` with LLVM - **TODO: reference?**
  - Main entry point: [`rustc_codegen_ssa::base::codegen_crate`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/base/fn.codegen_crate.html)
    - This monomorphizes and produces `LLVM-IR` for one codegen unit. It then
      starts a background thread to run LLVM, which must be joined later.
    - Monomorphization happens lazily via [`FunctionCx::monomorphize`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/struct.FunctionCx.html#method.monomorphize) and [`rustc_codegen_ssa::base::codegen_instance `](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/base/fn.codegen_instance.html)
